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The coastal mink on Vancouver Island, British Columbia Halter, David Francis 1976

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THE COASTAL MINK ON VANCOUVER ISLAND, BRITISH COLUMBIA by DAVID FRANCIS HATLER B.S., University of Alaska, 1964 M.S., University of Alaska, 1967 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of Zoology We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April, 1976 David Francis Hatler, 1976 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d that c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depa rtment The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date ABSTRACT The mink (Mustela vison evagor) which frequents the P a c i f i c coastal shores of Vancouver Island, B r i t i s h Columbia, forages p r i m a r i l y i n the marine i n t e r t i d a l zone, feeding mostly upon small crustaceans and f i s h . Decapod crabs of the family Cancridae are taken throughout the year, but e s p e c i a l l y i n summer when they move into i n t e r t i d a l waters to mate and moult. Kelp crabs (Pugettia) and most f i s h species appear to be most vulnerable i n winter, when storms create i n s t a b i l i t y i n t h e i r near-shore habitat. Water depth, substrate p a r t i c l e s i z e , and the degree of protection from heavy wave action are among the most important factors i n f l u e n c i n g the success of mink hunting for these organisms. Along these food-rich shores, most mink hunted at success rates which would have provided t h e i r d a i l y requirements i n less than two hours of hunting a c t i v i t y . Nevertheless, observations of i n d i v i d u a l s which hunted with less than average success, under various conditions, in d i c a t e that a c c e s s i b i l i t y to food varies with place and time, e s p e c i a l l y r e l a t i v e to tide l e v e l . Males r e g u l a r l y outnumbered females i n a l l areas studied, and the proportion of juveniles i n study populations was lower than expected, averaging less than two young per adult female i n a l l seasons. The mating season, i n t h i s area, peaks during l a t e May and the f i r s t h a l f of June. Despite the fact that t h i s i s two months or more l a t e r than has been recorded for mink elsewhere, there i s evidence that the delayed implantation c h a r a c t e r i s t i c of the species hasbbeenrretained. Although both mating and p a r t u r i t i o n occur when general food a v a i l a b i l i t y and climate are favorable, the apparently short delay (10 to 15 days) does not appear to enhance t h i s timing. I t i s speculated that the delay constitutes a period of convalesence i for female mink between the rigors of the mating season and the demands of maternal functions. Results from fur farm studies on mink productivity, interpreted within the framework of observations from the wild situation, suggest that productivity varies inversely with the frequency of contact which female mink have with other mink before and during the mating season. Populatiotiadens.ities ranged from about 1.5 to more than 3 animals per kilometer of shoreline on the Vancouver Island study areas. As has been the case with mink populations studied elsewhere, turnover was rapid with losses of 50 per cent or more between successive (4 month) seasons. There was l i t t l e evidence of emigration and most losses are believedcto have been due to mortality. Individual mink were known or suspected to have died from encounters with other mink, prey species, and predators, but there was no evidence that these were regular sources of mortality. A die-off which occurred in 1970 may have been related to a local build up of paralytic shellfish toxin. Animals examined were relatively free of parasites although sinus worms, which caused severe skull damage in some cases, and an unidentified mite, which apparently caused open wounds on the hindquarters of many mink in summer, both occurred fre-quently enough to have had population consequences. Summer inanition, characterized by deteriorating condition of numerous individuals and increased incidence of livetrap deaths during the period May-July, is believed to have been related to stresses imposed by reproductive activity and moulting, both of which occur at that time; this may have been the prime factor contributing to population turnover. Most individuals were relatively sedentary, ranging over only small areas even during the breeding season. Males had larger ranges (mean=0.72 km of i i shoreline) than females (0.41 km) and, due to a few longer d i s p e r s a l move-ments (up to 9 km), juvenile males emerged as the most mobile c l a s s . There i s evidence that range s i z e was inversely r e l a t e d to the q u a l i t y of l o c a l hunting habitat. Although much overlap occurred, ..individuals maintained d i s t i n c t ranges ( t e r r i t o r i e s ) . Intrusions on portions of a t e r r i t o r y occurred p r i m a r i l y when the resident t e r r i t o r y holder was not present to assert himself, although avoidance rather than confrontation appeared to be the primary method by which the animals keep, separate. The r e s u l t s of en-counters, and other evidence, indicate that mink society favors the adult male. Rela t i v e to population regulation, the emerging hypothesis i s that a l l members of a population are i n competition for some l i m i t e d resource, probably s u i t a b l e year-round hunting spots i n the case of the l i t t o r a l foraging mink of t h i s study and, due to the dominance of adult males, females suff e r increased mortality and decreased p r o d u c t i v i t y when th i s competition i s intense, i . e . whentthe population i s high and/or when the sex r a t i o i s high to males. A management implication of the hypothesized regu-l a t i o n mechanism i s thathharvest systems designed to s e l e c t males are the most l i k e l y to enhance p r o d u c t i v i t y . i i i TABLE OF CONTENTS GENERAL INTRODUCTION 1 I. THE STUDY SITUATION THE STUDY AREA GENERAL DESCRIPTION PHYSIOGRAPHY 5 CLIMATE 7 VEGETATION 10 INTERTIDAL HABITAT Tides 11 F l o r a , Fauna, and Substrates 13 SPECIFIC STUDY AREAS VARGAS ISLAND 14 TOFINO INLET 17 BROKEN GROUP ISLANDS 17 SEASONS OF STUDY 19 THE STUDY ANIMAL TAXONOMY AND PALEONTOLOGY 21 DISTRIBUTION 22 PHYSICAL CHARACTERISTICS SIZE 23 PELAGE 25 I I . FEEDING ECOLOGY FOOD HABITS INTRODUCTION METHODS AND MATERIALS ANALYSIS OF FECAL CONTENTS F i e l d C o l l e c t i o n Laboratory Methods EXAMINATION OF FEEDING MIDDENS DIRECT OBSERVATIONS ANALYSIS OF DIGESTIVE TRACTS REPORTS FROM OTHER OBSERVERS RESULTS PREY TAKEN Crabs Other Crustaceans Other Invertebrates Fi s h Birds Mammals Debris DISCUSSION COMPARISON WITH OTHER AREAS THE NATURE OF PREDATION BY MINK Factors A f f e c t i n g Prey Selection Preference Size of Prey 27 27 28 31 34 35 36 37 37 41 52 53 55 61 65 66 66 69 71 74 i v V u l n e r a b i l i t y and Abundance of Prey 76 Crabs 77 F i s h 81 B i r d s 87 Mammals 89 A v a i l a b i l i t y of Prey : Conclusion 90 E f f e c t s of Pred a t i o n 91 E f f e c t s on Marine Organisms 92 E f f e c t s on Seabird Colonies 93 SUMMARY, FOOD HABITS , 95 FOOD GETTING INTRODUCTION AND METHODS 97 HUNTING HABITATS Small P a r t i c l e Beach 98 Boulder Beach 99 Rockweed Shore 99 Eelgr a s s F l a t s 99 Estuary 102 RESULTS AND DISCUSSION HUNTING METHODS 102 Bird-dogging 102 Poking 105 . D i v i n g 107 HANDLING PREY 112 E a t i n g 116 Caching 116 ACTIVITY PATTERNS INTRODUCTION 122 METHODS 122 RESULTS DIRECT OBSERVATIONS 126 TELEMETRY 134 DISCUSSION 139 ENERGY REQUIREMENTS AND INTAKE INTRODUCTION AND METHODS 142 RESULTS HUNTING SUCCESS 145 THE DAILY REQUIREMENT 150 DISCUSSION 152 WATER REQUIREMENTS AND INTAKE INTRODUCTION AND METHODS 155 RESULTS AND DISCUSSION 156 SIZE DIMORPHISM AS A TROPHIC STRATEGY INTRODUCTION AND METHODS 158 RESULTS 160 DISCUSSION 164 v III. POPULATION ECOLOGY DESCRIPTION AND EVALUATION OF TECHNIQUES 167 LIVETRAPPING AND HANDLING METHODS 167 RESULTS Trapping Success 173 Differential Vulnerability to Trapping 175 Individual Response to Livetraps 180 DISCUSSION 181 AGE DETERMINATION METHODS 184 RESULTS AND DISCUSSION 188 REPRODUCTION INTRODUCTION 192 METHODS 195 RESULTS AND DISCUSSION SEX RATIO 197 BREEDING SEASON 203 MATING 208 PRODUCTIVITY 215 DELAYED IMPLANTATION 219 FACTORS AFFECTING PRODUCTIVITY 224 MORTALITY, CONDITION, AND PATHOLOGY INTRODUCTION AND METHODS 228 RESULTS MORTALITY 230 CONDITION 234 PATHOLOGY 237 Tooth Breakage and Wear 239 Body Wounds 239 Ectoparasites 244 Endoparasites 245 DISCUSSION 249 PARALYTIC SHELLFISH POISONING 250 SINUS WORM INFECTION 252 TAIL WOUNDS 254 SUMMER INANITION 255 MORTALITY PATTERNS OF SMALL CARNIVORES 256 POPULATIONS INTRODUCTION AND METHODS 259 RESULTS AND DISCUSSION RESIDENTIAL STATUS 260 MOVEMENTS AND RANGE 263 SOCIAL BEHAVIOR General 267 Territoriality 271 POPULATION DENSITY AND TURNOVER 275 POPULATION REGULATION 283 IMPLICATIONS FOR FURBEARER MANAGEMENT 287 vi IV. SUMMARY: THE COASTAL MINK ON VANCOUVER ISLAND, BRITISH COLUMBIA 289 LITERATURE CITED 296 APPENDIX 313 v i i LIST OF TABLES TABLE " PAGE 1 Climatic data from Tofino Weather Station, west coast of Vancouver Island, British Columbia, 1968-1972 9 2 Tidal extremes, Clayoquot Sound (Tofino), British Columbia, 1968-1972. 12 3 Seasonal food habits of mink on islands and shores within (Tofino Inle'.t) and at the mouth (Vargas Island) of Clayoquot Sound, Vancouver Island, B.C., 1968-1972. 39 4 Spring and Summer 'foods of mink in Barkley Sound, Vancouver Island, British Columbia, 1971-1972, as determined by fecal analyses. 40 5 Foods in digestive tracts of west Vancouver Island mink, May-July 1971 and 1972. 43 6 Predominant foods of wild (North America) and feral. (Eurasia) mink (Mustela vison). 67 7 Proportional seasonal occurrence of sculpins (Cottidae) and blennioid fishes (Stichaeidae and Pholidae) in fecal samples from mink, Vancouver Island, British Columbia. 84 8 Hunting methods used by mink in various littoral habitats, Vancouver Island, British Columbia, 1968-1972. 103 9 Handling of prey byvimink, Vancouver Island, British Columbia, 1968-1972. 113 10 Observed activities of mink in relation to tide levels, Vancouver Island, British Columbia, 1968-1972. 131 11 Evidence of differential niche utilization by the sexes, Vancouver Island mink, from observations of free-ranging animals. 161 12 Summary of mink livetrapping results, west coast of Vancouver Island, British Columbia, 1968-1973. 174 13 Comparison of age determinations from field and laboratory criteria, wild mink from Vancouver Island, British Columbia. 191 v i i i TABLE PAGE 14 Seasonal sex ratios observed in mink populations along the west coast of Vancouver Island, British Columbia, 1968-1973 198 15 Age ratios of wild caught mink. 218 16 Incidence of non-accidental trap deaths among live-trapped mink, west coast of Vancouver Island, 1968-1973. 235 17 Incidence of external pathological symptoms and ecto-parasites in wild mink, Vancouver Island, British Columbia, 1968-1973. 238 18 Incidence of tail wounds among captured and observed mink, Vancouver Island, British Columbia, 1968-1973. 241 19 Incidence of endoparasites in Vancouver Island mink, 1968-1973. 246 20 Residential status of known mink, as determined by live-capture histories, on Vancouver Island, British Columbia, 1968-1973. 262 21 Movements and range of mink along marine shores of Vancouver Island, British Columbia, 1968-1973. 266 22 Mink population numbers,composition and turnover on Vargas Island, British Columbia, 1968-1970. 277 23 Mink population age structure as determined from a sample of collected specimens, Vancouver Island, British Columbia, 1968-1973. ' 282 ix LIST OF FIGURES 1 Mink study areas, west coast of Vancouver Island, British Columbia. 6 2 Long-term climatic records, Tofino Weather Station, west coast of Vancouver Island, British Columbia. 8 3 Study locations in Clayoquot Sound, Vancouver Island, 1968-1973. 15 4 Study locations in Barkley Sound, Vancouver Island, 1968-1973. 16 5 Mean weights of livetrapped mink (all seasons) on the west coast of Vancouver Island, 1968-1973. 24 6 Incidence of unusually light-colored pelage among mink, west coast of Vancouver Island, 1968-1973. 26 7 Seasonal food habits, as determined by fecal analyses, of mink in Clayoquot Sound, Vancouver Island, 1968-1973. 38 8 Prey animals observed being captured by mink in littoral habitats along the west coast of Vancouver Island, British Columbia, 1968-1972. 42 9 Annual and bimonthly variations in midden occurrences of crabs eaten by mink, Vancouver Island, 1968-1972. 44 10 Histograms of crab size classes eaten by mink, Vancouver Island, 1968-1972. 50 11 Crabs commonly eaten by mink, Vancouver Island, British Columbia. 51 12 Duration of 264 underwater dives by hunting mink, Vancouver Island, British Columbia. 109 13 Incidence of hunting, traveling, and sleeping of mink in relation to time of day and the time of the lowest tide. 129 14 Results of 84 census transects north Vargas Island, 1968-1969. from Rassier Point, 133 15 Activity of radio-tagged mink in relation to levels of the tide and time of day. 135 16 Activity of radio-tagged male mink along the waterfront in Tofino, British Columbia, February.1972, as determined by continuous monitoring. 137 17 Hunting success (net weight of prey taken per minute of hunting) for Vancouver Island mink, 1968-1972. 147 18 Mink hunting success in relation to several environmental variables, Vancouver Island, British Columbia. 148 19 Seasonal variation in livetrapping success at Chalk Island, Barkley Sound, April 1972- May 1973. 176 20 Proportional incidence of the sexes among livetrapped mink during the seasons of study, Vancouver Island, British Columbia, 1968-1973. 179 21 Monthly reproductive condition of livetrapped mink, Vancouver Island, British Columbia, 1968-1973. 204 22 Monthly evidence of reproductive activity in wild mink, Vancouver Island, British Columbia, 1968-1973. 206 23 Water crossings by male mink during t?he mating season, Barkley Sound, British,Columbia, 1971-1972. 214 24 Proportion of livetrapped mink released alive in poor or jjpa-if. condition, west coast of Vancouver Island, 1968-1973. 236 xi LIST OF PLATES Plate Page Frontispiece A male mink on a Vancouver Island intertidal shore. 1 Study Areas 18 2 Pelage; Hunting habitats 100 3 Hunting habitatss 101 4 Steps in the handling of a livetrapped mink 171 5 Female neck wounds incurred during mating 212 6 Aspects of mink reproduction 220 7 Examples of tail wounds in Vancouver Island mink 242 8 More examples of tail wounds 243 9 Aspects of condition in Vancouver Island mink 248 x i i ACKNOWLEDGMENTS As i s always the case at t h i s stage i n the " g e s t a t i o n " of a manuscript, one becomes a c u t e l y aware of the ass i s t a n c e provided by other people along the way. I have, before me, a l i s t of more than 40 names, and gnawing at my innards i s the c e r t a i n t y that I have probably f o r g o t t e n some and, i n any event, can't mention them a l l . So, f i r s t , to those I do not l i s t below, I nevertheless extend my g r a t i t u d e . Among those who can not be f o r g o t t e n , Dr. Ian McTaggart Cowan i s at the top of the l i s t . He was always a v a i l a b l e to d i r e c t and a s s i s t , when d i r e c t i o n and as s i s t a n c e were needed, but he p a t i e n t l y gave me the r e i n s at other times. One could not ask more of a superv i s o r . His p r o f e s s i o n a l i s m , though o f t e n beyond my c a p a c i t y to understand, l e t alone emulate, was a sti m u l a n t , and the breadth of h i s experience a constant i n s p i r a t i o n . But, perhaps most important was the r e v e l a t i o n : once, as I floundered on a s i d e -t r a c k of my l i f e , I discovered, i n Dr. Cowan, a warm, understanding human being. My f a m i l y has done w e l l c o n s i d e r i n g t h a t , mostly, i t has done without. My w i f e , Mary E t t a , l i v e d i n shacks, trapped, skinned, nursemaided, cooked, weathered storms, compiled data, e d i t e d , and through i t a l l challenged me to stay human. I needed tha t . S p e c i a l thanks are also due: Michael M i l e s and James Biggar, f o r con s c i e n t i o u s and competent help i n t h e i r c a p a c i t i e s as summer a s s i s t a n t s i n 1968 and 1971, r e s p e c t i v e l y ; Dr. J.R. Adams, Kathleen Stewart, and Dr. P. Zuk f o r i d e n t i f i c a t i o n of p a r a s i t e s ; Daphne Hards, f o r p r e p a r a t i o n of x i i i h i s t o l o g i c a l material; Delores Lauriente, for assistance i n some of the data analyses; fellow students, G. Calef, R.W. Campbell, S.R. Johnson, and others, for occasional f i e l d assistance, regular encouragement, and fin e companionship; west coast residents, e s p e c i a l l y the Arnets, Barrs, Gibsons, Palms, Parlees, Sarlunds; Seymours, Peg Whittington, and Joe Wilkowski for continuous moral, f r a t e r n a l , and often l o g i s t i c a l support; Jack Todd, for information and mink specimens from southeastern Vancouver Island; Drs. J.M. Taylor, H.D. Fisher, and W.D. K i t t s , f o r t h e i r c r i t i c a l readings of the manuscript; Mrs. H o l l y Linden, for typing the f i n a l copy, and for her patience while doing so. F i n a l l y , despite my basic b e l i e f i n the innate p e r v e r s i t y of inanimate objects, I must here acknowledge "Minkboat", my twelve-foot, pneumatic, l i n k with l i f e . Her dogged determination to stay at the top of the water column was the ultimate key to the completion of t h i s project. x i v "My daddy studies the mink...;. poor ole mink." Marec'a Hatler, age 2 1972 FRONTISPIECE : A male mink on a Vancouver Island i n t e r t i d a l shore XV GENERAL INTRODUCTION This is a report on a field study of wild mink (Mustela vison Schreber) in a marine shore environment. The study is "old-fashioned biology" in that is has focussed more on the organism and its habitat, in a broad sense, than on particular ecological problems or processes (see Dobzhansky, 1966). The preceding statement is neither an apology for my approach nor a denigration of the more concentrated, specialized endeavor which is current in ecological work. As Dobzhansky (loc.cit.) points out; The strategy of biological research is to discover the patterns first, then their components, and finally the functional and adaptive meaning as well as the evolutionary origin of the particular ways the components are combined in the patterns. In the case of the mammalian order Carnivora, the patterns have begun to emerge only recently because members of this group are difficult and often expensive to study. Indeed, Chitty (1964) has argued that i t is best to work out ecological principles on simpler, more abundant, and easier to handle species. He admits, however, that numbers of reputable theoreticians have studied the "easy" species intensively for more than 30 years, and have failed to produce answers. Work on the less tractable species, can scarcely be less successful than that, and i t may provide a new and valuable perspective. Since, by ecological definition, predators are less abundant than their prey, we have an opportunity to observe them over wider areas without being overwhelmed by numbers. Like a large-scale map, this can provide more pertinent detail. 1 2 Relevant recent studies on c a r n i v o r e s i n c l u d e work by Hornocker (1969, 1970) on the mountain l i o n ( F e l i s c o n c o l o r ) , Mech (1970) on the wolf (Canis l u p u s ) , Jonkel and Cowan (1971) on the black bear (Ursus americanus), S c h a l l e r (1972) on the A f r i c a n l i o n (Panthera leo) and Kruuk (1972) on the spotted hyena (Crocuta c r o c u t a ) . Among the few i n t e n s i v e f i e l d s t u d i e s of m u s t e l i d populations are work on marten (Martes  americana) i n Montana (Newby and Hawley, 1954; Hawley and Newby, 1957; Weckwerth and Hawley, 1962), weasels (Mustela erminea and M. n i v a l i s ) i n Scotland ( L o c k i e , 1966), and f e r a l ranch mink i n southern Sweden ( G e r e l l , 1967a, b; 1968; 1969; 1970; 1971). A l l of these papers, and o t h e r s , provided bases f o r comparison with my f i n d i n g s , a l l o w i n g s p e c u l a t i o n on the g e n e r a l i t y and s i g n i f i c a n c e of observed p a t t e r n s . The mink was chosen as a study animal f o r s e v e r a l reasons: 1. ) Academic - I wanted to conduct a f i e l d study on a c a r n i v o r e and, among t h i s group, the mustelids appeared to be the l e a s t w e l l known. Study of a r e p r e s e n t a t i v e m u s t e l i d , t h e r e f o r e , appeared to o f f e r the g r e a t e s t p o t e n t i a l f o r s i g n i f i c a n t new f i n d i n g s . A d d i t i o n a l l y , the mink i n h a b i t a t i n g marine shores had not been studied at a l l , and questions r e l a t i n g to i t s adaptation to t h i s environment were of s p e c i a l i n t e r e s t . 2. ) Economic - Small m u s t e l i d s , e s p e c i a l l y the mink and the marten, have long been important resources i n the f u r trade; b e t t e r b i o l o g i c a l understanding of these animals i s necessary f o r management, and enhanced management c a p a b i l i t y has economic i m p l i c a t i o n s at l e a s t l o c a l l y . For example, on the Yukon - Kuskokwim D e l t a of A l a s k a , annual revenue from 3 sale of mink pelts has averaged over $400,000, and some villages took in 25 per cent, or more, of their annual income from this source (Burns, 1964a). In British Columbia mink and/or marten have regularly been among the top four fur species in terms of total annual value of sales (records of British Columbia Fish and Wildlife Branch, Victoria). 3. ) Practical - Mink occur in numbers along the entire British Columbia coast, thus i t was possible to select island study areas which were accessible, yet "wild". Island study areas are desirable since they enable more realistic definition of population boundaries than is usually the case on large land areas. A further practical consideration in- the choice of the mink as a study animal is that,' through interest in fur ranching, a wealth of information on its biology (morphology, reproduction, nutrition, pathology, and even genetics) is available in the literature for comparison with field findings. 4. ) Personal - Perhaps as important as any of the above reasons is the fact that, since my early days as a schoolboy - trapper, I have long had an intense interest in mink. When I began the field work in May 1968, I was armed with a variety of ecological "problems" which I hoped to solve. After a year's work i t became apparent that our knowledge of the coast mink had been so scanty that we hadn't even known what the pertinent questions were. For example, interest in dispersal of young had to be abandoned, as a focus of study, in favor of an inquiry into why there were so few young to disperse. Objectives generally settled into a "natural history" framework: 4 - what features of the coast environment are of p a r t i c u l a r importance to mink? - what foods are used, and under what conditions? How do mink obtain these foods? - what factors a f f e c t the health of individuals? - what are the main influences on numbers i n populations? - how do mink divide up habitat among themselves and, since they are p r i m a r i l y s o l i t a r y i n habit, how do they keep separate? These questions might a l l be summarized into a single objective: To gather information on a l l aspects of the l i f e h i s t o r y of coast mink which r e l a t e to the welfare of both the i n d i v i d u a l and the population. The f i r s t step was to observe and record. This i s natural h i s t o r y . Along the way, and i n following pages, I also have indulged i n "natural philosphy", i . e . , a consideration of the meaning of the record I ob-tained (see Sears, 1944). Most of my data have been analyzed s t a t i s t i c a l l y , but I w i l l here point out that I prefer to use s t a t i s t i c s as a guide rather than a crutch. Hence, I have not chosen an a r b i t r a r y l e v e l of p r o b a b i l i t y upon which to basemy/cconclusions. Rather, i n most cases I have simply stated the p r o b a b i l i t y obtained and i n v i t e the reader to decide whether he w i l l accept my conclusions at the observed l e v e l . B i o l o g i c a l s i g n i f i c a n c e , rather than s t a t i s t i c a l s i g n i f i c a n c e alone, has been my goal. 5 I. THE STUDY SITUATION THE STUDY AREA GENERAL DESCRIPTION o Most field work was conducted in the vicinity of 49 North latitude and 125-126° West longitude, within the Estevan Coastal Plain (Holland 1964) on the west coast of Vancouver Island, British Columbia. Study activities were centered at two of the marine inlets which dissect the plain, Clayoquot Sound and Barkley Sound (see Figure 1), although I obtained scattered observations from many adjacent areas. As described by Rigg and Miller (1949), "the region is characterized by rugged, surf-beaten shores, large tidal range, strong tidal currents, cold water, heavy rainfall, cool air, a low percentage of sunshine, and considerable fog". Specific features of this environment, especially those pertinent to the biology of mink, are detailed in the following pages. PHYSIOGRAPHY The geology of the area has been described by Holland (1964). To summarize from his work, the Estevan Coastal Plain is a narrow band of lowland, less than 2 miles wide along much of its length, which fronts on the open Pacific Ocean on Vancouver Island. Much of this plain is flat and featureless as a result of late Tertiary erosion of the relatively soft Oligocene and Miocene sandstones which underlie i t along much of its length, although irregular hills and isolated knolls appear in areas where i t is underlain by harder, volcanic rocks. Most of the plain lies at elevations less than 50 m above sea-level, and even the hills described above rarely exceed heights above 100 m. The exposed coastline is irregular and rugged as a result of differential resistance to wave erosion, and1 is characterized face page 6 Figure 1. Mink study areas, west coast of Vancouver Island, B r i t i s h Columbia. The area covered i s indicated i n the inset. The main study areas are shown i n greater d e t a i l i n Figures 3 and 4. 7 by great expanses of small p a r t i c l e beaches and small to extensive bare rock headlands. Numerous i n l e t s , bays, and small archipelagos provide some protection from the open sea and, with beaches of boulders and t i d e f l a t s supporting marine vegetation, o f f e r shorelines a t t r a c t i v e to mink. Many of the i n l e t s extend inland for 15 km or more and penetrate the Vancouver Island Mountains which, r i s i n g abruptly to elevations of 1600-2000 m, form the inland border of the coastal p l a i n . CLIMATE Data gathered at the Tofino Meterological Station, approximately mid-way between Clayoquot Sound and Barkley Sound on the Esowista Peninsula, are depicted i n Figure 2 i n the form of long-term averages. S p e c i f i c data for the years of study are l i s t e d i n Table 1. Total annual p r e c i p i t a t i o n i s high, averaging over 325 cm, and at l e a s t some can be expected on more than h a l f the days i n any given year. Summers however, e s p e c i a l l y during July and August, are c h a r a c t e r i s t i c a l l y dry. Record r a i n f a l l s for a 24-hour period have exceeded 10 cm i n eight months of the year, with a maximum of 17.5 cm once i n January. Snowfall i s t y p i c a l l y l i g h t , averaging just 42 cm annually over a 17 year period. During the years of study (1968-72), p r e c i -p i t a t i o n was approximately 25 per cent above the average i n 1968 and 25 per cent below i t i n 1970, but was near average i n the ..other three years. Unusually heavy snowfalls (185 cm and 130 cm respectively) occurred i n 1969 and 1971. Temperatures are t y p i c a l l y mild and uniform and, although extremes of from -1.5° C to 33° C have b een recorded, the annual range averages about 20 degrees. Even during December and January, the coldest months, temperatures below freezing occur on less than h a l f of the days. The record minimum, given above, occurred during t h i s study (January 1969); although i t was one of the coldest winters on record, the mean annual temperatures for that year and for face page 8 Figure 2. Long-term c l i m a t i c records, Tofino Weather Station, west coast of Vancouver Island, B r i t i s h Columbia. a) Monthly p r e c i p i t a t i o n , 17-year average to 1970. b) Mean monthly temperatures, 17-year average. c) Frequency of winds from four main d i r e c t i o n s , 7-year average (1959-1966). The numbers along the h o r i z o n t a l axis denote the mean proportion (per cent) of calm days per month. d) Mean monthly windspeeds, 1959-1966. 8 Table 1. Climatic data from Tofino Weather Station, west coast of Vancouver Island, British Columbia, 1968-1972. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Temperature (°C) Mean3 4 6 6 7 10 13 14 14 13 10 7 5 9 1968 4 6 7 6 . 11 12 15 14 13 9 7 2 9 1969 -1 4 6 7 11 14 14 13 13 10 8 7 9 1970 4 7 7 7 9 13 13 14 12 9 7 3 9 1971 3 3 4 7 9 11 14 15 12 9 6 2 8 1972 2 4 6 6 11 12 15 15 11 8 7 3 8 ecipitation (cm) -Meana 41 32>. 31 26 10 9 8 8 15 .",37. 42 45 306 1968 61 24 42 23 10 15 7 17 26 72 46 39 381 1969 30 27 54 41 14 3 5 14 38 29 54 36 322 1970 24 14 22 32 13 3 9 4 21 20 29 47 238 1971 56 33 47 24 8 19 5 11 21 50 50 31 354 1972 32 39 70 33 6 11 18 3 15 7 39 49 323 a17 year average, to 1970. 10 the other years of study varied l i t t l e from the long-term average. Wind, with i t s : influence on wave action, is probably the most important climatic feature for organisms which hunt at the edge of the sea. As shown in Figure 2, westerly winds predominate in summer, when average velocities are lowest. Those from the northwest usually accompany fa i r , dry weather and on most summmer days do not begin blowing until late morning or early . afternoon. Fog.is common during the calm mornings of that season. The storm winds in the area are easterlies, especially those from the southeast, and these are the predominant winds from September through April. Southeasterlies, which commonly attain sustained velocities of 50 km per hour, are almost always accompanied by precipitation. VEGETATION The study region l i e s within the Coastal Western Hemlock zone of Krajina (1965). Detailed descriptions of the various plant communities and subcom-munities characteristic of exposed mainland shores south of Tofino and the islands of Barkley Sound have been produced by Bell and Harcombe (1972, 1973). The forest fringing the shore in this region is typically composed of large trees with a dense shrub understory, the combination excluding light, retaining moisture, and thus providing a dark, humid environment for mink at ground level.. The main forest community is that dominated by western red cedar (Thuja plicata) and western hemlock (Tsuga heterophylla) and these species, together with Sitka spruce (Picea sitchensis) in many locations, support mink dens, runways, latrines and middens among exposed root systems and other natural cavities. The primary shrubs in forested areas are salal(Gaultheria shallon), salmpnberry (Rubus spectabilis), and several species of Vaccinium. Salal grows to heights of 10 feet or more at many locations and usually so densely that i t acts as an almost impenetrable b a r r i e r to a l l except small animals. Mink using the runways which commonly p a r a l l e l the beach through these shrub zones are doubtlessly afforded protection against most p o t e n t i a l predators, e s p e c i a l l y raptors. Where s o i l i s poor and/or shallow such as on small rocky i s l e t s , c o n i f e r s grow only sparsely i f at a l l , and shrub communities usually dominate. In such cases burrows used for dens and middens may be dug amid the roots of shrub clumps; however, the root systems of trees ap-pear to be more commonly used. On i s l e t s or headlands where heavy shrub cover i s absent, mink run-ways and middens are commonly found i n other plant formations, of which clumps of a fern, Polypodium s c o u l e r i , and patches of wild rose (Rosa  nutkana) or beach rye (Elymus mol l i s ) are the most common. The fern has no common name, but I informally r e f e r r e d to i t as "mink fern" since i t was r a r e l y found free of mink sign. The leaves of t h i s fern and pieces of beach rye, other, smaller grass species, mosses and seabeach sandwort (Honkenya peploides) were plants i d e n t i f i e d i n nests prepared by mink. In summary, the wet, mild coastal climate supports a v a r i e t y of plant communities, of which those f r i n g i n g the ocean shore are of s p e c i a l i n t e r e s t i n t h i s context i n that they provide mink with protection from exposure and predation. INTERTIDAL HABITAT Tides The t i d a l cycle i n the region of study.is the semi-diurnal mixed type, with two low tides and two high tides of unequal height occuring d a i l y (see Dohler 1966). Throughout the remainder of t h i s report, the four tides w i l l be r e f e r r e d to as the lower low (LL), higher low (HL), lower high (LH) and 12 Table 2. Tidal extremes, Clayoquot Sound (Tofino), British Columbia, 1968-1972. (Summarized from Canadian Hydrographic Service, 1968-1972). Highest Tides (m) Lowest Tides (m) No. Minus Tides/Year3 During Study Month Mean Range Mean Range '68 '69 '70 '71 '72 Jan 3.96 (3.85-4.03) 0.15 ( 0.03-0.30) Feb 3.75 (3.63-3.90) 0.22 ( 0.03-0.42) Mar 3.64 (3.48-3.81) 0.26 ( 0.03-0.48) Apr 3.67 . (3.57-3.78) 0.01 (-0.15-0.21) 2 3 2 May 3.61 (3.54-3.75) -0.09 (-0.21-0.06) 4 3 2 4 Jun 3.65 (3.54-3.84) -0.11 (-0.18-0.03) 3 5 2 4 Jul 3.65 (3.51-3.81) -0.07 (-0.21-0.06) 5 3 2 Aug 3.62 (3.54-3.72) 0.11 ( 0.06-0.18) Sep 3.68 (3.63-3.78) 0.32 ( 0.27-0.42) Oct 3.86 (3.75-3.90) 0.17 ( 0.06-0.42) Nov 4.00 (3.93-4.08) 0.12 (-0.03-0.30) 1 Dec 4.05 (3.93-4.17) 0.11 ( 0.00-0.24) ^or month and year (1968-1972) indicated, the number of days on which tide f e l l below sea level datum (0.00). ^higher high (HH). As is indicated in Table 2, the tidal range in the area is approximately 4.5 m, with only a few of the LL tides falling below sea level datum annually, and these primarily in the months April-July. The highest tides typically occur in winter. In relation to time of day, the annual distri-bution of LL tides is such that they occur largely during early morning hours in summer and during late afternoon and evening during winter. According to Bousfield (1957), shore waters of the outer coast are of year-round high salinity (30-32 0/00) and of moderate annual temperature range (6-14° C). Flora, Fauna, and Substrates Marine plants and animals which can be found in intertidal habitats both within and adjacent to my main study areas have been described by Ricketts and Calvin (1962), Shelford et al. (1935), and Bousefield (1957) ; observations from other Pacific shores also pertain, especially those of Rigg and Miller (1949) for the northern Washington coast, Stephenson and Stephenson (1961a,b) on southern and eastern Vancouver Island, and Nybakken (1969) on Kodiak Island, Alaska. As Stephenson and Stephenson (1961b) suggested and Nybakken (1969) largely confirmed, shore flora and fauna from Alaska to lower Cali-fornia have many common features, especially in terms of zonation but also including consideration of actual species present. Bousfield (1957) characterized the Pacific coast shore fauna as "exceedingly rich in numbers of species and individuals", especially in comparison with shores along the Canadian Atlantic coast. Among the factors he felt are responsible for this fauna! wealth are moderate water temperatures (always ice free), moderate and uniform air temperatures, large tidal range, and prevalence of onshore winds to produce wave action and insure circulation in all shore zones. As is evident int.all of the above cited papers, different 14 o r g a n i s m s , t y p i c a l l y o c c u r a t d i f f e r e n t l e v e l s i n t h e i n t e r t i d a l h o r i z o n . M o s t o f t h o s e r e g u l a r l y e a t e n by m ink o c c u r i n t h e " l o w e r i n t e r t i d a l " and " d e m e r s a l " zones o f R i g g and M i l l e r ( 1 9 4 9 ) , i n a s s o c i a t i o n w i t h some o f t he l a r g e r m a r i n e m a c r o p h y t e s s u c h as r o c k w e e d s ( F u c u s ) , l a r g e brown ( L a m i n a r i a ) and r e d ( G i g a r t i n a , I r i d a e a ) a l g a e , and e e l g r a s s ( Z o s t e r a ) . T h e s e a r e e x p o s e d to s h o r e h u n t i n g p r e d a t o r s a t t i d e l e v e l s b e l o w abou t 1 m, a r e s t i l l m o d e r a t e l y a c c e s s i b l e f r o m t h a t d e p t h t o abou t 2 m, b u t c a n be r e a c h e d o n l y by deep d i v e s a t h i g h e r t i d e l e v e l s . P r o b a b l y more i m p o r t a n t t h a n t i d e l e v e l i n d e t e r m i n i n g what s p e c i e s may be a v a i l a b l e i n a g i v e n i n t e r t i d a l s i t u a t i o n i s t he n a t u r e o f t h e s h o r e l i n e . R i g g and M i l l e r (1949) f o u n d t h e g r e a t e s t number o f s p e c i e s " w h e r e t h e r e l a t i v e l y s h e l t e r e d , s h e l v i n g s h o r e w i t h numerous b o u l d e r s and an abundance o f t i d e p o o l s p r o v i d e s t h e g r e a t e s t v a r i e t y o f h a b i t a t s " . More e x p o s e d s i t e s " o b v i o u s l y c o n s t i t u t e a more r e s t r i c t e d e n v i r o n m e n t " . S P E C I F I C STUDY AREAS M o s t i n t e n s i v e r e s e a r c h a c t i v i t i e s were c o n d u c t e d i n two a r e a s o f C l a y o q u o t Sound ( F i g u r e 3) and a s i n g l e a r e a i n B a r k l e y Sound ( F i g u r e 4) a l t h o u g h , as t h e s e f i g u r e s show, o b s e r v a t i o n s o f an a t l e a s t o c c a s i o n a l n a t u r e were made a t many a d j a c e n t l o c a t i o n s . The t h r e e ma in s t u d y a r e a s may be c h a r a c t e r i z e d b r i e f l y as f o l l o w s : VARGAS ISLAND 2 T h i s i s a l a r g e ( a b o u t 30 km ) , l o w - l y i n g i s l a n d a t t h e mouth o f C l a y o q u o t Sound ( F i g u r e 3 ) . E x t e n s i v e sand b e a c h e s and numerous r o c k y o u t c r o p s a r e t h e p r e d o m i n a n t s h o r e t y p e s a r o u n d i t s 35 km p e r i m e t e r , and n e a r - s h o r e w a t e r d e p t h s a r e t y p i c a l l y s h a l l o w . The w e s t s h o r e s a r e e x p o s e d t o t h e open P a c i f i c O c e a n , w h i l e t h o s e on t h e e a s t s i d e a r e p r o t e c t e d f r o m face page 15 Figure 3. Study locations in Clayoquot Sound, Vancouver Island, 1968-1973. Stippled areas indicate expanses of sand (exposed beaches) or mud (protected bays and tide-flats) . Place names mentioned in the text are shown, and shore areas of most intensive study are indicated by shading. Dots along shaded sections represent regularly used livetrap sites. face page 16 Figure 4. Study l o c a t i o n s i n Barkley Sound, Vancouver I s l a n d , 1968-1973. Every i s l a n d shown except Cree I s l a n d i s known to have harbored at l e a s t one mink at some time during the study. I s l a n d s of most i n t e n s i v e observations are shaded. The primary study area ( T u r t l e I s l a n d Group) i s shown i n l a r g e r s c a l e i n the i n s e t ; dots i n d i c a t e r e g u l a r l y used l i v e t r a p s i t e s . 17 most summer surf, but face d i r e c t l y into southeast storm winds and may be very inhospitable, e s p e c i a l l y i n winter. Most study a c t i v i t i e s on Vargas Island were concentrated along an approximately 8.5 km section of t h i s southeast-facing shore, a view of which i s shown i n Plate 1(a). TOFINO INLET Protected rock beaches and t i d a l mudflats on the Vancouver Island mainland shore i n the immediate v i c i n i t y of the v i l l a g e of Tofino and on adjacent i s l e t s to the north (Figure 2), were the most studied habitats i n t h i s area. Shallow waters and strong t i d a l currents are the r u l e i n Tofino I n l e t and i s l e t s are small (most less than 2 km^ i n area); the t o t a l shore-l i n e studied, i n t e n s i v e l y was somewhat over 2.5 km. BROKEN GROUP ISLANDS This archipelago of more than 80 islands, i s l e t s and reefs, l y i n g about 52 km south of the Clayoquot Sound study areas, occupies an area of approxi-2 mately 100 km ( of which about 15 per cent i s land) i n c e n t r a l Barkley Sound. The t y p i c a l l y rocky shores i n t h i s area are rugged, often sheer and much dissected where exposed to heavy waves, although many of the i s l e t s and reefs serve as breakwaters for others so that r e l a t i v e l y calm waters and protected boulder beaches are common. Most work was c a r r i e d out on such protected shores at f i v e named islands ( T u r t l e , W i l l i s , Dodd, Walsh, and Chalk), hereafter c a l l e d the " T u r t l e Island Group", i n the north-c e n t r a l portion of the Broken Group (Figure 4). The area of t h i s i s l a n d 2 sub-group i s about 5.7 km , and just under 10 km of shoreline were r e g u l a r l y covered during operations within that area. An a e r i a l view of t h i s part of the Broken Group i s shown i n Plate 1 ( c ) . face page 18 Plate 1: Study Areas a) East Vargas Island, showing approximately 3 km of the study area shoreline. I s l e t s 1 and 2, res-p e c t i v e l y , are M.E. I s l e t and Mink Den I s l e t , as l a b e l l e d i n figure 3. Note the long expanse of sand beach. b) More of the East Vargas shoreline, looking north. The i s l e t s shown are the same as those i n "a". c) Broken Group Islands, with portions of the T u r t l e Island Group i n the foreground (1-Dodd I., 2-W i l l i s I., 3- T u r t l e I., 4-Turret I.) and with the Effingham Group i n the background. d) A closer view of the N. T u r t l e Island shore, showing i t s rocky nature and the closeness of the forest edge to the t i d a l margin. The f l o a t i n g camp was used as the main base of operations during work i n the Barkley Sound study area. 19 SEASONS OF STUDY Field work was conducted on a year-round basis nearly continuously from May 1968 through November 1972, and was then terminated after further observations in April through July of 1973. Intensive study began on Vargas Island and continued there until late summer 1970, at which time a severe population decline made further work impractical. Sporadic observations were made in the Tofino Inlet area during the first two years and these continued throughout the study, but with most intensive effort from about October through March of both 1971 and 1972. Serious study in Barkley Sound began in summer 1971 and continued through summer 1973, although weather and water conditions limited winter study in these islands. The four conventional seasons "spring, summer, f a l l , and winter" are not applicable in this region. The wet, windy weeks of winter are distinct from the dry weeks of summer, but spring and f a l l grade imperceptibly into these two seasons. For the purposes of this study, i t was more practical to recognize three 4-month seasons, characterized by aspects of mink life history and features of the environment as follows: 1) April through July (AMJJ) - One month before to one month after the mink mating season; crabs used as food are mating and moulting in nearshore waters; period of calm, relatively dry weather, long days, and lowest tides of the year. 2) August through November (ASON) - Period of recovery from stresses of the mating season and the time in which mink young-of-the-year achieve independence from their mothers; crabs moving to deeper waters; weather deteriorating, days shortening, and tides less extreme than in previous period. 20 3) December through March (DJFM) - Time of preparation for ensuing mating season (e.g. weight increasing, gonadal development beginning); crabs moving back toward shore; weather improving, days lengthening, tides about as in previous period. Documentation for the biological trends indicated above is provided in appropriate sections of this report. 21 THE STUDY ANIMAL TAXONOMY AND PALEONTOLOGY A complete classification, as given by Simpson (1945) to genus and by Hall and Kelson (1959) below genus, recognizes the American mink as Mustela (Lutreola) vison Gray. In the first taxonomic review of this semi-aquatic weasel (Bangs 1895a), four subspecies were recognized. In years following, several new forms were described and a revision by Hollister (1913) listed ten subspecies of M. vison and a separate monotypic species, M. macrodon. The latter, the extinct "sea mink" from the Atlantic coast, has subsequently been reduced to subspecies (Manville 1966) so that this, together with four new subspecies listed by Hall and Kelson (1959) and another described by Burns (1964b), brings the total to 16 subspecies within the single species M. vison. According to Cowan and Guiguet (1963), two subspecies are known in British Columbia. The subject to the present study is largely M. v. evagor, the Vancouver Island mink, which was judged by Hall (1932) to be distinct from M. v. energumenos, the mink which occurs over the rest of the province including both inland and coastal habitats. According to Hall (1951), the paleontological record fails to show the precise ancestry of Mustela. The oldest known fossils are of Pleistocene age, and these specimens are conspecific with living members of the genus. Three extinct genera, Miomustela from the Lower Pliocene or Upper Miocene of southern Montana, Martinogale from the Pliocene of Sherman County, Kansas, and Pliogale from the Lower Pliocene of Humboldt County, Nevada are of interest as possible ancestors of the genus, but the incompleteness of remains known for these fossil genera makes firm conclusions impossible. The paleontological history of the mink, itself, is not well known, and the 22 earliest record of which I am aware is from a 4000 year old archeological site in Maine (Waters and Ray 1961). DISTRIBUTION The mink is distributed virtually everywhere in North America, except the higher arctic, with records from nearly all of the United States and the provinces and territories of Canada, from Nova Scotia (Northcott ejt al. 1974).south to Florida (Schwartz 1949) in the east, west to Texas (Taylor 1944) and New Mexico (Yarbrough and Studier 1968), and from California (Hall 1929) to northern Alaska (Burns 1964b) in the west. Records in the north extend almost to the Arctic coast (Cowan 1948). Although.it is indigenous to North America, escapes from fur farms have resulted in estab-lishment of healthy populations of feral animals elsewhere, e.g., England (Thompson 1968), Scotland (Hewson 1971), Sweden (Gerell 1967), Norway (Wildhagen 1956) and parts of the Soviet Union (Aliev and Sanderson 1970; Benkovsky 1971). Most records in all areas are from fresh water habitats. The extinct sea mink (M. v. macrodon) off.the Atlantic coast was reportedly a seashore animal almost entirely, feeding upon marine organisms, and the subspecies M.v. mink is also said to show a propensity for seacoasts (Manville 1966). Hall (1929) collected a mink at some distance from freshwater along the California coast and considered its occurrence there unusual. Svihla and Svihla (1931) caught a mink on a beach along the Olympic Peninsula in northern Washington, probably the southern limit for regular occurrence on Pacific shores. From there north, especially along the rocky coastline of British Columbia (this study) and southeastern Alaska (Harbo 1958;^Croxton 1960), mink which forage in the ocean littoral zone are common. At least 23 some of the feral mink in Scandinavia have also become established along the seacoast (see Wildhagen 1956; Gerell 1967). PHYSICAL CHARACTERISTICS SIZE A basic description of the mink can be found elsewhere (Coues 1877; Hall and Kelson 1959) and need not be duplicated here. Of the several sub-species, M. v. evagor of Vancouver Island appears to be second in size only to M. v- ingens of the Yukon-Kuskokwim Delta in western Alaska; in comparison with the.other British Columbia subspecies, M. v. evagor is larger and is typically lighter in color (Cowan and Guiguet 1965). Appendix 1 lists body measurements and weights of specimens obtained during this study. Age classes indicated are those determined by the laboratory technique described on page 186. Since many of the animals obtained as specimens apparently died of inanition, the mean weights indicated are minimal. The series of weights obtained during livetrapping operations, summarized in Figure 5, provide more representative comparisons. The strong sexual dimorphism in size is especially evident, with adult males averaging 59 per cent heavier than adult females. Indeed, some of the largest males (1450 g or more) were almost twice as heavy as the largest females. Size comparisons between areas can be made only on the basis of the live-trapping weight data. As shown in Figure 5, adult males from Vargas Island were substantially lighter than those from Tofino Inlet (t=2.9, df=62, P<0.01), which averaged, in turn, slightly lighter than those from Barkley Sound. Weights of adult females, on the other hand, were similar for all areas (P>0.30 for all comparisons). The apparent differences indicated for juveniles, especially males, are due at least partly to bias arising from face page 24 Figure 5. Mean weights of livetrapped mink ( a l l seasons) on the west coast of Vancouver Island, 1968-1973. Age determined by f i e l d c r i t e r i a ; numbers under sex and age classes are sample sizes f o r the four pertinent data points, r e s p e c t i v e l y , and v e r t i c a l bars are 5% confidence l i m i t s . Vargas Island Tofino Inlet Adult Males T ( 3 9 - 2 5 - 7 8 - 1 4 7 ) J Juvenile Males J- ( 3 1 - 2 4 - 3 1 - 9 8 ) T 1 T 1 Adult Females ( 2 1 - 5 - 1 9 - 4 7 ) 0 Juvenile Females 1 ( 1 6 - 7 - 1 2 - 3 6 ) Barkley Sound Al l Areas 25 the fact that most Barkley Sound individuals were weighed in the spring when they were nearly full-grown, while measurements from the other areas were made throughout the first year of l i f e . When they first begin entering livetraps in October and November, most young males weigh 800-900 g. The average is near 1000 g by January and 1100 g by May, but few, especially in Clayoquot Sound, will exceed 1200 g by July, when they are one year old. Adult males in good condition usually weighi1100 g or more, with a few individuals exceeding 1500 g (up to a maximum of 1650 gr-fdr one from Barkley Sound). Weights of up to 950 g were recorded for females in the la te stages of pregnancy, but most non-pregnant adults weighed less than 800 g and many weighed less than 700 g. Juvenile females weighed 400-500 g when they first began entering livetraps in the autumn. PELAGE Although a number of animals, especially juveniles, do attain the rich, dark pelage for which mink are known, many are light brown to reddish even in winter. Just prior to the summer moult, many animals become unusually light, turning to a tan or golden color. Plate 2 illustrates a female in the process of moulting from this light pelage to a darker summer coat. As shown in Figure 6, the peak frequency of the light pelage occurs in May among females and July among males, reflecting the different moult schedules for the two sexes. Females were recorded in this pelage (29% of 82 animals) more often than were males (157» of 243 animals;Xvt=6.5, df = 1, p<0.02). There was l i t t l e difference in incidence between areas or between age classes within sexes. face page 26 Figure 6. Incidence of unusually l i g h t - c o l o r e d pelage among mink, west coast of Vancouver I s l a n d , 1968-1973. 60 c « 50 0) CD 40 _g a) Q_ O) 30 w 20 x u c 11 0 J F M sample CT 9 23 28 size <J 2 3 15 1 A 27 10 M 64 13 { Female M a l e J 36 9 J 23 10 A 1 1 T— r • s 1 1 o 16 9 "I—r N 14 10 D 5 1 -r 27 II. FEEDING ECOLOGY FOOD HABITS INTRODUCTION A l i s t of the seasonal foods of an animal may imply a number of other things about the animal's lif e : Evidence of the habitats i t hunts, the times of day it is active and, therefore, the degree to which i t may be exposed to various environmental contingencies, the extent of its seasonal movements, the degree to which i t might limit or be limited by its food supply, the extent to which i t might be expected to compete with other species, parasites to which i t might be susceptible, occasionally its economic importance, and even its temperament may often be inferred from the nature of its diet. No other single aspect of lif e history provides a better preview to an understanding of where an animal fits in the scheme of a local community. Following is an;.account of foods used by mink on my Vancouver Island study areas. There have been no previous food studies of ocean littoral-foraging mink anywhere in North America, although Gerell (1968) has documented the diet of some feral mink on a coastal island in southern Sweden. METHODS AND MATERIALS Foods eaten by coast mink were determined by five methods, listed in descending order of importance as follows: 1. Careful analysis of 1752 scats and cursory observations of several hundred others. 2. Examination of remains in dozens of feeding middens. 3. Direct observations of 193 successful hunts by foraging mink. 4. Analysis of contents of 29 digestive tracts. 5. Reports from other observers. A number of factors, including 28 the vagaries of weather and apparent flucuations in mink numbers, combined to make systematic collection of these data impossible and, as a result, they are distributed unevenly over both time and space. I do not believe that this inequality of sample size has introduced any serious bias into my analyses. The following paragraphs describe the circu.mstances under which the five study methods were employed, outline the precise methods of collection, analysis, and presentation of data, and discuss some of the biases and limitations associated with each. ANALYSIS OF FECAL CONTENTS The fecal-passages (scats) of mink are usually small cylinders less than 10 mm in diameter and comprising volumes of about 5-10 cc. As noted by Gerell (1968), they are usually distinctive, both in appearance and in the characteristic locations of deposition. I am certain that all of the scats I collected were those of mink and not those of the two other small shore carnivores in the area, the river otter (Lutra canadensis) and the raccoon (Procyon lotor).As pointed out by several authors, food items pass rapidly through a mink's digestive system, as fast as one hour according to Waller (1962) although Slawinski e_t al_. (1962) l i s t a passage rate of two to seven hours after experiments with captive animals. It was common for three or more scats, to appear at den latrines overnight and on one occasion a small female mink deposited seven scats at the mouth of her den in less than 15 hours. Field Collection Scats were collected only i f they could be reliably dated. This limited collection to those which had not yet dried in most cases, although occasionally scats were known to have been deposited in an area since a recent previous visit and these were taken even i f they were dry. Scats 29 were labelled and put into separate plastic bags whenever possible. This was a simple straight-forward matter for those found singly on trails and along beaches, but was much less so for those in latrines (sites of scat concentrations). In fact, latrines were enigmatic in many ways, and a discussion of the extent of their occurrence together with an account of the rationale behind my treatment of them is warranted. The largest mink latrines, sometimes 35 cm or more in diameter and incorporating several hundred scats, occur near den sites. These are usually situated at the entrance of some cavity such as a burrow at the base of a tree, a hollow log, or a small cave. Smaller latrines often occur near feeding middens which, like the den sites, are areas of intensive use. Other latrines, rarely exceeding 15 scats, may be found scattered throughout the habitat. As Gerell (1968) and Waller (1962) point out, these are frequently located on prominent terrain features such as boulders, stumps and small knolls, but I have seen several in rather homogeneous surroundings. It seems likely that.the presence of one scat may stimulate the deposition of others in the same place. Whether more than one mink contributes to a latrine is unknown. Certainly most of the droppings in a den latrine re-present the output of the occupant mink which, as I will show, may spend up to 20 hours or more of each day at that location. Scat piles at middens are probably deposited largely by the midden "owners", although other mink, especially transients, may visit middens looking for scraps, and may use the latrines at the same time. This is most likely to occur when the midden is located at some distance from the den. The other, more vaguely situated latrines could be checkpoints on the travel routes of individual mink. Schnell (1964) and Schladweiler and Storm (1969) excavated mink dens in other areas and found that parts of the burrow system were used as 30 latrine sites, In my area many latrines occurred underground near dens in the great networks of caverns and hollows amid the roots of sea-fringing conifers, and I could not hope to find the main latrines of a l l , or even most, of the mink present. Neither could I make any unbiased assessments of the relative representativeness of those latrines which I could find. Thus, to count each scat in a latrine at face value, equal to those found singly or in small series, is to risk overemphasizing the feeding habits of a sample of the population which may not be representative. Waller (1962) has acknowledged this problem previously. I will show later that the feeding habits of adjacent mink in a small local area can differ considerably, their diets apparently reflecting differing prey availability in their respective hunting areas. A second major difficulty was that feces in latrines tended to become fused, especially after being rained upon. Even i f one chose to ignore the sampling problem mentioned previously, he would find i t a virtually impossible task to identify, separate out, and determine the age of indivi-dual scats from most latrines. Despite these problems, scats in latrines contained information about large numbers of mink meals and they could not be ignored.,. Usually they were assessed in a general way. along with midden remains, and items were listed as present or predominant. When scats were collected from latrines, the procedure was as follows: (1) Gathering, in normal, manner, of fresh single droppings scattered about the area. (2) At piles of fused feces, collection singly of those uppermost scats which were recognizably distinct. These were usually the four or five most recently deposited. (3) Separation, from the fused pile, of those feces which appeared fresh. This step was taken only when lower levels were clearly older and could 31 not be dated reliably. If all feces in the pile appeared fresh or i f all were known to have been deposited since a previous visit, the pile was left intact. (4) Thorough mixing of all fresh feces. (5.) Collection of a sample (75-100 cc) of the mixed material. I estimated the number of scats contained in the entire mixed matrix based on my experience with the volume of single scats. To allow for the possibility of larger scats in latrines, and to provide a guard against over-emphasis of one time or place, I purposely made this estimate conservative; in all cases, the number of scats recorded as being represented by latrine samples was 50 per cent of the number actually estimated present. Though arbitrary, the above procedure produced the desired effect, that of minimizing the data contribution of the source of scats most likely to be biased. From my observations i t was evident that mink often defecated during periods of hunting and travelling along the shore, and all mink in a local area might be represented by a. series of the single scats deposited at such times. However, as indicated above, each latrine appears to be produced mostly by a single individual, and not all individuals are likely to be represented in latrine samples. The largest recorded latrine sample was 35. In a l l , the 1752 scats analyzed included 7 46 (42.67») from latrines; the rest were collected singly. Laboratory Methods All scats:and latrine samples collected through December 1971 were examined under a binocular dissecting microscope at low power (8x). Most had been air dried prior to study,l-but some were fresh. Those collected in 1972 were examined fresh in the field, using a small 5x hand lens when necessary; scats containing bird or mammal remains were retained for labo-ratory study. Mink chew their food thoroughly, a fact noted previously for 32 this (Korschgen 1958) and other Mustela species (Day 1966). I identified all items as precisely as possible. Remains of crabs and other arthropods could usually be identified to species with l i t t l e or no magnification; early identifications were facilitated by use of a reference collection. Fish were occasionally identifiable to family when scales and certain diag-nostic bones were present. Regardless of whether or not such an identifir-cation was possible, each fish occurrence was assigned to one or more size categories (small, medium and large) determined from the size of skeletal elements present. Birds were identified on the basis of feather structural characteristics as described by Day (1966). The keys of Adorjan and Kole-nosky (196.9) and Day (1966) were useful in identification of mammal hairs recovered from scats. The method of slide preparation used by Carter and i Dilworth (1971) proved more convenient and successful than those proposed by the previously mentioned authors. The methods of expressing data on food habits from fecal analyses have been discussed by innumerable authors. Summarily, no way or combi-nation of ways which have been employed are perfect in the sense that they are exactly representative of the actual diet. In studies with caged foxes, Lockie (1959) showed that only "weight of undigested matter" gave results with errors constant enough for the application of correction factors. How-ever, this method would be impractical for analyses involving hundreds of small scats, many requiring segregation of small fragments of more than one prey item. If most of the items eaten possess at least some undigestible parts, the most commonly adopted method of expression,"frequency of occur-rence", will give a measure of how often a given food is eaten.. In another series of studies with captive foxes, Scott (1943) found that frequency 33 was satisfactory in this regard. If few of the items eaten possess undigestible parts, fecal analysis is of no value. Most previous studies of mink food habits, including the admirable work of Gerell (1969), have used only frequency of occurrence, and I have adopted that method (the percentage of scats in which an item occurred). I have also included a measure of quantity eaten,, the "mean volume" (the average volume attained by an item in those scats in which i t occurred). The volume determinations were estimated by eye and listed as one of the five following volume categories: (1) 1-5 per cent (2) 6-25 per cent (3) 26-50 per cent (4) 51-75 per cent and (5) 76-100 per cent. The categories were employed because consistently precise estimates by eye are not possible and mechanical separation and measurement was not practical. The above applied to single scats; latrine samples were exceptional in that greater care was taken in the volume estimates. I manually segregated individual items to as great a degree as possible for these, and then made an ocular estimate to the nearest five per cent. Items which were present, but only in minute quantity, were given a value of one per cent. These estimates were then used to calculate both frequencies and mean volumes for the number of scats represented by the sample. The manner and rationale of this calculation, which enables the latrine sample results to be lumped with those from single scats, is given in Appendix 2. Probably chief among sources of bias in my results is differential digestion of different foods, a factor which has been apologized for in nearly all scat-based food.studies. In this case, the remains of arthropods, especially crabs, contained a high proportion of undigestible matter and this undoubtedly resulted in a tendency to overestimate the volumetric 34 c o n t r i b u t i o n of these animals when they occurred i n the same scats with more d i g e s t i b l e animals such as f i s h . This bias i s o f f s e t , but to an unknown degree, by d i f f e r e n t i a l resistance to weathering i n the f i e l d . Scats containing f i s h or b i r d material were far more cohesive than those containing arthropods and were less l i k e l y to be driven apart by heavy r a i n or wind. Further, a scat containing crab remains may take on an old appearance i n less than three days under dry conditions, while a b i r d or f i s h bearing scat, under the same conditions, w i l l remain fresh-looking for a week or more. Thus, I was more l i k e l y to pick up the f i s h and b i r d scats and there may be a tendency toward over-representing the frequency of these items i n the mink d i e t . Despite these problems, I am confident that my scat data are s u f f i c i e n t to give a general p i c t u r e ; o f the range of foods used by mink i n the coast environment, and provide good evidence as to the r e l a t i v e importance of each i n the d i f f e r e n t seasons and areas I have studied. EXAMINATION OF FEEDING MIDDENS Accumulations of p a r t l y eaten or in e d i b l e remains of prey, here termed feeding middens, may be found i n and near den s i t e s and at s t r a t e g i c l o c a t i o n s , u s u a l l y d i r e c t l y upshore from hunting spots, along the shore. These middens afforded opportunities for more s p e c i f i c i d e n t i f i c a t i o n s of prey items than could u s u a l l y be made from scat remains, p a r t i c u l a r l y i n the case of f i s h and bi r d s . On a number of occasions I found fresh, i n t a c t specimens of these i n storage at den middens. Most middens consisted l a r g e l y , often e n t i r e l y , of the carapaces and other exoskeletal fragments of crabs. In such cases I usually did l i t t l e more than to note the species present and to indicate which predominated. P e r i o d i c a l l y I made more thorough examinations of accumulations of crab remains (76 middens 35 over the years of study). At these times I measured, to the nearest m i l l i -meter, each specimen present and summarized the degree to which the appen-dages had been u t i l i z e d . The l a t t e r was accomplished by counting the number of i n t a c t and p a r t i a l walking legs and chelipeds present. Whether the examination of a crab midden was s u p e r f i c i a l or thorough, i n a l l cases I broke a l l carapaces present to insure that I would not measure them again and to give a basis for dating remains found i n subsequent v i s i t s . Whenever remains of f i s h were found, they were examined c a r e f u l l y and i d e n t i f i e d to species i f pos s i b l e . Intact or nearly i n t a c t specimens were measured and occ a s i o n a l l y weighed. B i r d remains were i d e n t i f i e d and examined for conditions which might have contributed to t h e i r v u l n e r a b i l i t y . In those cases i n which f i s h or birds had been p a r t l y eaten, I took notes on the manner i n which the eating was done. Examination of middens, alone, could not provide an accurate p i c t u r e of the coastal mink's d i e t because of the heavy bias toward those prey items which have a large porportion of undigestible material, those which are large, or both. These examinations were valuable, however, when used i n conjunction with the other .methods of food study.- Indeed, most of my i d e n t i f i c a t i o n s of f i s h and birds eaten by mink were f i r s t made i n middens. DIRECT OBSERVATIONS The number of opportunities I had to a c t u a l l y watch mink securing prey i s unprecedented. I saw mink hunting on 452 occasions, and those included the handling of 222 d i f f e r e n t prey items. I usually observed hunting animals from a small boat, at distances of from about 5 m up to 100 m or more, but o c c a s i o n a l l y I was able to approach and observe from land. Since s p e c i f i c i d e n t i f i c a t i o n s , e s p e c i a l l y of f i s h , were often impossible, these observations were of l i m i t e d value i n providing data 36 on foods eaten. Also, far more observations were made in late spring and summer, when the low tide providing best hunting is in the morning, than in the f a l l and winter when the low tide is at night. However, the supplementary information which observations provided was most useful. On a few occasions I saw mink hide fish after catching them, and my sub-sequent identification of these led to additions on the known food species l i s t . Other observations of value include instances in which mink were seen abandoning one kind of prey in preference for another, instances in which potential prey species were ignored, and instances in which certain kinds of prey proved difficult to handle. These, plus identification of the kinds of habitats and circumstances in which various kinds of prey were caught, provided insight into the reliability of the results obtained by the other methods. ANALYSIS OF DIGESTIVE TRACTS From May througn July during 1971 and 1972, I collected specimens for reproduction and pathology studies. Most were collected with a 12 gauge shotgun using number 4 pellets at a preferred distance of 30-40 m. A few were taken with Conibear snap traps, and some specimens were obtained after being killed by automobiles. Many of these had food in their stomachs and intestines and this was analyzed as follows. The volume of stomach contents was determined by displacement in a graduated cylinder, then the contents were thoroughly washed and drained on a fine mesh screen. Items were identified as specifically as possible, and frequency of occurrence and relative volume data were recorded in exactly the same way as described previously for scat analysis. Intestinal contents were handled in the same way as described for stomachs except that no in i t i a l volumetric determi-nation was made. 37 The most useful data obtained from these analyses were probably the stomach volumes, which indicate how much a mink may eat in one meal. Also of interest is a comparison of the foods eaten by the two sexes. The l i s t of foods eaten is probably biased in that my most successful stalking was in a single kind of habitat, the boulder beach type, in which fish are the items most often caught. The fact that these specimens were taken mostly in one season further limits the usefulness of these data. REPORTS FROM OTHER OBSERVERS To,encompass the full range of foods eaten by mink in this environment I had to rely; on observations from other people for a few of the apparently l i t t l e used . and unusual items. I had many reports of observations similar to mine which, though tney served to reinforce my feelings about my data, were often deficient in one way or another, usually in lacking a date, time, tide level or. specific prey identification, and I have used none of these in my final data compilations. RESULTS PREY TAKEN Most of the fecal samples examined in this study were obtained from Clayoquot Sound where, as shown in Figure 7, crabs and fish were by far the main items eaten by mink during a l l seasons. Frequencies of occurrence and mean volumes of specific items recorded in this area over a four year period are listed in Table 3, which is, itself, a summary of data (given in Appendices 3 and 4, respectively) for each of the two Clayoquot Sound study areas. Scats collected in Barkley Sound from March through September 1971 and'1972 (Table 4) were also dominated by crab and fish remains. These data indicate that crustaceans comprise the most intensively used food group in this coastal region, a conclusion corroborated also by direct observations f a c e page 38 Figure 7. Seasonal food habits, as determined by f e c a l analyses, of mink i n Clayoquot Sound, Vancouver Island, 1968-1972. The three seasons indicated are described.^in the text. The numbers i n par-entheses preceding the bars i n the "Crabs" section r e f e r to sample sizes (number of scats) for each time period,jandtthese p e r t a i n to the following food categories ( f i s h , b i r d s , and other) as w e l l . The h o r i z o n t a l span of each bar indicates the frequency of occurrence for a given food during the time period indicated, and t h i s may be read from the percentage, scale above. The extent of shading of each bar depicts the average volume attained by the item i n i t s occurrences during that period. For these volume designations, each bar i s divided v e r t i c a l l y into f i v e sections and the number of these which are blackened indicates the mean volume as follows: one (mean v o l J = 1-5%; two (6-25%); three (26-50%); four (51-75%); f i v e (76-100%). 38 Clayoquot Sound C R A B S , 9 6 » (0) 1969 (132) 1970 (is), 1971 (133): 1972(275) Dec -Mar Apr -Jul Aug -Nov FISH BIRDS OTHER 1968 1969 1970 1971 1972 1968 1969 1970 1971 1972 i1968 1969 1970 1971 1972 s D B I H U ALL YEARS CRABS FISH BIRDS OTHER N =588 N= 504 N = 324 i i 39 Table 3. Seasonal food habits of mink on islands and shores within (Tofino Inlet) and at the mouth (Vargas Island) of Clayoquot Sound, Vancouver Island, B.C., 1968-1972. Frequency (7.)a and Volume Category (Vol.) b Dec-Mar Apr-Jul Aug-Nov Vargas Tofino Vargas Tofino Vargas Tofino (n=298)c (n=290) (n=363) (n=141) (n=212) (n=U2) Food Items 7. Vol. % Vol. % Vol. 7. Vol. 7. Vol. 7. vol. MOLLUSKS T (3) CRUSTACEANS Decapod a Hemigrapsus sp. Cancer productus  Cancer magister  Cancer gracilis  Cancer oregonenis unident. Cancer  Telmessus cheiragonus  Pugettia producta other Pugettia  Petrolisthes sp Isopoda Amphipoda unident. crustaceans INSECTS FISHd Small fish Medium fish Large fish BIRDS Anatidae Seabirds e Laridae Passeriformes unidentified birds MAMMALS Microtus townsendi  Peromycus maniculatus  Mustela vison  Procyon lotor unidentified mammals DEBRIS 71 (5) 61 (5) 1 (5) 1 (2) 16 (4) 25 (5) T (5) 5 (5) 13 (5) 1 (5) 1- (3) T (5) 1 (4) T (1) 48 (4) 22 (5) 3 (3) 10 (4) 4 (4) T (5) 1 (2) 49 (4) 50 (5) 41 • - (3) 18 (4) 6 (4) 5 (5) 6 (4) 27 (5) 15 (4) 7 (4) 9 (4) 3 (4) 1 (4) T (5) T (5) 5 (4) 2 (4) 1 (3) T (4) T (5) 1 (2) 1 (3) 5 (3) 89 (5) 76 (5) 2 (4) T (5) 35 (5) 64 (5) 1 (5) 1 (3) 47 (5) 14 (5) 13 (5) 9 (3) 1 (2) 3 (3) 3 (4) 1 (2) T (3) 30 (4) 32 (5) 27 (4) 16 (4) 7 (4) 3 (4) 3 (3) 17 (5) 2 (4) 2 (5) 1 (2) 1 (3) ^. T v (5) T (2) T (1) 3 (4) 77 (4) 55 (5) 1 (1) 33 (4) 47 (5) 3 (5) 2 (5) 1 (1) 1 (1) 22 (4) 21 (4) 5 (4) 1 (4) 1 (2) 1 (3) 1 (3) 2 (2) 51 (4) 55 (5). 42 (3) 39 (4) 10 (4) 5 (5) 8 (4) 16 (5) 12 (4) 6 (5) 5 (4) 5 (5) 5 (5) 5 (3) 4 (2) 3 (2) 1 (1) 5 (3) 5 (2) aFrequency of occurrence, expressed as percentage (nearest whole per cent) of scats in which item occurred. . 'The average volume the item attained in those scats in which i t occurred, expressed as one of six volume categories: T=trace (less than 17.); 1=1-57.; 2=6 - 257.; 3=26-50%; 4=51-757.; 5=76-1007.. cThe number of scats in sample. ^Fish size categories defined in text. eSeabirds - Gaviidae, Podicipedidae, Phalacrocoracidae and Alcidae. Table 4. Spring and Summer Foods of Mink in Barkley Sound, Vancouver Island, Brit ish Columbia, 1971 - 1972, as determined by Fecal Analyses. Food Items (n=37)c 7. Vol. Frequency (7.)a and Volume Category (Vol . ) 1 1971 Mar - Apr May-Jun Jul-Sep Mar-Apr 1972  May-Jun (n=33) % Vol (n-3) 7. Vol (n=100) % Vol Jul-Oct (n=81) 7. Vol . Mollusks Crustaceans Decapoda Hemigrapsus sp. Cancer productus  Pugettia producta  Petrolisthes sp. Unident. crabs Isopoda Amphipoda F i s h d Small Fish Medium Fish Large Fish Birds Passeriformes Unident. Birds Mammals . - Microtus townsendl (2) (3) 89 92 92 33 91 93 76 92 92 33 61 93 22 (3) 3 (1) 4 (3) 62 (4) 92 (5) 80 (5) 33 (5) 53 (5) 93 22 (4) 1 (5) 2 (2) 1 (5) 30 (5) 8 (3) 28 (5) 3 (1) 3 (2) 22 18 17 33 22 9 8 (4) 8 (3) 15 (4) 33 (5) 14 (4) 9 11 (2) 33 (5) 4 (4) 5 (3) 13 (3) 3 (5) 5 (5) 3 (2) 2 (3) 3 (2) 2 (3) 3 (5) 3 (5) (5) (5) Debris 22 (4) aFrequency of occurrence, expressed as percentage (nearest whole per cent) of scats in which item occurred. bThe average volume the item attained in those scats in which i t occurred, expressed as one of six volume categories: T=trace (less than 1%); 1=1-57.; 2=6-257.; 3=26-507.; 4=51-757.; 5=76-1007.. cThe number of scats in sample. ^Fish size categories defined in text. 41 of hunting mink (Figure 8) and by analyses of digestive tracts of animals collected in Clayoquot Sound. (Table 5). As I have suggested in the des-cription of methods, the preponderance of fish among the Barkley Sound sample of digestive tracts (also shown in Table 5) is probably an artifact of the sampling situation. Following paragraphs describe the extent and circumstances of use of mink prey species identified on my study areas. Crabs Figure 9 (a) depicts the relative occurrence of' several crab species among the remains at mink feeding middens during the years of this study. The red crab (Cancer productus) was the most often encountered species, , and i t was fairly stable in its occurrence over all years. Close behind i t in apparent importance is the northern kelp crab (Pugettia products), which showed a high incidence in 1970 and 1971, but was taken less often in other years. The Dungeness crab (Cancer magister) and the slender crab (C. gracilis), lumped together in the category "other Cancer", showed the greatest fluctu-ation in use, falling from a high in 1968 to virtual absence in 1970, and then rising again to greater importance near the end of the study period. The helmet crab (Telmessus cheiragonus) showed a peak in 1969, but was rather stable at just under 15 per cent of all occurrences over most years. All other crab species were used l i t t l e by mink, appearing as less than four per cent of the midden occurrences and in only a few scats during all years. Seasonal differences in occurrence during the study period are shown in Figure 9 (b). More detailed data on crab carapace counts from closely examined middens on both a seasonal and annual basis are listed in Appendix 5. The general pattern, in common with .ithat shown by the scat analyses, is one of a predominance of Cancer crabs, especially in summer and f a l l , with kelp crabs increasing in importance in early winter and becoming dominant face page 42 Figure 8. Prey animals observed being captured by mink i n l i t t o r a l habitats along the west coast of Van-couver Island, B r i t i s h Columbia, 1968-1972. Relative frequencies of occurrence for each item can be interpolated from the percentage scales at the top. Numbers i n parenthesis are sample sizes (number of captures observed) for the bar charts, below. Items C and F (other crabs; other f i s h ) include crabs (or f i s h ) not s p e c i f i c a l l y i d e n t i f i e d and species other than those l i s t e d . U n i d e n t i f i e d prey may have been eit h e r f i s h or crabs i n some cases, but mostly involved very small animals which were eaten immediately a f t e r capture. Dec-Mar Apr-Jul Aug-Nov I 1 1 — — i 1 1 I 1 1 — — i 1 1 I 1 — i 1 1 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 mm Crabs a - r e d c r a b b - helmet crab c - other (including unidentified) Fish d - blennioids e - sculpins f - o f h e r Unidentified Prey 43 Table 5. Foods in digestive tracts of west Vancouver Island mink, May-July 1971 and 1972. Stomachs Food Items Freq' Vol. Intestines Freq.' Vol. CLAYOQUOT SOUND (16 Stomachs, 15 Intestines) CRABS Cancer productus 37.5 51.8 Pugettia producta 6.2 25.0 Telmessus cheiragonus 43.8 64.1 Cancer gracilis 12.5 55.0 Hemigrapsus nudus 6.2 10.0 33.0 20.0 66.7 13.3 50.0 93.3 94.5 97.5 OTHER INVERTEBRATES Isopoda Amphipoda Polychaeta Mytilus sp. 6.2 6.2 6.2 1.0 95.0 5.0 6.7 6.7 5.0 1.0 VERTEBRATES Small fish 37.5 84.2 Rattus norvegicus 6.2 100.0 BARKLEY SOUND (13 Stomachs, 12 Intestines) CRABS Cancer productus 38.5 79.0 Pugettia producta 7.7 '60.0 Hemigrapsus nudus 7.7 25.0 20.0 13.3 58.3 8.3 8.3 50.5 86.4 100.0 VERTEBRATES Small fish 84.6 74.5 66.7 61.9 Frequency of occurrence (proportion of stomachs or intestines in which item occurred), expressed as a per cent. •"The average volume (ocular estimate) attained by the item in those stomachs or intestines in which i t occurred. face page 44 Figure 9. Annual (a) and bimonthly (b) v a r i a t i o n s i n midden occurrences of crabs eaten by mink, Vancouver Island, 1968-1972. Each crab species was given an "occurrence score" of two when i t was the pre-dominant item i n a midden, or a score of one i f i t occurred i n numbers equal to or less than other species. The numbers i n parentheses below the h o r i z o n t a l scale are the t o t a l occurrence scores obtained during the seasons indicated. 44 Clayoquot Sound Barkley Sd. Red Crab (80.4) All Cancer (51.0) Red Crab (36.4) Kelp Crab (34.4) Other Cancer (l4.6) Kelp Crab (15.4) Helmet Crab (l3.4) Other Crabs (l.6) Other Crabs (3.9) 1968 1969 1970 1971 1972-1973 X All Years X All Years (29) (44) (89) (115) (3l) (308) (5l) 45 until late spring. Helmet crabs, which were common among the outer islands of Clayoquot Sound but rare or absent among the Broken Group Islands of Barkley Sound, achieved their greatest importance to mink during the. summer months. Less used species such as the shore crabs (Hemigrapsus sp.) and the smaller spider crabs (Pugettia sp.) appeared in middens primarily in winter, especially late winter, and were scarcely taken in summer when the more commonly eaten crabs were most available. Among Cancer species eaten by mink, the red crab predominated in all seasons on both study areas, especially in Barkley Sound where i t is the only large decapod regularly seen in intertidal waters. In winter 1971-72, the slender crab appeared in unusual abundance in Clayoquot Sound and i t temporarily supplanted other-.; species among mink foods at that time, but by the following summer the red crab again dominated Clayoquot Sound middens. I also found red crabs predominating in summer and fa l l middens on islands in Georgia Strait off the southeast end of Vancouver Island, along rocky shores of Barkley Sound and Clayoquot Sound inlets and channels far removed from the main study areas, and on islets and headlands in Nootka Sound and in Nuchatlitz, Esperanza and Ououkinsch Inlets 50-150 km north of Clayoquot Sound. Reports from other observers indicate that mink along the northern mainland coast such as on the Bardswell Group Islands (CJ. Guiguet, personal communication) also feed regularly on this species, and in February 1971 I found evidence of heavy summer use of red crabs on Bonilla Island, in Hecate Strait south of Prince Rupert. The importance of the red crab to coast dwelling mink in British Columbia is therefore apparent, although evidence from expanses of sand and mud bottom shores, where this species does not occur in numbers, indicates that many individual mink subsist without i t . In early summer 1968, when this study began, Clayoquot Sound mink were feeding primarily upon red crabs and helmet crabs. However, on 19 July M. Miles and I found a concentration of hundreds of small Cancer crabs 46 (most less than 80 mm in carapace width) in a small estuary on north Vargas Island, and these predominated in nearby mink middens. We pre-sumed that they were juvenile Dungeness crabs, but i t later became apparent that I did not distinguish the similar slender crab from Dungeness crabs until more than a year later.' It was this realization, in retrospect, which led to my lumping the two species as "other Cancer" in my midden data (Figure .9), although results from subsequent years suggest that most of the occurrences in that, category are slender crabs. After I had learned to distinguish these species (late summer 1969), I identified just six Dungeness crab carapaces in middens through 1972, and saw mink with fresh, caught specimens on only a few occasions. Despite its low occurrence among mink foods there, the Dungeness crab was common in Clayoquot Sound through-out the study, especially in summer, and I saw i t frequently. For two years after the single high incidence of "other Cancer" reported above,I rarely found either species in middens and saw slender crabs in intertidal waters only.rarely. My first certain record of extensive use of the slender crab was.in November 1970, when I found carapaces in the ratio 40 slender crabs:60 red crabs in mink middens near the mouth of the Megin River (15 miles north of the main study areas, in one of the deep water inlets of Clayoquot Sound). Slender crabs were seen only sporadically near the Clayoquot Sound study areas until 20 December 1971, at which time J. Svoboda, Jr., a commercial fisherman, took 206 specimens up to 75 mm in. width from a single shrimp trap a few miles north of Tofino Inlet. He had never before caught this species in a shrimp trap, and in over 20 years residence in the area had not previously seen i t in such numbers. Within a few weeks I observed a dramatic increase in mink use of slender crabs 47 in the Clayoquot Sound study areas, especially Tofino Inlet where they pre-dominated among crab remains in fecal material (Appendix .4) and in middens from December through March. For a l l of Clayoquot Sound, only the kelp crab was used more often during that winter (see Figure 9). Of the larger decapods occuring in Clayoquot Sound, the kelp crab was second only to the red crab among food items consumed,annually by mink in that area. As is indicated in Figure 9(b) and Table 3, this species often predominated in winter and spring, but use declined considerably during the summer. A Vargas Island midden belonging to one male mink contained 118 kelp crab carapaces when examined on 17 November 1971, and 39 days later (26 December) had accumulated an additional 158. Large accumulations of kelp crab carapaces were seen as late as April (117 on the 14th of that month in 1970), but the largest after that time until late f a l l was 30 (31 May 1970 on Vargas Island). There were no counts larger than 20 between June and October. As shown in Figure 9(a), annual use of the kelp crab peaked in 1970 and was again high in 1971, and these results paralleled apparently unusual local abundance. Scuba diver R. Palm (personal com-munication) began reporting large concentrations of kelp crabs in Clayoquot Sound waters in December 1970, and commercial crab fisherman D. Arnet (personal communication) found unprecendented numbers in his Tofino Inlet Dungeness crab traps (8-10 per trap, or more) from February through May of that year, and again in November and.December 1971. An interesting aspect of mink predation upon the kelp crab was an apparent selection for females. Among remains found in middens it was un-usual": for evidence of males (v-shaped telson and/or large chelae) to appear in proportions higher than 10 per cent and the ratio was often much 48 lower. Appendix 6 details the observations pertinent to this relationship, discusses the biases which might have influenced the results, and speculates that this differential selection occurred as a result of the difficulty mink have in handling the pugnacious and powerful kelp crab males. The helmet crab appeared among Clayoquot Sound food materials primarily from spring to early f a l l (Tables 3 and 5; Figure 9b). During the years 1969-1971, when I frequented beaches there almost daily, earliest records of this species in feces and/or middens were 7 March, 14 April, and 30 March, respectively. The earliest date on which more than 20 carapaces were found in one midden was 6 June 1969 (23), and the highest count for a single midden was 44 (20 June 1970). During four of five years this species predominated among crabs eaten at Vargas Island between April and July; in 1969 i t was a close second to the red crab during that period and actually exceeded the occurrence of the red crab in the August through November period of that, but no other year (see Appendix 3). In Tofino Inlet the helmet crab was also most im-portant to mink during the April-July period (Appendix 4), but to a much lesser extent than at Vargas Island, Most occurrences were in the lower reaches of the inlet, in the immediate vicinity of Tofino, and i t appeared that this species occurs only rarely in protected inland channels of the area. Although I twice found moulted carapaces of the helmet crab in Barkley Sound during the summer months, on no occasion did I find evidence of its being eaten by mink there. The purple shore crab (Hemigrapsus nudus) was preyed upon by mink throughout the year, but at a frequency of two per cent or less in all seasons at Clayoquot Sound (Table 3). In Barkley Sound, where the variety 49 of crab species is smaller, this species was eaten more often and was found in more than one-fifth of the scats examined there in spring 1971 (Table 4). Other species which occurred in numbers under rocks along study area shores, including Hemigrapsus oregonensis, Cancer oregonensis, and an Anomuran (Petrolisthes sp.), were eaten by mink in only trace frequencies and with no evidence of seasonality in their occurrence. Two smaller spider crab species, Pugettia gracilis and, to a lesser extent, P. ric h i i were eaten occasionally in Clayoquot Sound, especially in the winter (December-March) period (Table 3). Figure 10 shows the sizes of crabs eaten by mink in my study areas, based on measurements of carapaces in feeding middens. Figure 11(a) gives a visual comparison of the relative sizes of the commonly eaten crab species, showing that the Cancer crabs, with their short, wide carapaces, are "smaller" than individuals of other species with the same measurements. Figure 11(b) shows the relationship of carapace width and intact body weight for some red crabs and kelp crabs. Most of the crabs regularly eaten by mink appear to spend their early li f e in hiding, often under boulders in the case of red crabs and kelp crabs (personal observations) and do not appear among mink food samples until they have attained sizes of 30-40 mm. Incidence of shore crabs as small as 15-20 mm in scats indicates that small crabs are at least sometimes acceptable, although as shown in Figure 10, most predation on these diminutive species is of large individuals (x = 36.8 mm). Mink ate red crabs ranging in size from 34 to 149 mm but, as Figure 10 shows, few were smaller than 60 mm or larger than 110 mm. The largest one I saw,anywhere, was"a 151 mm male taken in a crab trap from five fathoms in face page 50 Figure 10. Histograms of crab size classes eaten by mink, Vancouver Island, 1968-1972. These are a l l based on measurements of carapaces from mink feeding middens. Measurement;of Cancer species is the greatest width, including lateral spines, while that for the kelp crab and the helmet crab is width just anterior to the largest lateral spine, as shown in Figure 11. a. Red Crab (n=849) 50 1 X=85.7 C A R A P A C E c. Helmet Crab [n =161) WIDTH (mm) face page 51 Figure 11. Crabs commonly eaten by mink, Vancouver Island, B r i t i s h Columbia. a) Size r e l a t i o n s h i p s of four species with the same carapace measurement (M). b) Carapace width - gross weight r e l a t i o n s h i p of red crabs and kelp crabs. 51 (a) Red Crab Helmet Crab (b) 500r Kelp Crab 400 300 O) 200 Red Crabs • Kelp C r a b s A 100 I I I I I I I I 50 70 90 110 130 150 c a r a p a c e width (mm) Barkley Sound, Such evidence, from middens and from observations of crabs seen elsewhere, indicate that mink are able to prey upon i n d i v i d u a l s up to the maximum size for a l l species detected i n food samples with the probable exception of the Dungeness crab. At 900 g or more, many mature crabs of th i s species outweigh most female mink and approach the weight of many males. By comparison, the second largest species (red crab) r a r e l y attains a weight of 500 g and most i n d i v i d u a l s of the size r e g u l a r l y taken by mink are h a l f that, or less (Figure 11). Comparisons (Appendix 7) between seasons and between areas show that red crabs caught i n winter averaged.a b i t larger than those from other seasons at both Tofino I n l e t and Vargas Island, and that those from middens i n Barkley Sound were s u b s t a n t i a l l y larger than those i n Clayoquot Sound. The area., differences probably r e f l e c t habitat differences (the rocky nearshore areas of Barkley Sound providing better habitat for red crabs) rather than d i f f e r -e n t i a l s e l e c t i o n by hunting mink. Other Crustaceans Some mink, e s p e c i a l l y those i s o l a t e d on small rocky i s l e t s , fed r e g u l a r l y on a large isopod, the sea s l a t e r ( L i g i a p a l l a s i i ) . Highest use was recorded i n winter at Clayoquot Sound (Table 3), perhaps because other foods are harder to obtain there during that season. In Barkley Sound, where there are fewer a l t e r n a t i v e foods and/or higher populations of s l a t e r s , they were eaten r e l a t i v e l y often i n spring and summer.(Table 4). According to Care-foot (1973) these animals emerge at night from cracks and crevices among boulders above the high tide l i n e , and move into the i n t e r t i d a l zone to feed. At such times they may be taken s i n g l y by mink, but they are probably most vulnerable when tides are high at night. Under these conditions they 53 aggregate on rocks near their crevices (Carefoot 1973). When slaters occurred in mink feces, there were usually several present. Pinkish colored amphipods, most of them apparently Orchestoides sp., occasionally occurred in the fecal material from both Clayoquot and Barkley Sounds. As was the case with sea slaters, individual scats containing amphipods usually contained large numbers, suggesting that mink fed upon them primarily when they encountered concentrations such as those which often occur in piles of beach debris. I saw impressively large concentrations of amphipods among rotting algae at several locations on Bonilla Island (more than 350 miles north of the study areas on the northern British Columbia coast) in February 1971, and several mink droppings there con-tained these animals (Appendix 8). In one small latrine at Bonilla, amphipods actually predominated. On a few occasions, fragments of small shrimp-like animals, probably Spirontocarls sp., were found in Clayoquot Sound fecal material. Ghost shrimps (Callianassa sp., Anomura) are acceptable as food, as indicated by the fact that more than a dozen of these animals which I had collected •for fish bait were pirated and eaten by a mink. However, although these animals were abundant locally (especially on Vargas Island), I found no trace of their use among the natural mink food samples. Other Invertebrates Some mollusks are probably eaten without ingestion of hard parts and would therefore not be detected in fecal analyses, but evidence from exami-nation of middens and from observations of hunting mink, as well as that from fecal samples, support the conclusion that predation upon mollusks 54 is rare. One mink pirated several large horse clams (Schizotherus nuttalli) from me, but was unable to open their shells and ate only the siphons. Among the natural food samples, use of bivalves was limited to isolated occurrences of Mytilus and Protothaca. In Barkley Sound, abalones (Haliotis kamschatkana) up to 110 mm in greatest diameter appeared in mink middens on several occasions. I have found these animals lying loose among marine vegetation on some extreme low tides, and i t is likely that they are not susceptible to predation by mink except under such circumstances. Use of other univalves such as the turban snails (Tegula sp.) was suspected but not confirmed. The large moon snail (Polinices lewisii) is common on study area waters, and I often saw i t stranded by the receding tide in summer, but I have only two records of its being eaten by mink. On two occasions I saw hunting mink pass, with apparent disinterest, within a few centimeters of fully expanded individuals of this species. Use of other marine invertebrates was infrequent: I twice found the remains of sea urchins (Strongylocentrotus franciscanus) which appeared to have been eaten by mink; R. McLeod (personal communication) told me that he once watched a mink make several trips to remove and carry away a variety of sessile animals including the white anemone (Metridium) from a recently beached logging boom; a mink found dead near Tofino in July 1971 had a large volume of unidentified polychaete worms in its digestive tract; in winter 1969 I found evidence that a mink had chewed and perhaps eaten some empty casings from the tubeworm Eudistylia polymorpha. On the other hand, mink were twice seen in contact with tide-stranded specimens of the giant sea cucumber (Stichopus californicus), but did not attempt to use them as food. 55 Land invertebrates were used l i t t l e . A species of beetle (Carabidae) often appeared on b a i t i n my l i v e t r a p s and on items i n middens, and also o c c a s i o n a l l y showed up i n f e c a l samples. Most may have been eaten i n -advertently, although on one occasion several beetles occurred i n a single scat. F i s h F i s h was second only to crab among foods used by Clayoquot Sound mink (Figure 7). Both at Vargas Island and at Tofino, f i s h occurred i n h a l f of the scats c o l l e c t e d i n the August-November and December-March periods ( i . e . f a l l and winter) but i n the A p r i l - J u l y period (spring and summer) frequency of occurrence f e l l to 30 per cent (Table 3). F i s h was also the number two ranking item among Barkley Sound mink, but i t occurred somewhat less often there, at a frequency of 20 per cent or less (Table 4) i n the periods of study. However, Table 5 shows that some Barkley Sound mink, p a r t i c u l a r l y those c o l l e c t e d on low tide boulder beaches, were preying heavily upon small f i s h . F i s h remains found i n feces were not often i d e n t i f i a b l e to taxonomic categories, but a l l were l i s t e d under one or more of three s i z e categories: (1) small (centra of largest vertebrae less than 2 mm i n diameter), (2) medium (centra of largest vertebrae 2-5 mm i n diameter), and (3) large (centra of vertebrae larger than 5 mm, or, coarse f i s h bones present, but no vertebrae). As the data i n Tables 3-5 show, by far most f i s h occurrences were i n .the "small" category. Appendix 9 l i s t s the length, weight and cen-trum diameters for several i n t e r t i d a l and subtidal f i s h to bring the actual meaning of the size categories into perspective. As these figures show, 56 pricklebacks (Stichaeidae) and gunnels (Pholidae) up to 132 mm are well within the small category and even at 166 mm have only begun to approach the upper limit of this size range. The more squat fishes such as sculpins (Cottidae) are more than double the weight of pricklebacks. of the same length and achieve "medium fish" rating (on the basis of centrum diameter) at much shorter lengths. One northern clingfish (Gobiesox maeandricus), at just 90 mm total length, is in the medium category. These figures, combined with the results of Table; 3, indicate., that most individual fish taken by mink weigh less than 15 g. It appears that most small fish taken are intertidal forms, particularly pricklebacks, gunnels and sculpins, although fishes which might more properly be described as subtidal, such as juvenile rockfish (Scorpaenidae) and greenlings (Hexigrammidae), are also taken. On the basis of the presence of diagnostic bones, as determined from preparation and use of a reference collection, 115 Clayoquot Sound fish occurrences in the fecal samples could be identified to family (74'-sculpin, 36 prickleback and/or gunnel, and 5 rockfish). Because diagnostic bones were not always present and occasionally because fishes not represented in my reference material appeared, such identifications were possible in just i+0%> of the fish-bearing scats. Thus the figures 74:36:5 given above probably do not represent precisely pro-portional use of the groups listed. However, othere evidence suggests that sculpins, pricklebacks and gunnels are indeed the most regularly eaten fishes in my study areas. Many species in these groups are sparsely scaled or possess only very small scales (see Clemens and Wilby 1961). Several species known to have been eaten by mink, including Ascelichthys rhodorus. , Clinocottus embryum , Leptbcottus armatus. and Xiphister atropurpureus , have no scales at a l l . The fact that I found recognizable fish scales in only 26 (9.TL) of 268 fish-dominated scats examined carefully (all from Clayoquot Sound) confirms that mink used non-scaly fish more regularly than scaly forms in that area. As I shall show later, mink hunt and catch fish largely in the inter-tidal zone, and this fact plus the information given above (most intertidal forms bear few scales and few scales appeared in the fish-bearing scats analyzed) are consistent with one another and with the results of scat analyses based on skeletal material, as listed earlier. While I can there-fore state with some confidence that sculpins, pricklebacks and gunnels were the most important food fish for my Vancouver Island mink, I cannot demonstrate satisfactorily that' any particular species among these groups was more im-portant than others except in some purely local situations. I was able to identify individual prey species on a number of occasions, however, both from direct observations of hunting mink and from examination of mink-caught specimens at middens and den-sites, and the following paragraphs summarize these specific findings. All fish nomenclature in this paper follows Hart (1973). The great sculpin (Myoxecephalus polyacanthocephalus) was the fish species most often identified among mink food items (14 records involving a minimum of 21 fish), but this was primarily because it is a large species and its easily identifiable head was conspicuous in middens. Occurrences of this species, recorded in all seasons but only in Clayoquot Sound, mostly involved fish estimated to have been 250-300 mm long. Another large cottid which appeared frequently in Clayoquot Sound middens was the staghorn sculpin 58 (Leptocottus armatus), a species which was common along mudflats and sand beaches in the area. A 192 mm specimen which had been cached intact by a mink was the largest recorded. The red irish lord (Hemilepidotus hemilepidotus) was identified in middens and scats in both Barkley and Clayoquot Sounds. Four mink-caught specimens I measured ranged in size from 75-250 mm. Among the smaller, more typically intertidal cottids, the rosylip sculpin (Ascelichthys rhodorus) was the most commonly identified among mink foods, occurring cached in middens (78-112 mm, N=4) and in scats at both study areas and in^all seasons. The calico sculpin (Clinocottus embryum) was identified only once among mink food items, a 97 mm specimen from a midden on Wickaninnish Island, but i t is reportedly common in shallow waters along the west coast of Vancouver Island (Clemens and Wilby 1961) and was probably eaten more often. Many scats, collected throughout the year and in all areas, contained tiny green bones suspected to have come from small sculpins. In a few cases these included preopecular bones bearing bifid spines and were al-most certainly from the tidepool sculpins Oligocottus maculosus and/or 0. .:  snyderi. Two dark-colored Blenniiform fishes, the black prickleback (Xiphister  atropurpureus) and the rock prickleback (Xiphister mucosus)were. seen commonly under boulders in all study areas, and both were eaten by mink. Two speci-mens of the black prickleback cached by mink measured 139 and 152 mm, while a partly eaten rock prickleback was estimated to have been about 120 mm long in l i f e . Mink were frequently seen with Xiphister specimens, especially in summer, but i t was not possible to determine whether one was taken more often than the other since identification from a distance is impossible. I did not 59 certainly identify any other pricklebacks (Stichaeidae) among mink foods, but on one occasion the high cockscomb (Anoplarchus purpurescens) was the most abundant species under a boulder at which a mink had hunted success-fully for several minutes. In the Blenniiform family Pholidae, the penpoint gunnel (Apodichthys  flavidus) was the species most often eaten by mink. It was especially common along Vargas Island in spring and early summer, and on numerous occasions during that season mink were seen with specimens 80-120 mm long. Blennii-forms estimated to have exceeded 250 mm in lengtth were caught twice during my observations and in both cases they were presumed, on the basis of colo-ration, to have been ..this species, sin July 1971, a mink caught a 118 mm crescent gunnel (Pholis laeta) from a tide-exposed Vargas Island eelgrass bed. This was the only other gunnel which I am certain was caught by mink on my study areas, although I sometimes found other species, especially the saddleback gunnel (Pholis ornata), under intertidal boulders at which mink were known to have hunted, and these are probably taken occasionally. Fishes which occur mostly in deeper waters, below the intertidal zone, appear to be eaten only rarely... Juvenile rockfish (Sebastodes sp.) occurred in large schools amid the kelp beds off rocky islets and headlands along the entire west coast of Vancouver Island, but I saw a mink with one of these just once, a black rockfish (S. melanops) estimated at 100 mm near Turtle Island in April 1972. Other occurrences, including small individuals in a few scats, remains of a 300 mm specimen in a Vargas Island midden, and a report of a Barkley Sound mink swimming with a live specimen (estimated one kg) in tow (B. Hillier, personal communication) were recorded. Two species of hexigrammids, the kelp gre'enling (Hexagfammus decagrammus) and the rock 60 greenling (H. lagocephalus), were common along rocky shores in habitat similar to that described for rockfish. Both species were probably taken occasionally, although only H. decagrammus was identified among mink foods; a 124 mm specimen was found in a midden and on 25 November 1971 I saw a small female mink dragging a s t i l l struggling kelp greenling, 273 mm long and weighing 375 g. This was the largest fish known to have been caught by mink during this study. All five species of Pacific salmon (Oncorhynchus sp.) spawn in streams on Vancouver Island's west coast, but there was no stream;., heavily used by salmon in my main study areas. Coho salmon (0. kisutch) spawned in small numbers in at least two streams on Vargas Island, but I have no evidence of their use by mink either by direct predation or by scavenging of spawned-out fish. The chum salmon (O.keta) is probably the most abundant salmon in Clayoquot Sound Streams and dead individuals of this species occasionally drifted onto study area shores. On three occasions tracks indicated that mink had visited such specimens, but had not eaten from them. Many dead and dying chum salmon were present in the estuary at the mouth of the Megin River, north Clayoquot Sound, in November 1970, but mink there were observed catching mostly crabs and intertidal fish, as usual. Local middens consisted mostly of crabs, although two salmon were found under shore-fringing shrub-bery and were presumed to have been dragged there by mink. A small portion of one of these had been eaten,but the other was intact. Observations of sign and some trapping along salmon streams indicated that marten (Martes  americana) were using salmon more intensively than were mink in all areas. It is apparent that mink do not exploit the annual salmon runs in the areas I studied, although individuals readily pirated salmon from the holds of 61 commercial fishing vessels moored in Tofino and Ucluelet. Scattered records of other fishes eaten by mink were obtained. The northern clingfish was common among intertidal boulders in both Clayoquot and Barkley Sounds. On a few occasions I saw mink catch this species, and I found a 120 mm specimen at the mouth of a mink den at Wickaninnish Island in mid-August 1970. Another intertidal form, the tidepool snailfish (Liparis florae) was recorded once, a 138 mm individual found in a midden. It appeared to be rare in the area, as I saw i t on only one other occasion. Flatfishes (Pleuronectiformes) were reported among the items in a midden found on a fishing boat in Tofino one winter (S. Eide, personal communication), a Pacific sandfish (Trichodon trichodon) was cached by a mink on Wickaninnish Island in mid-July 1969, and in May 1973 I found the fresh remains of a small dogfish (Squalus acanthias), estimated 400-500 mm in l i f e , in a mink midden on Turtle Island. Mink also had occasion to feed upon fish they had not caught. I noted several instances in which they had fed upon remains left by otters (Lutra canadensis), especially large specimens of cabezon (Scopaenichthys marmoratus) and lingcod (Ophiodon elongatus). I found a number of other fish including pelagic and benthic species, which had washed up on beaches of the study areas, and such fish are no doubt scavenged as opportunity allows. Birds As shown in Figure 7, birds were eaten only very rarely during the summer months, but occurred in up to 15 per cent of the scats from Clayoquot Sound during the other seasons. Based on downy barbule characteristics of feathers found in scats (family criteria as described in Day (1966) and confirmed by my own reference collection), water species', especially anatids, were the birds 62 eaten most regularly (see Table 3). Specific identifications, which came mostly from remains found in middens, support the above conclusion and are summarized in following paragraphs. I found evidence of mink feeding on Canada Geese (Branta canadensis) on three occasions, a l l in November, but i t was evident in each case that the birds were scavenged and not captured by "the mink involved. The only other anatids specifically identified were the Surf Scoter (Melanitta perspicillata), from middens in September, October and January, and the Bufflehead (Bucephala  albeola), one specimen in January and two in March. However, many other species occur in the area and the fact that more than half of all bird occur-rences in the fecal samples were ducks (see Table 3) makes i t likely that some of these were also taken.. Indeed, I often found the mottled brown covert feathers of dabbling ducks (Anatinae) in and near middens, although never enough for identification to species. Because micro-characteristics of feathers did not enable easy distinction between the families Gavidae (loons), Podicipedidae (grebes), Phalacrocoracidae (cormorants), Alcidae (murres, auklets, puffins), Procellariidae (shearwaters, fulmars), and Hydrobatidae (storm petrels), I lumped all six into a "seabirds" category for the fecal analyses. Specimens of the last two mentioned families occasionally drift ashore and are available for scavenging, but I had no evidence that either these or loons were eaten during my study. Representa-tives of the other three families were identified in middens.. Cormorants, which appear to be the most shore-bound among the groups listed, were the seabirds most often identified; individuals of unknown species appeared in middens in November 1969 and April 1971, while three fresh, partly eaten Brandt's Cormorants (Phalacrocorax penicillatus) were found in middens on. 63 closely adjacent islets in Tofino Inlet on a single day in November 1972. I suspected that these birds had been shot by a waterfowl hunter and scavenged by the two mink involved, but mink do prey upon cormorants at least occasionally. J. Wilkowski (personal communication) watched a mink stalk, catch and k i l l a roosting Pelagic Cormorant (P. pelagicus) near Turtle Island in winter 1971. Other seabirds found in mink middens during the study include a Horned Grebe (Podiceps auritrs) and a Common Murre (Uria aalge) in November 1969, another murre in April 1971 and a Rhinoceros Auklet (Cerorhinca monocerata) in July 1972. I have seen mink hunting on the same beach with gulls (Laridae) on many occasions. The gulls move aside a few meters to let a travelling mink pass, but often they and northwestern crows (Corvus caurinus) were seen standing near feeding mink, waiting to scavenge remains. Never did I see mink show any interest in these birds, although other evidence indicates that they do eat gulls occasionally. I found remains in middens on three occasions in October and November and twice in April. For the three cases in which identification was possible, the birds taken were Glaucous-winged Gulls (Larus glaucescens), the common resident species in the area. I have two records* of mink feeding upon downy young of Glaucous winged Gulls, both from off the study area. In early August 1973, I visited the gull and cormorant colonies on a rocky islet off Long Beach, a few miles south of Clayoquot Sound, and found that a mink had spent a few days there some weeks earlier. In addition to several crab carapaces, its midden contained the remains of two young gulls and fragments of some gull egg-shells. The 12 scats in its latrine contained crab, fish, bird, and isopod in that order of abundance. 64 A more interesting record was provided by R.W. Campbell (personal communication), who monitored nesting by gulls and cormorants on the Chain Islets, near Victoria off the south end of Vancouver Island, during summer 1973. On 23 June he discovered the active midden and latrine of a mink on one of the islets. Cached at this site at that time were a Glaucous-winged Gull, a Pelagic Cormorant, and several shorebirds including three Ruddy Turnstones (Arenaria interpres), four Black Turnstones (A. melanocephala), and a Black Oystercatcher (Haematopus bachmani). Two days later there were fresh scats at the site and these contained mostly crab material; the above listed birds had not been eaten and no new ones had been added to the cache. By 25 July the mink was s t i l l present on the islet and, although fecal remains consisted mostly of crabs, the downy feathers of young gulls were evident in several scats. I found no evidence of heavy predation on shorebirds on my study ateas despite the fact that, especially during spring migration, large numbers of several species may often be seen roosting on mink-inhabited islets. In such cases they appear vulnerable 'and Campbell's observations on the Chain Islets (above) indicate that mink are occasionally able to exploit them. I found feathers which I suspected to be shorebirds in a few scats, but identified only one Black Oystercatcher among midden remains. Occurrences of passerines in my mink food-habits material was limited to feathers in two scats and midden remains of a Yellow-bellied Sapsucker (Sphyrapicus  varius), a Song Sparrow (Melospiza melodia), and a Fox Sparrow (Passerella  iliaca), a l l in winter. The stomach of a road-killed mink found in November 1971 contained feathers of an apparently scavenged grouse -(Bonasa umbellus). 65 Predation upon domestic birds was also recorded, with mink killing 30 chickens in one case and 4 in another at Tofino and Ucluelet respectively; J. Todd (personal communication) told me that, over a period of years on Sidney Island, near Victoria in the Strait of Georgia, mink had caused serious losses among ground-nesting exotic birds he had introduced there, and had actually prevented establishment'of some. Mammals Mink in my study areas rarely ate mammals. A scat from Vargas Island in ^January 1969 and another picked up fresh at Dodd Island (Barkley Sound) on 12 July 1971 contained vole (Microtus townsendi) hair. The latter was deposited about 100 m from the edge of a small estuarine meadow supporting a colony of this species. A scat found on Vargas Island in June 1969 was not fresh, but contained deer mouse (Peromyscus maniculatus) hair and bone, including epiphyseal caps (indicating a young individual). This was my only record of this species as mink food. Over the years of my study, I was aware of at least five mink which were crushed on the highway adjacent to the Ucluelet Dump, a center of Norway rat (Rattus norvegicus) abundance; the only one of these mink which I was able to examine contained remains of a rat in its digestive tract. A mink caught in a Tofino chicken-house also contained rat remains, and Mrs. T. Gibson (personal communication) saw a,mink carrying a dead rat,along the Tofino waterfront in April 1969. Mink hair occurred in mink feces, as would be expected, but I have no evidence of cannibalism. Three of five occurrences were in the July-August moulting period. One scat contained the hair of raccoon (Procyon lotor), certainly taken as carrion. On two occasions I found freshly killed rac-coons, apparently shot, on Clayoquot Sound shores. 66 Debris Most occurrences l i s t e d as debris involved vegetation, both land and marine. I believe that most was taken a c c i d e n t a l l y as i t adhered to animal food being eaten, although on a few occasions an e n t i r e scat was composed of vegetable matter. I t i s l i k e l y that mink occ a s i o n a l l y chew and swallow plants much as domestic dogs are seen to do, but I do not think that any plants are eaten for t h e i r food value. A few debris occurrences were of a dark, mineral material which may have been the remains from the digestive glands of mollusks. DISCUSSION COMPARISON WITH OTHER AREAS The establishment of v i a b l e populations of the North American mink at several locations on other continents, for example S i b e r i a (Benkovsky 1971), other parts of the U.S.S.R. (Aliev and Sanderson 1969), Sweden {(jGerell 1967a), Norway (Wildhagen 1956) and portions of England, Scotland and Wales (Thompson 1968), attests to the a d a p t a b i l i t y of t h i s animal. C e r t a i n l y one of the features of i t s l i f e h i s t o r y which contributes much to t h i s a d a p t a b i l i t y i s the degree to which i t i s a g e n e r a l i s t i n i t s feeding. A l i s t of a l l prey species known would be very large.,, and would include mostly animals smaller than the mink, although mammals up to the siz e of hares (Lepus sp.) and birds of cormorant siz e or larger are taken. Hamilton (1959), a l o n e , i d e n t i f i e d nine f a m i l i e s of f i s h and three of amphi-bians, t h i r t e e n genera of mammals and two of crustaceans, eight orders of insects and assorted b i r d s , r e p t i l e s , mollusks and other invertebrates i n a single series of mink stomachs from New York. Table 6 presents r e s u l t s of the major mink food habits studies of which I am aware, showing the Table 6. Predominant foods of wild (North America) and feral (Eurasia) mink (Mustela vison). The f irst and second ranking classes of food animals in each area and season are listed (M=mammal, B=bird, A=amphibian, F=fish, C=crustacean, I=insect). Underlines in the body of the table show the period of time (months on top scale) over which the data pertain). Winter Spring Summer Fal l AREA HABITAT D J F M A M J J A S O N SOURCE New York several? M,F M.F Hamilton (1936) New York marsh M.F Hamilton (1940) Michigan several M, A Sealander (1943) Pennsylvania several F.C Guilday (1949) North Carolina marsh F,M Wilson (1954) Missouri several I.A Korschgen (1958) Interior Alaska river- M,F Harbo (1958) New York marsh several F,M M,F F,M Hamilton (1959) Iowa marsh M,B B.M M,B M.B Waller (1962) Western Alaska river delta M,B F.C Burns (1964) - marsh F,M F.M M,B M.B lake-marsh F.M F.C C,B M.A river F,M F.C C.F F,M Sweden stream C F Gerell (1968) (nine areas) river C,F C,F C,F C,F stream C.F&M C,M river M,F M,F M.F river-lake M,F M,F M.F sea coast F.C F.C F.C F.C Sweden river F.B F.B&M F.B F.M Erlinge (1969) Russia F.I M,F Aliev & Sanderson (1969) British Columbia sea coast C,F C.F c This study (1968-73) 68 classes of animals which have predominated in different seasons and habitats (mostly terrestrial and fresh water) in a number of areas. The group of animals which has ranked first or second most often in these studies has been fish (43 of the possible 112 occurrences), and fish plus the other class of largely aquatic organisms, crustaceans (20 occurrences), were among the predominant items in 57 per cent of the cases. Classes of animals which are normally associated more with land habitats comprised the remaining 43 per cent of the occurrences, with mammals, birds, amphibians and insects listed 33, 11, 3 and 1 times, respectively. However, many of the high ranking mammal occurrences were dominated by the semi-aquatic muskrat (Ondatra zibethica), and occurrences of birds involved mostly species which nest near water or, in one case (Waller 1962), roost near water. Further, the amphibians taken were invariably from aquatic habitats, and the single high-ranking record of insects involved water beetles (mostly Dytiscidae). These results confirm that the mink tends to concentrate its hunting activities near water (see Coues 1877). In comparing the food habits of mink elsewhere to those reported here for coastal British Columbia, the most obvious contrast is in the use of mammal prey. Although eaten only rarely on my study areas, mammals pre-dominated among mink foods in many other places, especially during the summer months, and in all those areas for which there is no listing of mammals in Table 6, i.e., where they were not first or second in occur-rence of food items, they were third in at least one season. In most areas, fishes predominate among winter foods of mink (Table 6.) Gerell (1968) believed that in his study areas this was due to low .69 water temperatures which reduced the a c t i v i t y and a g i l i t y of f i s h , making them e a s i e r to catch. E r l i n g e (1969) a l s o r e l a t e d seasonal d i f f e r e n c e s i n f i s h caught by both r i v e r o t t e r s and mink to t h i s . e f f e c t of water temperature, and showed i n an e a r l i e r paper ( E r l i n g e 1968a) that slow-moving f i s h were indeed caught more o f t e n by r i v e r o t t e r s than were f a s t e r ones even when the f a s t e r ones appeared to be p r e f e r r e d . H i s conclusions were based on feeding experiments w i t h c a p t i v e o t t e r s ; f i s h w i t h m u t i l a t e d caudal f i n s were caught more f r e q u e n t l y than were i n t a c t f i s h of the same species, and the slower of two species f e l l prey to the o t t e r s more f r e q u e n t l y when both were i n t r o -duced together. That many f i s h e s do show dramatic decreases i n a c t i v i t y and v i t a l processes at lowered water temperatures i s evident from d i s c u s -sions i n F r y (1947) and N i k o l s k y (1963: 258-262). As i n d i c a t e d e a r l i e r , however, marine inshore water temperatures vary l i t t l e over the year, and there i s evidence that other f a c t o r s such as the e f f e c t of wave a c t i o n on rocky shore cover probably c o n t r i b u t e d more to the increased winter use of f i s h on my study areas. THE NATURE OF PREDATION BY MINK Facto r s A f f e c t i n g Prey S e l e c t i o n Because there i s some c h a r a c t e r i s t i c terminology a s s o c i a t e d w i t h the feeding ecology of predators, a b r i e f review i s i n order. In her exhaustive review of stu d i e s from a l l over the world, Ewer (1973) concluded that most ca r n i v o r e s are " o p p o r t u n i s t i c " i n their feeding. She c i t e s three main f a c t o r s , a v a i l a b i l i t y , p a l a t a b i l i t y , and experience which combine to determine what a given c a r n i v o r e w i l l eat i n any l o c a l context, and p o i n t s out that l i t t l e i s known of the l a s t two i n nature. Experience, a property of the predator, i s an i n d i v i d u a l matter and i s probably of l i t t l e importance i n the c o n s i d e r a t i o n of a species' food h a b i t s on a r e g i o n a l b a s i s . P a l a t a b i l i t y , 70 a property of the prey, refers to the relative pleasantness associated with the handling and eating of an item which in turn leads to an exhibition of preference by the predator. Preference may be a matter of taste, such as was apparently the case in the failure of Iowa red foxes (Vulpes fulva) to eat most insectivores and weasels they had killed (Scott 1943), but other factors such as relative toughness of integument(Erlinge 1968a) may some-times be involved. Some authors apply the term "preference" whenever a prey species is shown to be eaten out of proportion to its abundance relative to other potential prey, although in such cases i t may be that the heavily used prey is simply more vulnerable than others, but is not at all preferred. One of the primary factors contributing to availability is prey size in relation to that of the predator in question. Rosenzweig (1966) showed that a number of different carnivores specialize on prey of certain sizes and he theorized that the fact that many of these carnivores can exist in sympatry is due at least partly to these food size differences. Schaller (1972) has also shown the effects of prey size on a given carnivore's food habits, and has pointed out that solitary carnivores usually take smaller prey than those of the same size which hunt in groups. % Of the appropriate-sized prey species accessible to a ..Carnivore in any local area, i t is usual that some are taken more often than others. Often this is simply a function of numbers, the most abundant species being the most often eaten. Some authors, e.g., Scott (1943) use "availability" and "abundance" almost interchangeably, and most authors assume that abundance is the most important aspect of availability. However, two prey species may be equal in abundance but yet be preyed upon to different degrees i f , for instance, one is more easily caught and/or more easily handled than is the other. Various attributes of a prey species, such as speed, agility, pugnacity, hiding behavior, diel activity pattern, habitat preference and susceptibility to disease, render i t more or less vulnerable to predation from a specific carnivore species. This concept of prey vulnerability resulting from factors other than simple abundance has been developed especially by Errington (1943, 1946, 1954). In summary, any prey species which is eaten is "available", and the degree of availability is determined largely by one or more of three factors: (1) size (2) abundance (3) vulnerability. Preference, another factor, is best shown by the relative treatment of different prey species after they have been caught, although i t may act in conjunction with experience to influence what is caught. Earlier I have documented the use, by mink, of a number of food species along the west coast of Vancouver Island. In following pages I consider the food species themselves from the standpoint of "avail-ability" as discussed above, in an attempt to explain why each was eaten at the time and to the extent that i t was. Preference During the direct observations of hunting mink, I occasionally had the opportunity to watch animals confronted with more than one kind of prey. Indicating that at times mink do exercise some kind of choice in their feeding, preferring not to eat certain available items but subsequently eating others, these observations may be summarized as follows: 1. Crabs were accepted shortly after other invertebrates had been ignored (three observations, two involving sea cucumbers and one with a moon snail). 2. Large crabs were released and smaller specimens of the same species were subsequently taken (three observations). 3. Red crabs were released or discarded in favor of a slender crab (once) and a helmet crab (once). 4. Crabs were released in favor of fish (four observations, all involving red crabs). There are no data available on the relative nutritional qualities of these prey, but the fact that mink occasionally selected crabs of the same species as those they had just rejected suggests that selection was not based on nutritional differences. Pugsley (1942) found that crab meat (Cancer magister) was very similar in nutritive value to that of "non-oily" fish such as cod, haddock and halibut, producing roughly 100 calories per 100 grams of meat. Most crab meats are also similar to many fishes in protein content, yielding 20 per cent or more (Borgstrom 1962). I suspect that the main factor influencing the observed selections has been the ease with which the different prey items can be eaten. Erlinge (1968a) experimented with captive otters and found that they rejected cray-fish more often that they did fish, and large-scaled fish more often than small-scaled ones. He believed that rejections were due largely to differences in integument of the proffered prey animals, but acknowledged that other factors such as habit might have been operative. Among my listed mink prey, fish are more easily eaten than are crabs and they were selected in those observed instances in which there was a choice. While studying remains in middens, I routinely broke crab carapaces to ensure that I did not examine each more than once. From t h i s experience I can generalize, though su b j e c t i v e l y , about r e l a t i v e toughness of the exo-skeletons of species commonly eaten. In hardness and thickness, those of kelp crabs are by far the.ascendants, while those of the red crab are second. Carapaces of slender crabs are moderately hard, but are usually t h i n and easy to break, and those of helmet crabs, though tough, are leathery rather than hard and are probably the easiest to chew. F i n a l l y , f o r a l l of these species, carapace thickness increases with increasing s i z e . Again, the few pertinent observations of hunting mink are consistent, with animals s e l e c t i n g a softer crab (within or between species) when two were equally a v a i l a b l e . Perhaps a more objective measure of the attractiveness of various prey to mink i s the degree to which each i s used a f t e r i t has been caught. In Appendix 10, I have described the manner i n which d i f f e r e n t prey are eaten and, for crabs, have compared use of three species discussed above. To summarize, mink w i l l almost always eat a l l of the body meat and v i s c e r a from a crab, regardless of species; they r a r e l y touch the hard, small-diameter, tubular appendages of the kelp crab, but w i l l often consume at le a s t the proximal portions of red crab appendages, e s p e c i a l l y the thick, meaty chelipeds. The comparatively soft appendages of the helmet crab, however, are r e g u l a r l y eaten. Further post-capture evidence of apparent mink preferences includes incidents i n which mink caught and mutilated crabs (removed appendages), but then l e f t them near shore (mostly red crabs larger than 100 mm) and incidents i n which crabs were taken to middens but then l e f t to s p o i l (occasionally red crabs, but most often kelp crabs). Helmet crabs were never so treated. 74 Certainly the degree to which a mink may exercise choice depends upon the state of its nutrition. A very hungry mink will consume a specimen more completely than will a sated one, and most mink which I.saw reject prey had eaten or carried away a few specimens before they became selective. All evidence indicates, however, that when selection is appropriate, mink favor softer prey. There is a possible selective advantage ,in such choice, for softer prey would produce less tooth wear and, as Stirling (1969) suggested for Weddell seals, excessive tooth wear may lead to mortality. Size of Prey Coastal mink regularly preyed upon invertebrates weighing up to 350 g, and occasionally took individuals up to about 500 g. Operating within this weight range, they were capable of handling most individuals of almost all crab species which occupy near-shore waters in the study area. Failure of mink to make much use of the Dungeness crab, a common local species (see Spencer 1932) may be due in part to the fact that this species spends much of its yearly cycle in deeper waters. However, even when both this species and the red crab were moulting,- in numbers, on Tofino Inlet tideflats, the bulk of predation was on red crabs and relatively small Dungeness individuals. It appears to be mostly size which limits use of the latter species. Crustaceans eaten in other areas (Guilday 1949; Gerell 1968; Burns 1964), mostly fresh water crayfishes, aire within the size range given above. All investigators who have distinguished the sizes of fish taken by mink have found that small ones predominate (see Hamilton 1940; Wilson 1954; Korschgen 1958; Gerell 1968; Erlinge 1969). Most fish eaten on my study areas were intertidal forms weighing less than 50 g, often much less, although specimens estimated to have been 500 g or more, in l i f e , occasionally appeared 75 . in middens and one report was received of a mink catching a fish of about 1000 g. Among birds, specimens up to 4000 g (Canada Geese) were eaten but the largest which appeared to have been killed by mink were Brandt's Cor-morants which weighed about 2000 g. Most birds identified among the mink foods were smaller, few exceeding 400 g. This is in general agreement with results from other areas, although i t is evident that of the four prey groups regularly taken over the entire geographical range of the mink (mammals, birds, fish and crustaceans -- see Table 6), birds are the ones most often taken in sizes equal to or exceeding that of mink. I suspect that, once contact has been made, birds are less able, than large individuals of the other groups to either defend themselves or to escape. Muskrats (Ondatra zibethica), approaching the size of adult male mink at weights of 1000 g, or more, have often been reported among mink foods, but mature individuals are apparently not taken at will (see Errington 1943). Mammals up to the size of hares (Lepus) have been reported among foods from other areas (e.g., Gerell 1968), but rarely. The largest mammal prey from my study areas was the Norway rat (weights to 250 g). The lower limits for mink prey size are less clearly defined. On my study areas, the smallest animals taken other than incidentally were beach amphipods, which average about 10 mm long and weigh only a small fraction of a gram. When these occurred in food samples, they always occurred in numbers suggesting that they were attractive to mink only when concentrated. The isopod Ligia pallasii, attaining lengths to about 40 mm and weights of 900 mg or more (Carefoot 1973), was the smallest species which occurred singly wi any regularity although i t , too, was more commonly taken in numbers. Shore crabs (Hemigrapsus sp.), with many individuals in the size range listed for Ligia, were also taken. Water beetles (Dytiscidae) appear to be the smallest 76 prey taken regularly in other areas, e.g. Sweden (Gerell 1968), New York (Hamilton 1940), and Iowa (Waller 1962), and these are comparable in size to the arthropod species mentioned above. In general, i t is evident that mink rarely prey upon animals larger than themselves and, especially in my study areas, subsist primarily upon animals less than one-half their weight. On the west coast of Vancouver Island this does not set any severe limits to the potential food supply for mink sinceVtd6zens of species (invertebrates, fish, birds, mammals) of the appropriate size at maturity frequent littoral areas hunted by mink, and the young of larger species are also occasionally available. Factors other than size appear to be more important in determining what is eaten at any given time. Vulnerability and Abundance of Prey As discussed earlier, vulnerability relates to the special aspects of behavior or li f e history which place a prey in a position at which i t may be contacted and subdued by a predator. Abundance of the prey serves primarily as a modifier of vulnerability in determining the level of predation. For example, the rate of predation upon a given vulnerable species may be low i f that species is very rare in comparison to other potential prey. However, as pointed out repeatedly by Errington (1943, 1946), differential vulnerability exists within species as well as between; some individuals in a population, often those least able to compete, may be forced into vulnerability by the pressures of local abundance. Few of the species eaten by mink on the west coast of Vancouver Island have been studied intensively, although fragments of the life history patterns of some have been recorded incidentally to studies on other species, usually those of commercial value such as the Dungeness crab, and through scattered observations made during climatically favorable seasons. For the most im-portant food species, at least, published notes and my own observations are sufficient to provide some insight into patterns of predation as related to differential vulnerability and abundance. Crabs - Moulting is an essential part of mating in North Pacific coast Cancer crabs (Knudsen 1964a) and two species at least, the red crab and the Dungeness crab, carry out these activities near shore after well-defined movements in from deeper waters (see MacKay 1942). It is at this time that they are most accessible to beach predators. I have established earlier that the Dungeness crab is used l i t t l e by mink, apparently because of its large size; the red crab, on the other hand, was the most used food species in my study areas and i t warrants considerable discussion. From the observations of Spencer (1932), Hartnoll (1969) and the two authors cited previously, one may synthesize the following sketch of this species' li f e history as i t may relate to its potential vulnerability to mink. From about January through May, male red crabs can be found commonly in the intertidal zone, but most females are either hidden or are occupying adjacent deeper waters during this time. By late May and early June, females appear in numbers and may actually predominate among intertidal red crabs in late summer and f a l l . Moulting and mating occurs during that period. Both sexes are present during the late f a l l - early winter period (November-December) before the females again disappear. Clearly, red crabs occur in the intertidal zone throughout the year and should be continuously susceptible to predation by the littoral for-aging mink, but with maximum use in summer and f a l l when both sexes are present. Observed use of this species by mink in Clayoquot Sound, the only area where data were obtained'year-round, Is consistent with this expectation (Table 3; Figure 9). Overall, the red crab was the most widely and regularly 78 eaten crab there; i t was eaten less often in summer at Vargas Island than in Tofino Inlet, probably because of a differential buffering effect by helmet crabs between these two areas. Occurrence of the helmet crab among mink food samples was sharply seasonal, peaking in early summer and dropping to almost n i l from about September to April. Use by mink clearly parallels the species' occurrence in accessible waters. I rarely saw helmet crabs during seasons when i t was not being eaten, Smith (1928) found them in near-shore waters of south-eastern Vancouver Island only in early summer, and MacKay (1943) noted that along the lower British Columbia mainland, near Vancouver, recently moulted carapaces washed ashore primarily from April through early June. Its li f e history has not been recorded, but the above observations suggest that, like the Cancer crabs to which it is related in the family Cancridae, i t moves onshore in summer to complete its mating cycle. It apparently shows a stronger predilection for deep waters, away from the influence of land predators, during the rest of the year. As implied above, the species does not appear in Tofino Inlet to the extent that i t does along Vargas Island shores, probably because of habitat differences, and other species supercede it in use there. I have already speculated that where i t occurs, i t is preferred, both because of its relatively soft carapace and because its rather small chelae do not enable i t to defend itself to the extent that species of Cancer and Pugettia are able to. The kelp crab is another species whose lif e cycle is only incompletely known (Knudsen 1964a,b). It is apparently primarily herbivorous, feeding on several species of marine algae including the large browns (especially Nereocystis in the Vancouver Island area) to which i t clings during the 79 summer. I t i s r e l a t i v e l y i n c o n s p i c u o u s t o man, and a p p a r e n t l y a l s o t o m i n k , d u r i n g t h i s t i m e . As K n u d s e n (1964a) i n d i c a t e s , a f t e r l a t e f a l l and e a r l y w i n t e r s t o r m s h a v e removed most o f t h e s e a l g a e many i n d i v i d u a l s move i n t o i n t e r t i d a l w a t e r s where t h e y may become t e m p o r a r i l y more c a r n i v o r o u s . I t i s a t t h a t t i m e t h a t mink p r e d a t i o n on t h e s p e c i e s becomes s i g n i f i c a n t i n C l a y o q u o u t Sound ( T a b l e 3 , F i g u r e 9 ) . K e l p c r a b s p r e d o m i n a t e d among c r u s t a c e a n s e a t e n by m ink t h r o u g h o u t two o f t h r e e w i n t e r s . I t i s h o t known w h e t h e r t h e r e i s a d i s t i n c t m a t i n g s e a s o n , and w h e t h e r t h e r e a r e any movement o r b e h a v i o r p a t t e r n s a s s o c i a t e d w i t h m a t i n g w h i c h w o u l d i n c r e a s e t h i s s p e c i e s ' v u l n e r a b i l i t y t o m i n k . K n u d s e n h a s f o u n d e g g -b e a r i n g f e m a l e s d u r i n g a l l months e x c e p t May , Sep tember and O c t o b e r i n P u g e t S o u n d , and I have s e e n them f r o m November t h r o u g h J u n e i n C l a y o q u o t S o u n d . I t a p p e a r s l i k e l y t h a t t h e a b o v e - m e n t i o n e d movement f o l l o w i n g s e a s o n a l h a b i t a t changes ( i . e . , t h e d i s a p p e a r a n c e and r e g r o w t h o f s t a n d s o f b rown a l g a e ) i s t h e p r i m a r y f a c t o r a f f e c t i n g t h e v u l n e r a b i l i t y o f k e l p c r a b s t o m i n k . I n t e r e s t i n g l y , use o f t h i s c r a b f a l l s o f f c o n s i d e r a b l y by M a y , even t h o u g h i t s t i l l o c c u p i e s i n t e r t i d a l w a t e r s i n numbers a t t h a t t i m e . I saw c o n c e n t r a t i o n s o f 10 t o 20 a n i m a l s i n e e l g r a s s beds and on e x p o s e d mud bo t t oms o n . s e v e r a l o c c a s i o n s as l a t e as e a r l y J u n e . F a i l u r e o f m ink t o e x p l o i t s u c h c o n c e n t r a t i o n s may be a r e s u l t o f t h e p r e f e r e n c e s w h i c h I s u g g e s t e d e a r l i e r . I n a d d i t i o n t o b e i n g h a r d t o e a t , t h e k e l p c r a b i s v e r y p u g n a c i o u s and h a r d t o h a n d l e i n c o m p a r i s o n t o o t h e r s p e c i e s ( A p p e n d i x 6 ) . The l i f e c y c l e o f t h e s l e n d e r c r a b i s b e l i e v e d t o be n e a r l y i d e n t i c a l t o t h a t o f t h e r e d c r a b (Knudsen 1 9 6 4 a ) . The f a c t t h a t use o f t he s p e c i e s by mink d i d n o t p a r a l l e l t h a t f o r r e d c r a b s was a p p a r e n t l y due m a i n l y t o 80 differences i n abundance between the two species. As indicated e a r l i e r , the slender crab was r a r e l y seen during the f i r s t two years of study, either f r e e - l i v i n g or among mink food items, but unusual abundance i n winter 1971, as indicated by unprecedented observations of numbers of l i v e crabs, was followed by a dramatic " f u n c t i o n a l response" (Holling 1959) by l o c a l mink. The exact nature of mink predation on the slender crab i s not discernable from my observations; i t i s evident that mink w i l l prey heavily upon the species when they can, but p r e d i c t i o n of time and circumstances i s not pos-s i b l e to the extent that i t i s for species discussed previously. Most of the other crab species consumed by mink are l i t t l e known, not common, and the few instances of predation recorded apparently represent sheer opportunism. The shore crabs (Hemigrapsus nudus and H. oregonensis), however, were continuously abundant under i n t e r t i d a l boulders over both study areas. Low (1971) reports that these crabs may be found i n densities of up to 500 animals per square meter i n good habitat, and good habitat pertained on many beaches i n both Barkley and Clayoquot Sounds. A large population of raccoons i n Clayoquot Sound subsisted l a r g e l y on Hemigrapsus; 75 per cent of 330 raccoon scats c o l l e c t e d there throughout 1971 were dominated by t h i s genus (Hatler, unpublished). The fact that mink did not often eat shore crabs, even though they hunted on the same beaches with the raccoons, was apparently due to t h e i r i n a b i l i t y to catch them r e g u l a r l y . Mink apparently can not overturn most boulders which shelter shore crabs, or at l e a s t they do not t r y to do so often. Prey items beneath boulders are u s u a l l y caught i n the mouth. The dprso-ventrally fl a t t e n e d crabs, which back up into the t i g h t e s t crevices, are d i f f i c u l t to grab, i n t h i s manner. Raccoons, on the other hand, remove crabs from t i g h t spots with t h e i r fore-paws, and often move smaller boulders to expose prey beneath. Most mink 81 predation upon shore crabs involved large specimens, mostly males. This was probably due to d i f f e r e n t i a l behavior between the sexes since,as Knudsen (1964a) points.out,males often forage out on the tops of i n t e r -t i d a l boulders, while females are "more timid and remain along the lower sides of rocks where they can reach a hiding place i n case of emergency". F i s h - Due to the paucity of l i f e h i s t o r y information on most f i s h species eaten by mink on my study areas, i t i s d i f f i c u l t to determine t h e i r r e l a t i v e v u l n e r a b i l i t y . C l e a r l y some f i s h such as the salmons occur i n areas accessible to mink only seasonally, during a small part of the year. Others, such as small rockfishes and greenlings, may be present s u b t i d a l l y throughout the year, but are probably available, to mink only under uncommon circumstances such as when the two a c c i d e n t a l l y meet i n t i g h t quarters underwater. I t i s doubtful that such free-swimming f i s h are hunted syste-m a t i c a l l y . Since evidence has indicated that the prickleback-gunnel group (suborder Blennioidei) and the sculpins, e s p e c i a l l y i n t e r t i d a l forms, constitute the most important food f i s h i n my areas, I w i l l l i m i t further discussion to these two groups. Abundance among i n t e r t i d a l f i s h i s most d i f f i c u l t to assess. The number of organisms one might see on a beach i s influenced by many variables including slope and exposure of the beach, tide l e v e l , amount and nature of ava i l a b l e cover, nature of substrate, d i s t r i b u t i o n of food items, and perhaps w'eather conditions, season, and time of day. To these considerations one must add some purely p r a c t i c a l ones: Blennioids are hard to count. Af t e r the f i r s t rock has been turned, the animals are moving and counts from under adjacent rocks w i l l i n e v i t a b l y include duplications. Counts by removal cannot be made without severe d i s r u p t i o n of the habitat and maiming of the 82 f i s h . Most f i s h occur under rocks only i n the lower regions of the i n t e r -t i d a l zone (lower mid-tide and low tide horizons of Ricketts and C a l v i n 1962), thus they can be counted only during a short period, usually less than an hour, and only on some days. Exact replacement of rocks i s seldom possible, thus features of the habitat of importance to hunting mink are i n e v i t a b l y a l t e r e d . For the above reasons, because I f e l t i t more p r o f i t a b l e for me to spend low t i d e periods looking for mink rather than for f i s h , and because i t was not apparent that abundance per se was the most important factor governing " a v a i l a b i l i t y " i n t h i s case, I s e t t l e d for a less objective appraisal. I often turned over boulders, i n d i f f e r e n t areas and at d i f -ferent times of year, to see roughly what was present. Appendix 11 l i s t s some r e s u l t s of these observations, and allows the following generalizations. In the habitat being discussed, i . e . , low t i d e beaches of loose rocks and boulders (which I w i l l show l a t e r appears to be preferred mink hunting h a b i t a t ) , blennioids always predominated among f i s h , and c l i n g f i s h e s occurred more commonly than did sculpins. There are other kinds of i n t e r -t i d a l habitats; i n c l a s s i c a l " t i d e p o o l " s i t u a t i o n s , i . e . , where puddles of seawater are l e f t i n depressions a f t e r the t i d e has fallen,:. sculpins were the most abundant f i s h (or at least were the most conspicuous). The mink of my observations r a r e l y hunted such habitats, however they often dove i n waters adjacent to low tide beaches of a l l kinds, and i n these s i t u a t i o n s I also saw sculpins more often than I saw blennioids. The fact that sculpins were i d e n t i f i e d i n the f e c a l samples more than twice as often as were blennioids requires some discussion. I t was my impression that sculpins, having several diagnostic bones, are more e a s i l y 83 i d e n t i f i e d from s k e l e t a l material than are the other groups I have dealt with, and i t could be argued that t h e i r apparent predominance i s r e f l e c t i v e only, of t h i s f a c t . However, the force of t h i s argument i s reduced by the fact that they did not dominate i n a l l seasons. As the data i n Table 7 show, blen-n i o i d s were i d e n t i f i e d more often i n the spring-summer period ( A p r i l through J u l y ) , and sculpins p r e v a i l e d during the r e s t of the year. My data on r e l a t i v e abundance (Appendix 11) do not permit any conclusions regarding proportional seasonal occurrence of sculpins, but suggest that for blen-n i o i d s there are changes i n numbers p a r a l l e l to the apparent changes i n occurrence i n the d i e t , i . e . , the maximum i s attained during the A p r i l - J u l y period. This i s best shown by the seasonal d i s t r i b u t i o n of my superlative abundance r a t i n g "many" (from Appendix 11) i n the table below: Vargas Island No. Observations Dec-Mar 3 Apr-Jul 4 Aug-Nov 3 t o t a l s 10 No. "Many" 0 2 0 A l l Areas No. No Observations "Many" 4 1 7 5 4 1 15 Five of seven observations at the "many" l e v e l were i n the spring-summer period, and at Vargas Island where observations were from a very few beaches and are therefore more s t r i c t l y comparable, the only "many" occurrences were i n that period. 84 Table 7. P r o p o r t i o n a l seasonal occurrence of s c u l p i n s (Cottidae) and b l e n n i o i d f i s h e s (Stichaeidae and Pholidae) i n f e c a l samples from mink, Vancouver I s l a n d , B r i t i s h Columbia. Dec - Mar Apr - J u l Aug - Nov No. 7o No. . 7o No. % S c u l p i n s 40 13.8 12 7.8 22 11.9 B l e n n i o i d s 7 2.4 16 10.4 13 7.0 Other D > 244 83.8 126 81.8 160 81.1 T o t a l (n) 291 100.0 154 100.0 185 100.0 2 X (Dec-Mar vs A p r - J u l vs Aug-Nov) = 15.15, df = 4, p<0.01 aNo. B number of occurrences; % = per cent of t o t a l occurrences ( S c u l p i n s p l u s b l e n n i o i d s plus o t h e r ) . ^Other: l a r g e l y u n i d e n t i f i e d , many probably i n c l u d i n g u n i d e n t i f i a b l e occurrences of s c u l p i n s and b l e n n i o i d s i n va r i o u s p r o p o r t i o n s . 85 In the only published account I found which mentions relative seasonal abundance of a blennioid, Gibson (1967) noted that the Atlantic species Blennius pholis showed its largest seasonal increase in July through Sep-tember, and this resulted largely from an influx of immature fish. Qasim (1957) had reported that larvae of this species have a long planktonic li f e and do not adopt the adults' demersal existence until an age of five or six months. British Columbia blennioids may follow a similar pattern; Peppar (1965) noted that the first small young (25-30 mm) of the high cocks-comb which he encountered in his study.were in June. He found this species and one or both of the Pholis species,_P. ornata and P. laeta, spawning in January and February, and the only other British Columbian species for which there is published reproductive information, the penpoint gunnel and the moss-head warbonnet (Hart 1973), are also winter spawners. According to the above authors, young-of-the-year of both the- high cockscomb and the penpoint gunnel are present in the intertidal zone at lengths of 30-50 mm in summer. I saw mink eating blennioids of this size on several occasions at both Vargas Island and Barkley Sound. From the above information, i t is likely that the app-arent summer maximum in blennioid populations, (both from boulder beach observations and from fecal analysis results) is real, and that i t results from recruitment of young age classes of one or more species. The subse-quent decline in numbers no doubt reflects the normal diminution suffered by the inexperienced and not-yet-established young of most animal species (see Errington 1946). While the above evidence suggests!that differential seasonal mink predation on blennioids is largely a function of numbers, there is l i t t l e to 86 indicate that the same is true for sculpins. Indeed, Green (1971) found that two species which he studied were much like the blennioids in showing population declines in winter and spring, followed by recruitment-related increases in summer. Possibly the proportional decrease in summer use of sculpins, by mink, was due largely to a buffering effect by temporarily more available species, especially the blennioids. Certainly predation on sculpins at a l l times of the year was out of proportion to their abundance in comparison with blennioids. As Peppar (1965) has noted, the slender compressed bodies of blennioids are able to "slide beneath and between rocks where other fish and invertebrate species would not find access". Mink probably enter these tight spots less often than they do larger cavities occupied by stouter species such as sculpins. It is also evident that blen-nioids are much more adept at escape movements than are sculpins, both in the water and out. Those who have studied behavior in intertidal fish, including both blennioids (Peppar 1965; Gibson 1967) and sculpins (Green 1971; Williams 1957) have found that individuals usually show distinct homing tendencies (to particular tidepools, rocks, etc.) and that blennioids, at least, exhibit some territoriality during at least part of the year. Activity, including feeding, occurs when water covers the intertidal area; Williams (1957) reports that at least two species of sculpins he studied, and probably others, showed vertical migrations, moving up and down the intertidal slope with the rising and falling tide. Homing, he says, "is considered a mechanism by which shallow water fishes of rocky shore areas avoid being left by the tide in unfavorable situations, such as pools that drain ... as long as its 87 home area remains a favorable low tide habitat, the fish continues to home to i t " . Green (1971) reports that storms frequently caused disruption by removal of cover in tidepools which he studied, and this resulted in local declines of tidepool sculpins. In one instance a storm dislodged two large boulders from a pool and, because these had eonsituted the only cover, the sculpin population dropped from about 20 to zero. Upon removal or alteration of cover, fish must move elsewhere or risk exposure to a variety of contin-gencies, including predation. It was evident in my observations that mink hunting on boulder beaches could not gain access to the organisms beneath most boulders. Once they had been successful at a particular rock, they were likely to return to i t repeatedly during a given hunting session. During the summer, when weather and water conditions are relatively mild, beaches are probably rather stable and a mink's opportunities to hunt intertidal fish may be mimimal. As shown above, this is offset somewhat by an apparent influx of young and perhaps especially vulnerable fishes. In winter, however, storms are frequent and at least subtle changes to beach physiognomy may occur almost daily. The relative vulnerability of fish no doubt increases at that time. This, and not changes in the relative availability of crabs,, is probably why fish appeared in the diet at its maximum occurrence during the winter months (Figure 7, Table 3). Birds - Hatler e_t al. (In Press) recorded 247 species of birds in the vicinity of the study areas. Among these, 70 species are present through-out the year and some are eaten by mink at least occasionally. These include breeding residents, such as a few species of cormorants and gulls, and other species (e.g., Surf Scoter) which do not breed locally. Migrants, 88 species which appear only briefly during spring and/or f a l l , constitute the largest single category of west coast birds with 78 species, including 32 shorebirds, having been recorded. Some of these occur in enormous numbers when present and, although l i t t l e predation on these species was recorded in this study, i t is likely that individual mink do have oppor-tunity to exploit them at times. Species which use the area primarily for overwintering number 23 and, including several of the Anatidae, were the birds most important to mink,- An additional ,36 species, mostly passerines but including a few alcids appear in summer to breed, and some occasionally f a l l prey to mink. It appears that most mink predation on birds takes place under special circumstances, involving either incapacitated individual birds or unusually exploitable concentrations. Among the birds I found in middens were a cormorant which had become entangled in fishing line, and a murre which had lost part of it's- b i l l in an old injury. A Glaucous-wihged Gull wearing and perhaps affected by a patagial disc marker was caught and killed by a mink on southern Vancouver Island (J. Ward, personal communication). Further, there is much recreational hunting for waterfowl in Clayoquot Sound; un-doubtedly some of the birds I have recorded as being eaten by mink were first killed or wounded by hunters. Studies elsewhere have also found mink feeding upon vulnerable in-dividual birds, but not taking any species at will. The indirect effects of sport hunting were implicated in the increased local success of mink predation on waterfowl in at least one area in southern Sweden (Gerell 1968), and in some Iowa marshes (Waller 1962). The latter author also 89 found mink preying upon newly hatched coot (Fulica americana) chicks when they were available in large numbers, but he observed that they were unable to catch healthy adult waterfowl (coots and ducks) regularly. Schladweiler and Tester (1972) released hand-reared Mallards in Minnesota and 21 of 56 radio-tagged birds and at least 12 of 99 without radios were subsequently killed by mink within 21 days of release. These authors concluded that their birds were especially vulnerable due to their "lack of wildness". Similarly, Sargeant et al. (1973) reported significant predation on pen-raised ducks in North Dakota, the mink selecting "recently released incubator-hatched ducklings, females in the process of incubating, and adults and juveniles on a marginal food supply" in this case. Bird concentrations most often exploited by mink are migrant flocks and breeding colonies. Waller (1972) reported heavy predation on black-birds (Icteridae) in central Iowa marshes, especially during spring migration when the birds roosted in large concentrations in low marsh cover. Gerell (1968) also found increased predation on various birds during spring migration. I have seen very large flocks of migrant shorebirds roosting on isolated islets frequented by mink, and i t was probably such a circumstance which preceded the caching of 8 shorebirds by the mink of R.W. Campbell's obser-vations (see page'^ 64). Mink predation in nesting colonies will be considered later. Mammals - Among my results, the failure of mink to prey regularly upon Peromyscus is the best example of the inefficacy;of'abundance, alone, in attracting predation. These mice were ubiquitous, occuring abundantly on most of the coast islands I visited, and everywhere on the mainland. Bait depredation by deer mice was the single most disruptive influence during livetrapping operations, and I saw mice and their sign in the low inter-90 tidal zone as well as along beach margins. As an indication of relative abundance in the area, I took almost 700 deer mice as compared to 57 shews (Sorex sp.), . 17 Microtus and 2 Norway rats in 4100 trap nights for small mammals. Voles and rats were very limited in distribution (H'atler 1972) but both were eaten by mink more often than were deer mice. Published records from elsewhere, e.g. Sweden (Gerell 1968), Iowa (Waller 1962), New York (Hamilton 1936, 1940), Michigan (Sealander 1943), Pennsylvania (Guilday 1949) and Alaska (Harbo 1958; Burns 1964) indicate that the small mammals eaten most often by mink are microtines. This appears to be true also for other mouse-hunting carnivores including red foxes (Scott 1943), weasels (Hall 1951), marten (Lensink e_t al. 1955), and feral house cats (Pearson 1964), and probably reflects the relative ease with which these animals may be caught. Voles are rather slow, and are largely restricted in their movements to runway systems in which they may be easily intercepted. Deer mice, on the other hand, are unpredictable in the direction of their movements and can jump and climb with great, agility. Mink are not,known to catch them regularly anywhere. Availability of Prey: Conclusion Among the mink food studies cited earlier, those of Gerell (1967b, 1968) and Waller (1962), particularly, illustrate the tendency for diets to vary between habitats and to change with the seasons within a given habitat. These authors correlate differences between areas and seasons with apparent dif-ferences in prey availability. I have shown that availability-related differ-ences (size, abundance, vulnerability) in food habits also occurred both seasonally and locally in my areas. I have given less consideration to 91 micro-habitat differences, but in one case three male mink which lived on closely adjacent islets, but which hunted different kinds of shoreline, were found to be eating different foods (Appendix 12). Of special interest in consideration of prey availability is a comparison of the diet of my Vancouver Island mink with that of mink on coastal islands off western Sweden, as given by Gerell (1968). Mink have apparently inhabited the coast shores of western Canada and southern Alaska throughout recorded history, presumably having arrived there sometime after the retreat of the last glaciation. The Swedish coast mink, on the other hand, have descended from farm-raised animals originating primarily, i f not solely, in the eastern and northern parts of North America, and have been present in Sweden for only 25 to 35 years (Gerell 1967a). It is doubtful that there was any history of prior existence near salt water in these animals, yet they have settled into a diet dominated by intertidal fishes (especially sculpins), crabs, sea slaters (Ligia oceanica) and seabirds. The slater is the Atlantic congener of the isopod commonly eaten by mink in my study areas. The above results provide good evidence of the adaptive plasticity of the mink,;anddindicate-ithat certain kinds of animals, e.g., slow bottom fishes, crustaceans, and colonial birds (animals which occur almost everywhere) have characteristics which naturally expose them to a hunting mink. Effects of Predation In his classic treatment of the subject, Errington (1946) concluded that most predation among vertebrates had l i t t l e depressive influence on prey populations. General concurrence with this view is evident in most recent major work with carnivores, especially the large ones such as wolves (Mech 1970), mountain lions (Hornocker 1970), spotted hyenas (Kruuk 1972) and lions (Schaller 1972). Smaller carnivores prey largely on species whose population levels are difficult to assess, and there is less' convincing evidence available either for or against their influencing ; numbers by predation. -Errington (1954) showed the degree to which mink responded to increased vulnerability of muskrats, and he recounted similar observations (Errington 1946) for other rodents and several small gallinaceous birds and their predators. However, Pearson (1971) has reported that small carnivores reduced populations of microtines on a California study area during two cyclic declines. The elongate shape of Mustela sp. is regarded as an adaptation to enable entry into confined spaces, including the burrows of prey (see Brown and Lasiewski 1972), and these animals are generally very efficient predators. Maher (1967) reported very heavy, apparently almost annihilative predation by weasels (M. erminea) on a population of arctic lemmings (Dicrostonyx and Lemmus), and Schnell (1964) related an incident in which a single mink exterminated a population of about 40 cotton rats (Sigmodon hispidus) which had been introduced to a small island. Effects on Marine Organisms My own data are deficient in that I could not adequately census the many prey species taken. Subjectively, i t appears that the organisms which seek cover under boulders, including most of the fishes eaten, are safe from intensive predation pressure by mink and, except for crabs, other animals are taken so incidentally that population consequences would be unlikely. However, I am suspicious that mink may occasionally make significant inroads 93 into crab populations, e s p e c i a l l y l o c a l l y . For instance, one mink was known to have caught a minimum of 276 kelp crabs i n less than two winter months, and a l l of these came from waters surrounding an i s l e t scarcely 75 m i n diameter. Observations of i n d i v i d u a l mink catching four or more crabs during a single low t i d e were not uncommon, and many animals do a l l of t h e i r hunting along a r e l a t i v e l y short span of shoreline. Unfortunately, the degree to which crabs i n deeper waters are available to replace those removed from the i n t e r t i d a l zone, and the extent of recruitment from other sources are unknown. Much must be learned about the dynamics of crab popu-la t i o n s before the e f f e c t s of mink predation can be ascertained. E f f e c t s on Seabird Colonies , Guiguet (1971) speculated, probably c o r r e c t l y , that seabirds along the B r i t i s h Columbia coast are l i m i t e d to nesting on islets' which are not i n -habited by mink. According to C a r l et a l . (1951), an unauthorized i n t r o -duction of mink to Lanz Island o f f the northwest end of Vancouver Island i n the l a t e 1930's apparently resulted i n elimination of nesting seabirds there, i n a period of less than 12 years. Damage to coastal b i r d colonies has also been reported i n Sweden ( c i t a t i o n s i n G e r e l l 1968) and Norway (Wildhagen 1956). There are three small vegetated islands i n the v i c i n i t y of my study areas which support numbers of nesting seabirds. Burrow-nesting species, which one would think would be more vulnerable to predation by mink than those which nest on c l i f f s or out i n the open i n colonies, are common on two (Cleland Island west of Vargas Island and Seabird Rocks just south of Barkley Sound) and occur i n small numbers on the t h i r d ( F l o r e n c i a Island 94 at the south end of Long Beach). Glaucous-winged G u l l s nest i n numbers on a l l three. I have v i s i t e d each of these islands at least twice and, although I have searched c a r e f u l l y , I have never seen a sign of mink at any. Conversely I have examined a number of vegetated i s l e t s , including dozens i n Barkley Sound, which support mink but not seabird colonies. I t i s n o t c e r t a i n that birds would use a l l , or any, of these i f mink were not present, but on those few Barkley Sound islands where both nesting birds and mink do occur, the birds nest i n locations l a r g e l y i n accessible to mink (cormorants on c l i f f s and i n sea caves, g u l l s i n trees and on rock pinnacles at some distance from cover) or at low d e n s i t i e s (Black Oystercatchers on several coast islands and head-lands) . I t seems evident that a mink can not locate permanently on an i s l a n d lacking s o i l and vegetation or at least a hollow log i n which i t can make i t s den. There are several such i s l e t s along the west coast of Vancouver Island, and many harbour nesting birds such as g u l l s , cormorants, and oystercatchers i n summer, but l i e barren i n winter. These species are apparently more secure, therefore, than are the burrow nesting species which have some habitat requirements ( s o i l , vegetation) s i m i l a r to those of mink. The three islands mentioned e a r l i e r as supporting burrowing species are each two kilometers or more from the nearest mink-inhabited land mass, and t h i s distance appears to be s u f f i c i e n t to prevent natural establishejmht of mink populations. Both instances of predation on nesting seabirds recounted e a r l i e r , that on White I s l e t o f f Long Beach and that observed by Campbell on Chain I s l e t s near V i c t o r i a (see page 64 ), involved temporary v i s i t s by a single mink to 95 the unvegetated rocky type habitat described above. The long term damage which mink can do under such circumstances remains uncertain. In the case at White I s l e t , the midden and l a t r i n e were located i n the only cover a v a i l a b l e on the e n t i r e i s l e t , a hollow under a boulder, and t h i s was an obvious r a i n t r a p . The mink stayed only a few days, having done l i t t l e damage to the approximately 90 nesting p a i r s of birds (3 species) there. At Chain I s l e t s the mink found a dry nest s i t e under a p i l e of beach debris and was able to stay for several months. However, even i n that s i t u a t i o n , i t did not appear that production of young birds was s i g n i f i c a n t l y altered i n comparison with that on adjacent i s l e t s where no mink occurred (Campbell, personal communication). Again, residence was only temporary as a l l d r i f t logs and debris washed away i n winter storms. According to Campbell, there was no sign of the mink, or s u i t a b l e denning s i t e s , on the i s l e t when he r e v i s i t e d i t at the end of December. P o s s i b l y i f a mink could exert pressure on the birds of a s i n g l e i s l e t for more than one season, i t might have greater e f f e c t . On the whole, i t appears that free-ranging mink i n areas where birds are sparsely d i s t r i b u t e d do not r e g u l a r l y prey upon birds; where concentrations of nesting birds e x i s t , resident mink may be quite destructive, but those which cannot e s t a b l i s h permanent residence probably do not s e r i o u s l y impair an i s l e t ' s capacity to support nesting b i r d s . SUMMARY, FOOD HABITS Mink on the west coast of Vancouver Island r e g u l a r l y eat at l e a s t s i x species of crustaceans and twelve species of f i s h , and have been known to eat dozens of other animals including invertebrates, f i s h , birds and mammals at l e a s t o c casionally. Most of the animals eaten r e g u l a r l y , and some of 96 those which have been detected i n food habits material only now and then, must be classed as generally abundant i n or near l i t t o r a l waters along a l l of my study areas. Several animals such as sea cucumbers, sandworms (Polychaeta), ghost shrimp and pile-seaperch are common to abundant near shore, but were not known to have been eaten by mink. In terms of biomass present, i t i s apparent that food l i m i t a t i o n per.se . should not be a factor i n the l i v e s of coast mink. However, not a l l p o t e n t i a l foods are r e g u l a r l y a v a i l a b l e and even the commonly eaten foods vary i n a c c e s s i b i l i t y at d i f f e r e n t times and i n d i f f e r e n t habitats. Crabs are taken most often i n summer when they move into i n t e r t i d a l waters for mating and moulting. One species (the kelp crab) appears i n near-shore waters i n winter, a f t e r storms have removed the kelp beds which constitute i t s summer refuge, and i t i s preyed upon most often i n that season. I n t e r t i d a l f i s h occur least i n the die t i n summer, when t h e i r boulder cover i s most stable, and when other foods, e s p e c i a l l y crabs, are more a v a i l a b l e . Warm-blooded vertebrates are r a r e l y taken except when sp e c i a l circumstances permit; i n the case of b i r d s , the circumstances which appeared to be most i n f l u e n t i a l on my study areas, severe weather and the human hunting season,led!toJheaviest use i n f a l l and winter, although some e x p l o i t a t i o n of migrant concentrations i n spring i s suspected. Invertebrates other than crabs appear to be taken l a r g e l y when other food i s not immediately a v a i l a b l e , again less often i n summer than at other times. 97 FOOD GETTING INTRODUCTION AND METHODS Small mustelids are l a r g e l y nocturnal i n most areas, and observations of them i n the process of seeking food have been few. In her recent review, Ewer (1973) c i t e d a few published records from the f i e l d which indicated that tracking by scent and response to sounds are important weasel (Mustela sp.) hunting methods. She also concluded, as did Hewson and Healing (1971), that the most common k i l l i n g technique for these aminals i s a b i t e to the upper neck or head. Burns (1964) recorded s i x observations of mink catching small f i s h by div i n g from the i c y shores of sloughs i n the Yukon-Kuskokwim Delta, Alaska, and Errington (1967: 20-29) summarized h i s observations of mink i n contact with several prey species i n Iowa,marshes. There are few other accounts of hunting mink, and those I am aware of involve si n g l e anecdotal observations such as that reported i n Sargeant e_t _al. (1973) i n which a mink jumped from a riverbank upon a swimming brood of wild ducklings and k i l l e d or c r i p p l e d at least three of them. As indicated e a r l i e r , my own opportunities for watching active wild mink have been unprecedented. A basic d e s c r i p t i o n of observation techniques has been given i n the food habits section. During the period May 1968 through August 1972, I saw mink hunting 432 times. For a v a r i e t y of reasons, such as interference caused by my; presence and poor v i s i b i l i t y due to adverse weather or the nature of the hunting habitat, 278 (647o) of these observations were short and provided only minimal data (date, l o c a t i o n , time, tide l e v e l , habitat, s i z e , color, and sometimes sex of mink, and occasion-a l l y the hunting method used). An add i t i o n a l 56 observations (137») lasted only one minute or l e s s , being cut short i n most cases by the observed mink's 98 catching something, carrying i t away and not returning. Observations of 2-5 minutes duration numbered 46 (117o), those of 6-10 minutes and 11-30 minutes each occurred 17 times (47o' each), and observations of more than 30 minutes duration (up to a maximum of 80 minutes) numbered 18 (47o) . With-out exception, the longer observations included periods during which the mink had taken items away from shore to cache or eat them, and was out of sight. As long as recognizable mink kept returning to the same area to hunt, i t s a c t i v i t i e s were considered part of a single observation. In a l l cases I attempted to p a r t i t i o n the t o t a l observation time into time spent hunting, time spent eating, and time spent engaged i n other associated a c t i v i t i e s . In addition to the basic data l i s t e d e a r l i e r for observations of one minute or l e s s , the long observations provided information on methods of hunting, hunting success and foods taken i n r e l a t i o n to various habitat conditions. HUNTING HABITATS A l l hunting observations occurred on shores below the highest high water l i n e . Mink probably hunt t e r r e s t r i a l prey above t h i s l i n e at l e a s t occasion-a l l y , but I d i d not see them doing so. For the purposes of the observations, shoreline habitats were categorized into f i v e types, described i n following paragraphs. When a mink hunted jin water, near shore, i t s hunting habitat wasereeordedias'.;the shore habitat from which i t was d i v i n g . Small P a r t i c l e Beach Shores i n t h i s category are r e l a t i v e l y homogenous expanses of sand, s h e l l , or gravel, t y p i c a l l y exposed to heavy wave action and providing l i t t l e cover for near-shore marine organisms. Some small p a r t i c l e beaches consist of compacted, f i n e r p a r t i c l e s approaching s i l t s i z e , and i n Barkley Sound a number of beaches so categorized are composed l a r g e l y of wave-rounded rocks averaging up to 20 cm i n diameter. An example of a small p a r t i c l e beach i s portrayed, from a distance, i n Plate l a . 99 Boulder Beach This type i s composed p r i m a r i l y of loose rocks of various shapes and a l l s i z e s , but incl u d i n g many with diameters greater than 20 cm. The es s e n t i a l feature of these beaches i s that they provide cover for a v a r i e t y of i n t e r t i d a l organisms eaten by mink. In some cases, I examined beaches co n s i s t i n g almost e n t i r e l y of small rocks, averaging less than 20 cm i n diameter but i r r e g u l a r l y shaped and p i l e d upon each other i n such a way that chambers and c a v i t i e s suitable as refuge for small f i s h and crabs occur-red i n "honeycomb" fashion throughout. Boulder beaches, examples of which are shown i n Plates 2 and 3, t y p i c a l l y l i e i n r e l a t i v e l y sheltered waters on the lee sides of islands and ree f s , and along inland waterways. The boulders on these beaches may themselves l i e on substrates ranging from small p a r t i c l e s ( i n which case cover habitat for marine organisms i s limited) to bare rock. Rockweed Shore Much of the exposed outer coast and many headlands i n protected waters consist of bare, s o l i d rock. Below tide l i n e , cracks and crevices i n t h i s rock and various attached marine algae, e s p e c i a l l y rockweeds (Fucus sp.), provide hiding places for several of the prey species sought by mink (see Plate 2). Eelgrass F l a t s Exposed t i d a l f l a t s (Plate 3) and small mud-bottom bays bearing a growth of eelgrass (Zostera sp.) and other marine plants such as Ulva f i t i n t h i s category. They d i f f e r from other small p a r t i c l e shores i n that they usually occur i n protected waters, t h i s fact probably accounting for t h e i r a b i l i t y to support a plant cover. face page 100 P l a t e 2: Pelage; Hunting h a b i t a t s a) A moulting female mink, showing unusually l i g h t pelage being replaced by a new,darker summer coat. b,c) Mink hunting by "poking" on a t y p i c a l rockweed shore. d) Mink hunting i n a boulder beach h a b i t a t . face page 101 Plate 3: Hunting habitats a) Mink hunting on an eelgrass f l a t s b) Mink on a mixed eelgrass f l a t s - b o u l d e r beach. c) A t y p i c a l boulder beach. 102 Estuary Shores along the lower reaches of coastal streams, s p e c i f i c a l l y those portions exposed to marine t i d a l action, constitute t h i s beach category. Such shores are usually composed of small p a r t i c l e bottoms, usually sand or mud but o c c a s i o n a l l y gravel and larger stones, and most provide r e l a t i v e l y l i t t l e cover for i n t e r t i d a l animals. RESULTS AND DISCUSSION HUNTING METHODS Mink hunted by three main techniques, which I termed "bird-dogging, poking, and diving", but they also employed •-. v a r i a t i o n s of each and com-binations of one or more. As shown i n Table 8, the method used appears to depend mostly upon habitat, and a single mink w i l l employ several methods in one hunting session, changing as i t moves from one habitat type to the next. Following are descriptions of the circumstances and mechanics of methods used by mink to f i n d food, together with accounts of what prey animals were usually found i n these ways and how they were caught once having been found. Bird-dogging - This technique was used most often on eelgrass f l a t s , but I also saw i t on small p a r t i c l e beaches, e s p e c i a l l y along d r i f t debris l i n e s , and along expanses of homogeneous, sparsely-vegetated rockweed shore (Table 8). The c h i e f c h a r a c t e r i s t i c of t h i s techniques, as my name for i t implies, i s that the mink holds i t s nose to the ground, s n i f f i n g , as i t moves along. I t u s u a l l y maintains a semi-crouched posture, sometimes moving s u r p r i s i n g l y quickly i n t h i s way, but I also saw mink bird-dogging from a more upright p o s i t i o n . While bird-dogging, a mink may move forward i n a r e l a t i v e l y s t r a i g h t l i n e or may change d i r e c t i o n frequently, moving either i n a zig-zag or back-and fo r t h pattern; e i t h e r way, i t moves i t s head from side Table 8. Hunting methods used by mink in various littoral habitats, Vancouver Island, British Columbia, 1968-1972. Hunting Methods Bird-dogging Poking Diving More Than One Method3 No. % No. % No. % No. 7o Boulder Beach (n=106:86)b 3 2.8 72 67.9 31 29. 2 20 23.2 Poking + Diving = 18 Bird-dogging + Diving = Poking + Bird-dogging = 1 1 Rockweed Shore (ii=61; 5 3) 2 3.2 22 36.1 37 60. 1 8 15.1 Poking + Diving = 8 Eelgrass Flats (ri=61;43) 36 59.0 5 8.2 20 32. 8 18 42.9 Bird-dogging + Diving = Bird-dogging + Poking = Diving + Poking = 1 All three methods = 1 15 1 Small Particle Beach and Estuary (n=8; 6) 1 12.5 3 37.5 4 50. 0 2 33.3 Poking + Bird-dogging = Poking + Diving = 1 1 X* (Three hunting methods vs Boulder Beach + Rockweed Shore + all others) = 112.23, df = 4, p<0.001 Observations in which more than one hunting method was employed; the combinations noted for each habitat are listed. ^The two figures given for "n" are as follows: The first is the total number of records for the three hunting methods, including those from under "more than one method". The second is the total considering each entry under "more than one method" as a single observation, and is the total from which the per-centage under the category is calculated. 104 to side so that the area covered i n t e n s i v e l y i s wider than i t s body, and i t may slow down or stop frequently to extend i t s head i n some d i r e c t i o n , apparently to i n v e s t i g a t e ; ^ concentration' of-odor. Commonly i t i s moving quickly enough to over-run p o t e n t i a l food items, and on several occasions I saw bird-dogging mink stop short, then run back a few steps to relocate and capture detected prey. Crabs, e s p e c i a l l y Cancer sp. from spring through f a l l , are the prey animals found most often by mink on eelgrass f l a t s (Appendix 13). These are o c c a s i o n a l l y found l y i n g on top of the eelgrass, but are usually be-neath and have often- dug themselves into the mud so that only a portion of the carapace shows from above. Having found a crab, a mink follows roughly the following procedure to subdue i t . I f the crab i s covered with eelgrass (or other marine vegetation)j t h i s i s removed by a digging motion of the f o r e f e e t . On a few occasions I saw mink a s s i s t the digging by grasping vegetation with the mouth and p u l l i n g i t aside. Once vegetation has been cleared away, the mink attempts to get into a p o s i t i o n from which i t can f l i p the crab onto i t s back. I f the crab has buried i t s e l f , more digging with the forefeet i s required. The mink usually digs at several locati o n s around the perimeter of the crab u n t i l the crab commits i t s e l f by taking a defensive posture, r a i s i n g up on i t s walking legs and waving i t s chelae; at t h i s time the mink moves behind i t . The movement which f l i p s the crab over i s quick, and may be accomplished i n more than one way. In a l l cases I observed, crabs were grabbed i n the mouth and turned with a f l i p of the head, but on a few occasions i t appeared that the forefeet were used to give an a s s i s t i n g push. The exact manner i n which the crab i s grabbed may vary. Sometimes i t appeared that the mink b i t the carapace i n 105 straightforward manner and then tossed i t s head to the side, but at other times the mink turned i t s head over before b i t i n g the crab, so that i t s upper canines contacted the crab's ventrum and i t s lowers were on the crab's upper surface. In t h i s way i t could e f f e c t the desired twist of i t s prey by returning i t s head to normal p o s i t i o n . That more power may be applied by t h i s technique was suggested by an incident i n which a male mink, f a i l i n g three times i n succession to l i f t a red crab from a hole by the f i r s t method, employed the t w i s t - f i r s t v a r i a t i o n and not only wrenched the crab from the hole but also threw i t nearly h a l f a meter i n the process. On two occasions I saw mink catch small f i s h which had been stranded on eelgrass f l a t s by the f a l l i n g t i d e . The f i r s t time a male mink b i r d -dogging across a Vargas Island bay suddenly whirled, made three s t i f f - l e g g e d bounds back to one side, struck quickly and came up with a mouthful of eelgrass i n which was a struggling blennioid. I t was apparent that the mink had been attracted by sound, probably caused by the f i s h ' s wriggling. Another time a mink rushed forward and caught an u n i d e n t i f i e d f i s h i n s i m i l a r fashion. Again, i t was probably attracted by sound, although i t may have responded, by sight, to a movement i n the eelgrass. As indicated e a r l i e r , most other items appeared to have been detected by odor. Whether mink were usually following l i n e s of scent on the ground or gradients i n the a i r i s unknown, but they are capable of both. I saw mink following the t r a i l s of other mink which had passed several minutes e a r l i e r ( e s p e c i a l l y males on the t r a i l s of females during the breeding season), and have also seen mink respond to emanations from a one-spot source from some distance. Poking - On boulder beaches and on some rockwee'd shores there are numbers of cracks, crevices and natural c a v i t i e s into which hunting mink 106 thrust t h e i r heads. This "poking" was "the land hunting technique I saw most often (Table 8), because rockweed and boulder beach habitats were more common on my study areas than were the other kinds. In a sense, poking i s a v a r i a t i o n of bird-dogging i n that the nose i s being pushed out ahead and appears to be doing much of the business of prey detection. However, i t i s u n l i k e l y that there are many c a v i t i e s i n the i n t e r t i d a l zone, e s p e c i a l l y under boulders, which do not harbor odors tempting to a mink, and I suspect that once the animal has i t s head i n close, sounds and t a c t i l e stimulation are more important i n informing i t i f anything i s immediately a v a i l a b l e . F i s h and small, u n i d e n t i f i e d animals probably including isopods, are the prey taken most often by poking mink, although I have seen crabs caught on several.occasions i n t h i s way (Appendix 13). Crabs are usually dug out i n the same way as described for eelgrass f l a t s encounters. Other items are simply caught by the teeth and p u l l e d from t h e i r h i d ing places. I f the substrate allows, a^mink w i l l dig at the base of a boulder to gain access, and w i l l undertake considerable contortions in attempting to squeeze into t i g h t places a f t e r prey. I once came upon a mink which was l y i n g on i t s back, with the front two-thirds of i t s body under a boulder, i t s t a i l lashing back and f o r t h , and i t s hind legs f l a i l i n g the a i r w i l d l y . Occasionally b i t s of small rock flew from the opening, i n d i c a t i n g that i t s front legs were also "digging". I t caught nothing there while I watched, although I could hear f i s h wriggling and splashing and other organisms scurrying about beneath the rock. These sounds had probably contributed to the mink's obvious'--excitement. 107 Occasionally small rocks are moved when mink wedge their bodies . beneath or between them, and some small rocks are moved as a result of digging activity, but mink do not attempt to turn boulders over to get organisms beneath. In my experience, boulders of 25 cm or more diameter were never moved. It was evident that most of these provide ample refuge for the animals beneath; i f a mink catches something at one boulder, i t usually continues to try there for several minutes more,, and even after leaving i t to try other boulders, i t will likely return several times to one at which i t has had success during a hunting session. A variation of poking often;:occurs on sections of tide-exposed shore on which there is a heavy cover of marine vegetation, especially large red and brown algae. This involves the mink's "burrowing" into the vegetation and hunting among the various plant layers, or on the ground and among boulders beneath. Burrowing mink are out of sight most of the time and it is virtually impossible to determine their success. Diving - Mink were seen successfully diving for prey, especially crabs, in waters adjacent to a l l five habitat types.. Most dives were in water 2-3 m deep or less, but I observed a few deeper dives. The deepest recorded was on 3 June 1969 when a known adult male, swimming between two islets near Vargas Island, suddenly dove and surfaced about 30 seconds later with a helmet crab. The mean of four depth soundings taken in this area minutes later was .- 7.4 m (range = 6.9 - 7.8 m). The frequency of diving may be influenced by a number of factors, of which some are listed in Appendix 14. To obtain intertidal food, dives are especially necessary at higher tide levels. However, mink dove somewhat more frequently at low tide, probably 108 because most serious hunting is done then. There was also, a tendency for large (mostly male) mink to dive more frequently than small (mostly female) animals, but diving frequency varied l i t t l e between habitats and between seasons. The duration of dives depends upon a number of factors, of which three yielding statistical differences in Appendix 14 are tide level, seasons, and size of mink. Not surprisingly, dives lasted longer at high tide levels than at low, and were shorter during the summer months when prey animals, such as crabs, are abundant near shore and weather conditions are mild. Small mink made longer dives than did large ones. It may be that small animals have greater difficulty handling prey underwater, or that they are forced by competition with the larger ones to hunt in less productive areas. There were also slight differences in dive duration between habitats, with the shortest dives occurring on low profile beaches such as on eelgrass flats and sand beaches and the longest ones occurring in steeper areas such as along rockweed shores. As with tide levels, this result is probably a simple function of water depth. Other factors such as the number of prey present, whether or not one is caught or at least chased, "hunger", and perhaps condition of the mink, may influence the duration of any given dive, and common descriptive statistics (mean, range) cannot provide a complete picture of diving. Figure 12, a frequency diagram of durations for 264 dives I timed over a wide range of conditions, shows that most last between 6 and 20 seconds. Dives of less than five seconds usually involved attempts to get beyond thick, shore-fringing marine vegetation, although they occasionally culminated a series of face page 109 Figure 12. Duration of 264 underwater dives by hunting mink, Vancouver Island, B r i t i s h Columbia. The shaded area of each histogram represents the proportion of dives i n that category which res u l t e d i n prey captures. c 0) 50 u 40 30 u c 0 20 cr 10 o VO 0 -5 6-10 11-15 16-20 21-25 26-30 31-35 36-4041-4546-50 Dive Duration (seconds) 110 dives which had apparently chased prey up into shallow water near shore; nearly one-fourth of these short dives were successful. The longest dive recorded, 48 seconds,,was made in relatively shallow water (about 3m) and it failed to yield a prey. Of the 264 timed dives, just 44 (16.77«) were successful in the sense that prey was brought to the surface. As shown in Figure 12, the highest success rates occurred at some of the longest dive durations, especially between 26 and 35 seconds. This is probably because mink are reluctant to leave prey when capture seems imminent and extend themselves a bit longer. The mechanics of dives vary mostly with local water conditions. Most commonly, a mink enters the water, swims a short distance from shore, then surface dives, but underwater entry from wharfs, rockweed shores and similar locations in which deep water prevails are usually made directly. As a rule, mink do not, and probably cannot, hunt in heavy surf but I have seen animals enter water through low breakers (20-30 cm high) and moderate tidal surges. In such cases, the animal simply waits on shore, ducks under the oncoming wave and kicks out behind i t , often using the ebb to help its forward progress. Mink usually return to shore or at least to floating vegetation between dives, and may rest there for a few seconds to several minutes. Frequently a series of dives will be interspersed between bouts of bird-dogging or poking on shores. Most changes in dive locations are effected by movements on land along the shore rather than by swimming parallel to iti, Sometimes, apparently most often when prey has been spotted, a mink will dive repeatedly without returning to shore. Commonly in such cases the breathing time between dives will be very short, often only one or two seconds. I l l On no occasion was I able to see d i r e c t l y what was happening underwater. From f o l l o w i n g the paths of a i r bubbles r e l e a s e d by submerged mink, I have concluded that they hunt underwater much as they do above. When boulders are present beneath the su r f a c e , the bubble path o f t e n f o l l o w s a zi g - z a g p a t t e r n , suggesting that the mink i s moving from boulder to boulder and probably poking between and under them. Bubble paths on small p a r t i c l e bottoms are u s u a l l y s t r a i g h t e r and o f t e n f o l l o w v e g e t a t i o n l i n e s . Commercial fisherman D. Arnet (personal communication) watched a mink catch a red crab i n c l e a r water beneath a p i e r on the Tofino Waterfront. The mink dove, apparently swam d i r e c t l y to the crab, which was moving slowly across the mud bottom, and when the crab threw up i t s chelae i n defensive posture, the mink q u i c k l y swam around behind and p i c k e d i t up. The crab was c a r r i e d u p r i g h t . I t has also been by observation that crabs brought from the water are c a r r i e d up-r i g h t , but i t i s usual f o r the mink to l a y a crab down on i t s back a f t e r emerging w i t h i t from the water, and i f the prey i s to be transported f u r t h e r i t w i l l l i k e l y be c a r r i e d upside-down from that p o i n t . I t i s probably more d i f f i c u l t to fli>p a crab underwater, and may even be dangerous. I f i t gained a secure g r i p w i t h i t s chelae and wedged i t s body among boulders, a large crab could probably drown a mink. Arnet's o b s e r v a t i o n (above) suggests that mink hunt underwater c h i e f l y by s i g h t , and the f a c t that they o c c a s i o n a l l y b r i n g dead crabs and moults to shore i n d i c a t e s that response i s to shape r a t h e r than j u s t to movement. The a b i l i t y of mink to d i s c r i m i n a t e between o b j e c t s has been documented (Doty e_t _al. 1967), although t h e i r a c u i t y underwater i s poor i n comparison w i t h many other aquatic and semi-aquatic mammals ( S i n c l a i r et a l . , 1974)., I t i s 112 possible that some underwater prey detection i s also accomplished with the t a c t i l e sense, although I have no evidence that t h i s i s so, and would again suggest that t h i s could be dangerous where large crabs are present. A v a r i a t i o n of diving which I c a l l e d "herding" i s the technique which mink appear to use most often for catching f i s h i n water. I have seen i t i n use p r i m a r i l y at water's edge on eelgrass f l a t s . T y p i c a l l y the mink swims p a r a l l e l to the beach, ei t h e r underwater or above and about one meter o f f -shore, and chases f i s h toward shore. In t h e i r haste to escape, some f i s h beach themselves and are caught there before they can return to the water, or become confused and swim d i r e c t l y back into the jaws of the pursuing mink. I saw several mink catch u n i d e n t i f i e d blennioids and staghorn sculpins i n t h i s way, and one male which caught three f i s h i n about 10 minutes of hunting, l e f t l i t t l e room for doubt that he was applying the technique systematically and riot a c c i d e n t a l l y . On a few occasions I saw mink catch small objects, apparently f i s h , from standing pools of water and at these times they also appeared to use t h i s dive-and-corner technique. Judging from the observations of Errington (1967:27), f i s h hunting procedures are much the same i n fresh water. He wrote, "... along a stream the minks f i n d minnows or larger fishes i n r i f f l e pools, eddies, pools below mouths of t i l e s , pools under d r i f t e d debris, or i n watery c a v i t i e s i n banks or up under tree roots. I have seen minks making short dives and coming up with fishes now and then but they get many of t h e i r f i s h e s i n land-locked puddles or corner them somewhere, as i n c r a y f i s h burrows or under driftwood". HANDLING PREY Once prey i s caught i t may be handled i n one of two main ways: 1) I t may be eaten. 2) I t may be cached. A f t e r prey had been c a r r i e d out of 113 Table 9. Handling of prey by mink, Vancouver Island, British Columbia, 1968-1972. A. All observations B. Observations of two minutes or longer duration. A. Immediate Fate of Crabs Fish Crabs and fish Unident All Prey No. No. 7o No. 7o No. 7» No. % Eaten a b 29 Cached 66 30.5 69.5 24 49.0 25 51.0 53 36.8 91 63.2 52 80.0 13 20.0 105 50.2 104 49.8 2 X (crabs vs fish) = 4.8, df = 1, p <0.05 B. Immediate Prey Fate Order in which Prey Caught First or Second Third or Later No. % No. z x 2 df p c Eaten Crabs Cached 25 29 46.3 53.7 4 18 18.2 81.8 5.3 1 <0.05 Eaten Fish Cached 10 9 52.6 47.4 7 '9 43.8 56.2 0.3 1 cO.90 Eaten Both Cached 35 38 47.9 52.1 11 27 28.9 71.1 3.7 1 <0.10 a Eaten: newly caught prey partly or wholly consumed within view of the observer. ^Cached: prey temporarily hidden within view of the observer, or carried away to locations above the tidal high water mark. 2 Probability of a X so large under the hypothesis that there are no differences in handling of prey in relation to the order in which they were caught. 114 sight, i t was us u a l l y not possible for me to determine which of these two al t e r n a t i v e s a p a r t i c u l a r mink was following. Thus, for the purposes of t h i s discussion, "eating" r e f e r s to those occasions on which the mink ate the prey either immediately upon catching i t or r e l a t i v e l y near the hunting spot, but at le a s t remained within view of the observer. "Caching"j.on the other hand, r e f e r s to those instances i n which the prey was eit h e r temp-o r a r i l y l a i d aside nearby and r e t r i e v e d l a t e r , or was c a r r i e d immediately to some point above the t i d a l high water mark. In at least some of these cases the mink probably ate a l l or part of the prey before returning to hunt. I f one considers a l l prey captures which I saw (Table 9A), the frequency of eating was very nearly the same as that for caching, but t h i s i s due l a r g e l y to the r e s u l t s for u n i d e n t i f i e d prey items, which were eaten four times as often as they were cached. In f a c t , the reason why most of these items were u n i d e n t i f i e d i s that they were small enough to be taken completely into the mouth, chewed and swallowed on the spot. Most were probably small f i s h and invertebrates such as isopods, although a few may have been small crabs such as hemigrapsids. Among i d e n t i f i e d prey (crabs or f i s h ) , crabs were cached more than twice as often as they were eaten, while f i s h were eaten and cached i n about equal frequencies. The above information suggests that i d e n t i t y and size of the prey are important factors bearing on how i t w i l l be treated immediately upon being caught, but i t i s l i k e l y that there are also other factors involved. As shown i n Table 9B, crabs which were the f i r s t or second items caught by a mink i n a given observation were eaten s u b s t a n t i a l l y more often than were 115 crabs caught later. It would appear that relative "hunger" is the operative factor here, although the data show that fish were as likely to be eaten as cached, regardless of the order in which they were caught. As the fol-lowing figures show, crabs take an average of almost six times longer to eat than fish do, and this is probably the main reason for the differences in the way the two prey groups are handled: Time ( seconds) tt'aken to eat prey: Mean Range n_ crabs 358 60-1080 25 fish 57 15-210 13 As I will show later, I cannot demonstrate that caching takes any less time than eating, but i t does reduce the amount of time which a mink must spend in the open. This is less important on most boulder beaches, where a mink may take cover beneath a rock or under kelp,and eat at leisure. In other places, especially along sand beaches and on eelgrass flats, a mink feeding in the open inevitably attracts attention among scavengers such as crows and gulls, and this may, in turn, attract potential pirates and predators. I saw evidence of mink losing prey to eagles on two occasions. In addition to providing at least some security against predation and piratism, caching enables the capture of several prey items when they are most easily available, for consumption later when hunting conditions are less favourable (e.g., high tide). It also provides an animal with occasional opportunities to patrol its midden areas during its hunting session. On several occasions I saw mink run toward their dens with prey and knew that {-.'they encountered other mink (either on the way or when they arrived) by the sounds which followed. In at 116 least two cases I was able to determine that the observed mink had taken the offensive in such encounters. There are also some advantages to eating a prey item immediately; this requires less energy than transporting i t , enables the mink to maintain guard of its hunting area, and ensures that the item in question will not be eaten by anything else. Eating Eating is usually done with the mink in a lying down posture, with the forefeet assisting in holding food, although very small items may be eaten from any position. I saw one mink clinging to a sheer rock face below the tideline and repeatedly plucking small items from a crevice below i t . These were al l chewed and swallowed with the mink remaining in a near-vertical upside-down position. The degree and manner of consumption of various prey groups has been described in detail in Appendix 10. Caching "Storing" of food for later use, often by burying, has been documented for a number of carnivores including foxes (Scott 1943),bbears (see Figure 9, Craighead and Craighead 1970), and mountain lions (Hornocker 1970). Kruuk (1972) witnessed hyenas caching uneaten meat by submerging i t in standing water. As a group, however, the mustelids seem to display storing proclivities more than do other carnivores. Ewer (1973) cites references dealing with food storing in nine mustelid species^: not including the short-tailed weasel (Mustela erminea), the black-footed ferret (Mustela nigripes), and the American marten (Martes americana), and the storing habits of these are either stated or implied in Coues (1877), Hillman (1968) and Remington (1952), res-pectively. The mink is a swell ..known food-storer: Yeager (1943) found 13 117 muskrats, two ducks and a coot (Fulica americana) in a hollow log den; Schnell (1946) found ten dead cotton rats in a mink burrow he excavated; Schladweiler and Tester (1972) and Sargeant ej: al. (1973) reported finding dead waterfowl cached by mink at dens and other feeding sites; Errington (1967:28) stated that -"When they discover a wintering pool of frogs, or a lakeside spring full of desperate fishes,... minks may pack up to hundreds of pounds of frog or fish victims in the tunnels of a single snowdrift". I have referred to foods cached by coast mink a number of times in the food habits section of this report. The circumstances surrounding food storage by carnivores vary. For the larger ones, as evidenced in work cited above, "stored" prey usually is an individual which is too large to be eaten in a single meal. It is hidden, apparently to discourage scavengers (conceivably including conspecifics), and the particular predator responsible usually remains nearby for up to several days to defend and finish eating the carcass. Among small carnivores, food storage also often follows opportunities to obtain more food than can be eaten immediately, but usually involves a number of small items rather than one large one. In many cases these series of cached foods are composed wholly or largely of a single species which became unusually vulnerable to the predator at least temporarily. For mink, evident examples from references cited in foregoing paragraphs include an experimental population of cotton rats transplanted to a small islet with l i t t l e cover, concentrations of pen-raised and/or wing-clipped waterfowl, winter-numbed frogs and oxygen-starved fishes. I know of an incident in which a coast mink entered a small chicken-house and killed a ll of the more than 30 Bantams roosting there, no doubt 118 aided in this endeavor by the fact that the birds had no means of escape. In August 1962, E. Schallock and I anesthetized and tagged a sample of Arctic grayling (Thymallus arcticus), and released them in still-lethargic condition into a small backwater of the Alaskan river in which we were working. The following morning we found that a mink had caught 32 of the tagged fish and had cached them under a board nearby. Kruuk (1972) recounted an incident in which hyenas killed over 100 gazelle in a small area in one night and, while expressing belief that opportunities for such occurrences are rare, pointed out that animals which store meat could benefit from "surplus killing". Evidence given above indicates that both surplus killing and food storing are common among small mustelids and, in fact, I suspect that this is the pattern for secure exis-tence among these animals. As will be shown, they have high rates of meta-bolism and may respond to food deprivation by greatly increasing activity and further depleting lagging energy reserves. Therefore, situations such as weather-vxonditions severe enough to prevent hunting or temporary prey scarcity could place individuals with no stored food at a severe disadvantage. Glover (1942) observed that "weasels (M. erminea) in good condition will starve to death in about 48 hours", and I have seen the condition of indivi-dual mink deteriorate in surprisingly short periods. A consequence of continual food storage, especially during the warmer seasons in temperate areas, is that some prey which is caught goes uneaten. Criddle and Criddle (1925) and Schnell (1964) found spoiled prey in mustelid nests, and on several occasions I have seen spoiled items, especially crabs (which last scarcely a day after death in warm weather), at dens and middens of my study areas. Apparently the storing habit is of more benefit 119 in the long run than is saving prey individuals alive. As I have mentioned previously, mink occasionally catch prey and then neither eat nor carry i t away. This would seem to be a contradiction of the food storage tendency which I have contended is a vital part of the mink hunting pattern, and it should be discussed. Of the 176 items which I saw mink catch, they discarded just eight (4.5%); thus, this practice is rare. All eight discarded items were crabs, mostly large crabs, and the mink which discarded them had caught and either eaten or carried away an average of 2.4 items before catching them. This figure is minimal since in two cases the discarded crabs were the first items which I saw the respective mink catch, although in both cases I was certain that they had been hunting several minutes before I arrived and had probably caught other prey first. It is probable, therefore, that relative "hunger" affects storing tendency, and there may also be seasonal and sex differences. In experiments with ranch mink, MacLennan and Bailey (1969) found that in summer the animals, especially males, rarely stored food, while in the cold months both sexes stored con-siderable quantities. My eight observations involved six males, one female and one unidentified animal, and occurred in the months of May (1), July (2), August (4), and September (1). In summary, the littoral foraging mink on the west coast of Vancouver Island lives in an environment notably milder and perhaps providing a more bountiful food supply than is the case in most of the rest of the north temperate region (where the species presumably evolved), and i t might be expected to relax, somewhat, its dependence upon food storage. However, there is l i t t l e evidence that it has done so. Temporary caching is another aspect of food storage which deserves attention. Holling (1965) has shown': that predation involves a number of 120 additive components, and that time spent on any one, such as eating, will reduce the time available to spend on another, such as searching. Fisher (1951) watched young red foxes catching small items in a recently plowed field and caching them nearby, and concluded that the animals were hunting intensively at the best hunting time (just before dusk, immediately after a day's plowing) and were thereby catching more than would have been possible had they taken time to eat. This may be a valid explanation, although i t may be that so much prey was being uncovered by the farm machinery that the foxes were sated. In the case of my coast mink, I will show that an hour or so around the daily lowest low tide seems to be the prime hunting period, and i t would not be surprising to find behavior which would maximize huntingrtime then. Unfortunately, I was not able to determine the-'immediate'fateoof.,most prey which mink carried above the high water line and into the woods. In some cases i t seemed evident that the animals stayed long enough to have eaten all or part of the prey, although in some cases they may have been engaged in other activities. As a result, my data do not demonstrate any appreciable difference in time taken to store crabs.(mean 378 seconds, n=22) as opposed to the time taken to eat them (mean 358 seconds, n=25) and in the case of fish it actually took longer on the average to carry them into the woods and return (84 seconds, n=10) than to eat them (57 seconds, n=13). In four instances, however, mink were seen to catch food near the water's edge and then store i t temporarily a few meters away. In these cases the time taken off from hunting was 20 seconds or less. The animals involved each laid aside two or three small fish, and then took them to the woods before moving to another location, although one actually left 121 the hunting s i t e and did not return to r e t r i e v e i t s f i s h for more than an hour l a t e r , at which time the r i s i n g t i d e had nearly reached the temporary storage s i t e . O v e r a l l i t appears that temporary caching i s not common and may be, i n f a c t , a technique h i t upon only by some i n d i v i d u a l mink. On the other hand, a l l four of my observations of t h i s behavior were i n July and August and I leave open the p o s s i b i l i t y that i t may be more common i n other seasons, when both hunting hunting conditions and observation are more V d i f f i c u l t . 122 ACTIVITY PATTERNS INTRODUCTION The diel activity patterns of mink have been studied only once previously. Gerell (1969) monitored six radio-tagged animals (five males and one female) in southern Sweden, and found that the males were largely nocturnal in all seasons. The female, which was instrumented twice, was active mostly at night when she was pregnant, but greatly increased activity during daylight hours when lactating a month_later. Climatic factors seemed to have l i t t l e influence, although males appeared to increase activity with decreasing temperature; during lactation, the female was least active when temperatures were;lowest. The males were active for about 25-40 per cent (mean=32.2) of the day (24 hour period). The female was relatively inactive during pregnancy, spending less than 15 per cent of the time away from her den during that time but during lactation she increased her activity toe-. a level equal to that of the males. It was usually not possible to determine what animals were doing during periods of activity, but it was Gerell's impression that up to 90 per cent of all activity involved food-getting. METHODS I studied aspects of coast mink activity patterns both by direct observation and with the use of telemetry. Observations of free-ranging animals enabled identification.of the specific kinds of activity commonly engaged in and pro-vided insights into the degree to which these activities are dictated by various environmental conditions. It was not practical to catalogue all of the time I spent in potential observation situations, thus the direct observation data are subject to bias; l i t t l e consideration is given to times when animals were not seen. Partly to overcome this problem, I established a permanent census transect between Rassier Point and Kingfisher Point (shown in Figure 3) 123 just north of my Vargas Island study area. Almost exactly one mile in length, this section of shoreline features a rather steep rockweed shore habitat inter-rupted in a few places by small boulder beaches. The water at shore edge along most of the route is shallow, with a mud bottom supporting good growths of eel-grass and other marine vegetation. It is good mink hunting habitat, and den sites in the form of hollows among tree roots are 'abundant in adjacent woods. The maximum number of different animals seen along this route was five on July 10^ 1968; at least that number were present in the general area throughout the summer of 1969. Transect procedure involved cruising parallel to shore in a small boat, at a distance of about 30-40 m, and counting mink seen. A single pass took 8-12 minutes, depending mostly upon the number of animals seen; all animals were examined through binoculars for ear tags or other identification marks and this activity, requiring stops, resulted in longer transect times. My actual rate of movement along the shore was about the same for all runs. Animals were often disturbed by my passing boat, so I allowed at least three hours between counts on those few days when more than one count was made. When returning along the census route after completing a count, I usually saw fewer mink than during the count but on five occasions I saw one more and once I saw two after seeing none during the census. During the 1968-72 study period, a total of 119 transects were run along . this route. In 1968 and 1969, when the Vargas Island mink population was high, transects were run largely without regard to season, time of day or tide level, although weather conditions and darkness made winter counts at this remote location both impractical and unsafe. The population declined in the second half of 1969, and most counts 'from early 1970 on were made at low tide with the object of searching for tagged animals which had disappeared from the main 124 study area. For this reason, only the 1968-69 data (84 transects) were used for study of activity. Telemetry provided some useful supplementary information, particularly for times when direct observaton is difficult such as during darkness. Equip-ment used was 150 MHz stock purchased from Davidson Electronics, 2109 Glenwood Avenue, Minneapolis, Minnesota (squirrel-size" transmitter collars) and Wildlife Materials Inc., Route 3, Carbondale, Illinois (transmitters, 2-element Yagi antenna, AVM model LA-12 receiver). Field techniques were similar to those usually employed in telemetry studies (e.g.,see Gerell 1969) with the exception that most work was carried on from a small open boat or on foot. This resulted in special problems in this environment; in the boat i t was necessary to carry the receiver in a plastic bag in order to minimize corrosion from contact with sea water, and on shore the Yagi antenna frequently had to be dismantled to enable movement through dense shore-fringing shrubbery. Specific techniques as they apply to monitoring of activity are ..detailed in following paragraphs. Eight animals, four of each sex, were radio-tagged at Turtle Island during the summer of 1972, and an attempt was made to monitor the activity of these by an automatic recording system as described by Gilmer e_t al. (1971). In practice, mechanical difficulties plus the fact that mink often switched dens and occasionally wandered out of range of the automatic recording unit made i t impossible to interpret the results reliably. However, the pulsing transmitters were found to emit signals which were steady when animals were s t i l l and were variable, both in speed and intensity, when they were moving. This was usually detectable by ear, and i t was possible to classify 211 125 location, f i x e s on tagged animals as " a c t i v e " or " i n a c t i v e " . In February 1972, an adult male was monitored manually for 261 hours to determine h i s a c t i v i t y pattern. This mink had a rather l i m i t e d range of a few hundred meters along the Tofino waterfront, and i t r e g u l a r l y hunted from a nearby wharf. The r a d i o - c o l l a r was attached on 3 February and during the following two days dens were located and monitoring posi t i o n s were determined. Simultaneous observations and radio-tracking on one occasion while the animal was hunting provided experience with varying signal sounds under d i f f e r e n t conditions; for example I found that the signal became very intense and staccato when the animal was i n or very near the water, and stopped when the animal dove. I t therefore became possible to determine when the animal hunted by diving, and even to time the dives, without venturing from the camper-truck which served as a base of operations. By the evening of 7 February, when the continuous monitoring session began, the mink had apparently become accustomed to the c o l l a r and had s e t t l e d into i t s d a i l y routine. When observed that morning, i t had appeared to act normally i n a l l respects, and i t had not directed attention to the c o l l a r at any time while i n view. To monitor signals from t h i s animal I positioned the truck at one of three locations, depending on time of day and which of two main den systems was i n use. Signals could usually be received from both dens and the main hunting area at a l l three locations, but the l o c a t i o n g iving the widest coverage (used most nights) was poor during the day because of interference from outboard motors and automobile engines nearby; the other two' locations were nearer the respective dens. I r e g u l a r l y made loc a t i o n checks on foot, but took care not to approach the animal close enough to disturb i t . A l l a c t i v i t y was determined by ear; I l e f t the receiver playing while attending other duties i n the truck and noted and interpreted a l l 126 signal changes. To allow myself periods of sleep and occasional r e s p i t e , I plugged the receiver output to the input of an E l e c t r a r e e l - t o - r e e l tape recorder playing at a speed of 1 7/8 i.p . s . Up to four hours of continuous recording could be obtained i n t h i s way, and t h i s was l a t e r transcribed i n the following way. Since most s i g n i f i c a n t bouts of a c t i v i t y lasted ten minutes or more, the tape was scanned i n playback by using the fast forward c o n t r o l to provide output at the equivalent of ten-minute i n t e r v a l s . A count of the number of pulses per ten seconds and a subjective appraisal of the r e l a t i v e steadiness of the signal were compared with the previous count to determine whether a change had occurred. Whenever a change was indicated, the tape was reversed and further checks were made between the two pertinent counts. When a period of a c t i v i t y was i d e n t i f i e d , the tape was reversed u n t i l the beginning of the a c t i v i t y period was found, and t h i s was monitored continuously u n t i l the animal had again become i n a c t i v e . Monitoring terminated at about mid-day on 18 February. When the animal was recaptured for removal of the c o l l a r on 8 March, i t was i n good condition and was found to have gained weight during the period i t had been instrumented. RESULTS DIRECT OBSERVATIONS For each of 747 observations, the a c t i v i t y of the mink involved was c l a s s i f i e d i n one of eight categories. In several cases the observed animal was engaged i n a c t i v i t i e s f i t t i n g more than one category, i n which case that l i s t e d was usually the one which c l e a r l y predominated. In a few cases the category chosen was the one which I considered most noteworthy; for instance, nearly a l l observations of "mating" and "fighting" involved at least one 127 animal which: had been hunting during most of the period of observation. Frequencies and definitions of the various categories among my results are listed below. Activity Category Number of Occurrences Per Cent of Occurrences 1. Hunting 432 57.8 2. Traveling 101 13.5 3. Sleeping 85 11.4 4. Eating 20 2.7 5. Fighting 19 2.6 6. Mating 13 1.8 7. Other 23 3.0 8. Undetermined 53 7.2 Total 747 100.0 Definitions: 1. Hunting - diving, poking, bird-dogging, digging for prey, or otherwise handling prey below the highest high water line. 2. Traveling - moving from one location to another, either by land or by water, without stopping to engage in hunting or other behavior. 3. Sleeping - resting, sunning, or sleeping at or near dens, middens, latrines or other locations above the high water mark. (Note that this activity is "inactivity" for data obtained less directly, such as by telemetry.) 4. Eating - consuming prey above the high water mark or drinking at any location. (Eating of prey at locations where i t was caught, i.e., below high water line, was classified as hunting.) 5. Fighting - engaged in any agonistic behavior, whether or not direct contact occurred; an exception was behavior clearly related to mating. 6. Mating - animals engaged in mating chases, copulation, or post-copulatory grooming. 128 7. Other - activities not clearly belonging to any of the above categories; commonly these were interactions with me such as defense of young or attempts to steal trap bait, but functions such as defecation, urination, and grooming were also included. 8. Undetermined - category applied to those instances of activity in which observation time was too short, visibility was restricted, or for some other reason the specific activity could not be identified. Figure 13 depicts the distribution in time of the two most commonly observed activities (hunting and traveling) and of my only direct measure of inactivity (sleeping), as recorded during direct observations of mink on my study areas. Not unexpectedly, these data show a strong tendency for hunting to occur within an hour or so of low tide, although observations of hunting animals were obtained during most daylight hours and at virtually all tide levels. The other form of activity, traveling, also occurred mostly near low tide times, but not nearly to the extent as for hunting. Sleeping was also noted during most observation times, but especially at mid-day, regardless of tide levels. In relation to tide level, the least activity (almost no hunting and l i t t traveling) was observed between about 300 and 400 minutes before and after the lowest low tide, a time period corresponding to the daily peak tides (alternatingly high high or low high, depending upon time of year). The slight increase in activity indicated for times greater than 600 minutes befor and after the reference tide probably reflect the fact that waters drop to the daily higher low (HL) about 720 minutes from the lower low. The data in Figure 13 suggest that tide level was the most important determinant of activity during times when direct observation was possible; in terms of face page 129 Figure 13. Incidence of hunting, t r a v e l i n g , and sleeping by mink i n rel a t i o n * to time of day and the time of the lowest t i d e . " 1 0 =: ^ Oi °? i i i — Oi ro x w + O Oi o w . . - - O - - cr o • > • c © o =r. CO. Q • t> t> |_ 5" CO 0> D-s. 3 ' CQ 0 3 minutes before low low tide , ° n minutes after low low tide o -1 CO o o ro o © Ul o • o > O • o o T o o • • o • > • > • o > • T O " T " co a o + 0 0 © > > -0 • > > > • 0 © © 0" > > • • • 0 0 0 • > > > -• • • • 0 0 0 0 > > • • • 0 0 0 © • 0 0 0 • • 0 • 0 © CO Ul CD ro T T T P o o • > • © • • o © o © o o > • o o © > • © • • © © o > t> > • • • • o > • o • o > t> • > • o > • • > • > • • o • o • > • o o > • o • • • 621 130 frequencies of observations per nearest slack t i d e , a l l a c t i v i t i e s were seen most often near the low low t i d e , but t h i s was true for hunting to a s i g n i f i -c antly greater extent than for the others (Table 10). While the above information provides a general p i c t u r e of what coastal mink do as well as when and how often they do them, i t may nevertheless be biased i n some ways. Perhaps most importantly, the d i s t r i b u t i o n of sightings i s at l e a s t p a r t l y r e f l e c t i v e of my own timetable i n the f i e l d . Once I had learned how observable the animals were at low t i d e , I increased observation e f f o r t at such times. What th i s means i s that had I not gone out to look for mink on e a r l y morning summer low tides and evening winter low t i d e s , I would have seen far fewer animals than I did. I t does not n e c e s s a r i l y mean that I would have seen many more animals had I spent more time out at higher t i d e l e v e l s . In f a c t , during most summer f i e l d days I was out from about f i r s t l i g h t to at least noon and often longer. Thus, I was a v a i l a b l e for observations at the e m p i r i c a l l y determined optimum times, i . e . , about one hour before and a f t e r the slack low t i d e , but I was also present for four or more hours beyond those times. The 747 a c t i v i t y - c l a s s i f i e d observations l i s t e d e a r l i e r were made during a minimum of 798 f i e l d days, an average of under one sighting per day. The duration of " f i e l d days" varied g r e a t l y but a conservatively estimated average would be about f i v e hours. I t was possible to go for days, e s p e c i a l l y i n f a l l and winter when most daytime tides are high, without seeing any animals other than those caught i n l i v e t r a p s ; on the other hand, d a i l y sightings of f i v e or s i x d i f f e r e n t animals were not uncommon and on 18 June 1969 I saw eleven, a l l but one within an hour anda~hal.f ;_df the low t i d e . Table 10. Observed activities of mink in relation to tide levels, Vancouver Island, British Columbia, 1968-1972. Number (and per cent) of Observations per Tide Level 3 Activities LL LH HL HH Chi-Square df Hunting 376 (87) 17 (4) 28 (6) 11 (3) versus Traveling 62 (64) 16 (16) 10 (10) 9 (10) 36.3 3 <0.001 Sleeping 37 (44) 20 (24) 13 (15) 14 (17) . 91.1 3 <0.001 Eating 13 (65) 7 (35)e 6.0 1 <0.02 Otherc 27 (56) 10 (21) 6 (13) ..5 (10) 37.0 3 <0.001 Undetermined^ 26 (52) 9 (18) 7 (14) .,8 (16) 47.1 3 <0.001 Traveling 62 (64) 16 (16) 10 (10) 9 (10) versus Sleeping 37 (44) 20 (24) 13 (15) 14 (17) 7.3 3 < 0.10 Eating 13 (65) 7 (35)e 0.0 1 <0.90 Other 27 (56) 10 (21) 6 (13) 5 (10) 0.8 3 <0.90 Undetermined 26 (52) 9 (18) 7 (14) 8 (16) -•2.5 3 <.0.50 Sleeping 37 (44) 20 (24) 13 (15) 14 (17) versus Eating 13 (65) 7 (-35) e 2.1 1 < 0.20 Other 27 (56) 10 (21) 6 (13) 5 (10) 2.1 3 < 0.70 Undetermined 26 (52) 9 (18) 7 (14) 8 (16) 1.0 3 < 0.90 ^ide level = nearest slack tide: LL-Low low; LH-low high; HL-high low; HH-High high, bprobaiility of chi-square so large under hypothesis of no differences. c0ther = fighting, mating, playing, grooming, etc. (see textO a0bservations for which activities were not certainly identified. eFigure represents sum of observations for LH, HL, and HH tides.. 132 As shown in Figure 14, results of the Vargas Island transect counts confirm the main conclusion drawn from direct observations, i.e., that mink seen during daylight hours are seen primarily at low tides. Indeed, on 21 transects during which the tide level was at or below two feet, mink were seen on a l l but four, and most counts involving more than one mink were made in this tide range. The two animals seen at mid-day,with the tide level at nearly ten feet,were siblings apparently engaged in "play" at the edge of the woods. Analyzing transect count frequencies of zero, one, and two or more mink in relation to different conditions, i t is evident that the tendency for mink to be out and observable increases as the tide drops, (X2 = 20.87, df = 6, P<0.001), is greater within an hour of the slack low tide than at other times (X2 = 13.30, df = 2, P<0.001), and decreases with advancing time of day (X2 = 11.19, df = 4, P< 0.02) . Tide trend (whether rising or.falling) has l i t t l e influence on the activity of mink, judging from the transect data(X 2 = 0.63, df = 2, P<0.80), although i t was evident during observations that most animals began hunting on the falling tide. I sat and waited for known animals to arrive at their usual hunting spots on a few occasions, and they usually appeared about 40-50 minutes before the slack low. Many mink seen after the slack tide had probably begun activities earlier. There is some evidence that animals hunting low-profile beaches such as sand beaches and estuaries may regularly hunt on higher tides when the water rises nearer to cover. Prey animals hunted at such locations, usually crabs and small fish, commonly move up and down through the intertidal zone, with the tide, and would therefore be available. f a c e page 133 Figure 14. R e s u l t s of 84 census t r a n s e c t s north from R a s s i e r P o i n t , Vargas I s l a n d , 1968-1969. 133 o o o o o o o o o C D C <U CD 1/1 c moi O i . o 1— d) o -— CN CO _Q >• E II II II II L U z o e c • o O o o o o o o o o o o o € o o 0 D O O < o Q in CM in © 134 Telemetry As mentioned e a r l i e r , one l i m i t a t i o n of d i r e c t observations i n providing information about the a c t i v i t y schedules of mink was that they applied mainly to daylight hours. I was aware from a v a r i e t y of evidence that mink were also active at night. Tracks made on beaches a f t e r the recession of night high tides indicated animals had been active during darkness, most animals caught i n l i v e t r a p s entered them at night, and mink were often seen along the Tofino waterfront a f t e r dark. I attempted night counts by f l a s h l i g h t and found that animals could almost always be found hunting at; low t i d e , but not at other times. However, they were not e a s i l y seen under these conditions and were usu a l l y detected only by tapetum r e f l e c t i o n ; animals which became aware of me before the f l a s h l i g h t beam reached them could e a s i l y avoid being seen. Further, west coast forests are d i f f i c u l t to negotiate even during daylight, so i t was impractical to observe anywhere except along shorelines. Animals which were active elsewhere would not have been detected. The use of telemetry overcame some of the above d i f f i c u l t i e s but several f a c t o r s , among which was the hazard of night t r a v e l by small boat among the rocky islands of Barkley Sound, l i m i t e d the information which could be ob-tained. A far greater number of night f i x e s would have been desirable, and would have been po s s i b l e were i t not for the f a i l u r e of the attempted automatic recording system. Figure 15 (a & b) depicts the proportion, i n r e l a t i o n to tide and time of day, of active and i n a c t i v e f i x e s on eight radio-tagged mink i n the T u r t l e Island area i n summer 1972. As with other data sources, a greater proportion of a c t i v i t y at lowest tide l e v e l s i s indicated, although i n t h i s case mink were found to be i n a c t i v e more often than active at a l l times and tide l e v e l s . Again, there was a low incidence of a c t i v i t y during face page 135 Figure 15. A c t i v i t y of radio-tagged mink i n r e l a t i o n to l e v e l s of t i d e and time of day. In (a) and (b), open bars indicate observations (radio f i x e s ) i n which animals were i n a c t i v e , and cross-hatched bars represent fi x e s on animals which were moving. In ( c ) , only active f i x e s are considered; shaded bars represent nocturnal and crepuscular observations (evening to morning, 20:00-06:00) and open bars indicate.,; observations during the r e s t of the day. The number of pertinent observations i s shown at the top of each histogram. 100 80 60 40 20 135 X 2 = 6 . 6 8 ; df = 2; p <.05 88 67 11 I 20 (a) 17 22 C u 100 5 80 Q_ ^ 60 u « 20 D cr • 0 - 1 . 0 1.1-2.5 2.6 + tide level (m) X 2 = 9 . 0 6 ; df = 4; p < .10 44 31 19 13 1 / / 11 33 (b) 29 19 I 100 80 60 40 20 4:01-8:00 8:01-12:00 12:01-16:00 16:01-20:00 20:01-24:00 time of day X 2 = 3 .90; df =2; p < . 30 (e) < 1.0 1.1-2.5 2.6 + tide level (m), active fixes only 136 the middle of the day, but the greatest incidence of activity was not during the earliest and latest time periods, but during the period in which low tides were most frequent. This again suggests that tide level, not time of day, was the determining factor. To further evaluate the relative importance of tide and time, data for active fixes were examined separately. As shown in Figure 15 (c), the proportion of active fixes for higher tide levels was greatest during darkness or semi-darkness. A source of bias was the fact that most low tides at the time of year the Barkley Sound telemetry work was done occur during daylight. These data suggest, how-ever, that activity at night may be more independent of tide level than is that observed during daylight. As shown in Figure 16, the continuously monitored mink was active almost entirely at night. This is certainly at least partly related to the fact that the animal resided in an area much frequented by humans and dogs during the day, but i t is evident that i t did not need daylight activity (when most low low tides occurred during the observation period) to sustain itself. It weighed 1200 g (above average for Clayoquot Sound males) when instrumented in early February and 1300 g when the radio collar was removed a month later. During the period of study the tides passed from a "neap" situation (range between low low (LL) and high high (HH) less than 2m) to a series of more extreme "spring" tides (tidal range almost 4 m). The mink hunted primarily between the low high (LH) and high low (HL) tides for the first four days, a time period during which these two tides did not differ by more than 0.7 m. Thereafter the difference between EH and HL increased by more than 35 cm per day to a maximum of 3.3 m by 18 February. The face page 137 Figure 16. Activity of a radio-tagged male mink along the waterfront in Tofino, British Columbia, February 1972, as determined by continuous monitoring. Dotted lines indicate periods during which signals were not received, usually because of temporary equipment failure; for all other times shown, monitoring was in progress. Blackened portions of bars represent periods when the mink was active; narrow vertical bars crossed with horizontal dashes indicate short periods of activity, usually less than five minutes, during which the animal stayed in or near its den. A vertical arrow above -a period of activity indicates that the animal made one or a series of dives; arrowheads alone indicate the animal was in the water but no dives were detected. Letters below activity bars designate footnotes, as follows: A. Animal was hunting under a wharf and, attempting to watch him, I frightened him back to his den. Activity would have continued otherwise. B. Signal steady but very intense -- mink probably either resting or eating very near water, probably under wharf decking. C. Mink changed dens. D. Very strong winds and heavy rain -- mink apparently came out but went back in. E. Antenna blew down and signal was temporarily lost. F. Activity apparently due to disturbance from four children and two dogs seen playing in woods near the mink's den. G. Animal moved out of receiver range and stopped activity, going into the den farthest away, sometime during period indicated. H. A dog was found digging at the den system at this time, and was no doubt responsible for the short periods of activity indicated. I. Signal obliterated by some kind of interference, and activity stopped at unknown time during this period. Per cent activity is the proportion of monitored time during which the animal was known to have been active. Low Low High Low H i g h fligh m CO uj 12-13 per cent activity T I M E OF D A Y S U N S E T (17:19 - 1 7:37) S U N R I S E (07:36-07:18) 138 general trend after 11 February was for the mink to shift the onset of its night activity back toward the LL, which was slack during darkness by 14 February and was dropping to a level below 1 m during most days after that date. The final activity periods-each night usually occurred at or somewhat after the HL, but by 14 February (despite the fact that the lowest HL levels were reached after that date) this was no longer true, presumably because this tide had advanced into the morning daylight. Within the area frequented by this mink during the period of study, there are no beaches on which hunting would be greatly enhanced by tidal exposure. Indeed, this animal hunted largely, i f not exclusively, from a wharf under which water varied in depth from about 1.5 to 10 m, depending upon tide level and location. Food organisms, especially fish and crabs, were abundant under and around the wharf, but most could be caught only by diving, a hunting technique which was, in fact, detected during most of the night activity periods shown in Figure 16. From the standpoint of this mink, the difference of a meter or so in depth resulting from tide changes might mean l i t t l e in terms of hunting success. On the other hand, daily total activity time decreased on the last few days of observation, when the animal was able to hunt hear the LL, and this suggests i t was obtaining more food per unit time. A.complicating factor is that a mink, possibly this one, was reported to have stolen several pounds of fish from a fishing boat during the night of 14-15 February, and its low activity on subsequent nights may have been due to its having an adequate food supply in its larder, The weather during the monitoring period was characterized generally by a complete cloud cover with light to moderate winds and intermittent 139 rain. There was a single clear night (12-13 February), and i t was on that night that the mink was active for the iongest time. In a few cases negative effects of weather conditions may have occurred; a very heavy rain shower from about 22:00-23:45 on 10 February appears to have interrupted an activity session, and heavy rain plus storm winds gusting to 60 knots during the early morning hours of 12 February appeared to discourage the animal from leaving its den area (see Figure 16). In the latter instance it was known to have come out of the den and engaged in brief activity before going back in. On the other hand, most of the mink's activity bnithe night of 14 February occurred during a heavy downpour. From direct observations, my impression is that mink do restrict activity along the shoreline under storm conditions, especially when wind-generated waves are crashing over preferred hunting spots, but I have seen animals out under very severe conditions. An individual's response to weather probably depends upon a number of factors, especially its condition, its level of hunger, and the nature of its hunting area. DISCUSSION The feral mink observed by Gerell (1969) in Sweden were believed to have followed a basically nocturnal pattern of activity, largely in response to peak activity periods of prey species which, in this case, included a large proportion of rodents. Similar conclusions have.-, been reached for some other nocturnal and crepuscular carnivores including another mustelid, the least weasel (Price 1971), the red fox (Abies 1969) and three species of procyonids (Kavanau and Ramos 1972) . In the papers( cited, it was recognized that a variety of other factors also influenced the pattern and duration of activity 140 periods, including sex, age and reproductive condition of observed animals, effects of weather, seasonal differences such as changes in day length, and individual variation. Evidence from a variety of sources indicates that mink in coastal British Columbia hunt frequently, i f not,mainly, at the lower daily tides when the intertidal animals which constitute their chief food are likely to be most accessible. However, from data available there is no reason to suppose that the strong nocturnal tendency characteristic of Gerell's Swedish mink (and mink at most other locations so far as is known) is not also true of the British Columbia coastal animals. It is probable, in fact, that a nocturnal pattern is basic and that the observed tidal pattern is superimposed upon i t . Mink appear to be flexible in hunting behavior and, as suggested by Waller (1962), individuals may quickly learn to repeat sequences of behavior which result in feeding. I have previously recounted instances in which animals catching prey under a specific boulder were seen to return to that boulder repeatedly in subsequent observation. In the present context, animals may also discover, and lock onto, times most productive of feeding success. For most animals in my study areas this would be low tide, but I observed some which appeared to hunt regularly and successfully at high water. How animals resting in underground dens "know" when the most favourable tide level is reached is a question not answerable with data obtained in this study. The extent to which tide levels change and peaks shift from day to day would suggest that an endogenous rhythm (see Aschoff 1964) is unlikely. However, as I have pointed out,earlier, animals tended to appear at about the same times relative to the tide (apparently without making earlier trips to gauge tide level) so this possibility cannot be ruled out. 141 The single mink monitored during this study averaged less than half as much activity per day (24-hour period) than did any of the Swedish animals observed by Gerell (1969) except the female during pregnancy. During lactation the female also increased the duration of activity periods to an average of more than twice that of the coast mink. If, as appeared to be the case, the animals in both areas spent most of their active time in food getting, the difference between areas could be a reflection of the greater productivity of the marine habitat hunted by the British Columbia mink as compared to the mouse meadows and stream pools hunted by the animals in Sweden. Price (1971) experimentally increased total activity in captive least weasels by depriving them of food, thus considering activity duration as a function of food return (except during the mating season) seems valid. This has important implications for population studies based on livetrapping (pP.177 )• The most secure (best fed) individuals may be the least active and, therefore, the least likely to be trapped. A final point is that despite their generally high rate of metabolism (Iversen 1972) mink are like larger carnivores such as lions (Schaller 1972) and hyenas (Kruuk 1972) in that they may spend three-fourths or more of each day engaged in sleep and rest. 142 ENERGY REQUIREMENTS AND INTAKE INTRODUCTION AND METHODS There is l i t t l e imformation on the nutrition of wild carnivores other than that obtained through studies on a few species in captivity (see Golley et al. 1965). Research on ranch mink (Farrell and Wood 1968a; Iversen 1972) indicates that the basal metabolic rate of this species is not appreciably higher than that for other mammals (about 80 kcal/kg ^ ' ^ ) , although Farrell and Wood (1968b) found that the apparent digestible energy requirement for maintenance of female mink was almost three times the basal rate, or about 50 per cent higher than the value for most, mammals. They noted that confining their animals in smaller cages resulted in a decreased ;feed intake of about 20 per cent, and pointed out that this was apparently due to tne smaller area available for movement and not to increased quiescence in the small cages. These authors concluded that the unusually high energy requirement of the mink is associated largely with activity. . The National Research Council (1968) has outlined nutritional requirements for ranch mink, listing both qualitative and quantitative aspects of recom-mended- diets. A figure of 273 kcal/kg of body weight, somewhat higher than the approximately 240 kcal/kg which may be calculated from the results of Farrell and Wood (1968b) is given as the daily requirement for maintenance of an average ranch mink. Growing young, animals and pregnant females; are said to require up to 20 per cent more and lactating females s t i l l , more, depending upon the number and size of the young. Assuming mean body weights of 1200 grams for adult males and 750 grams for adult females (see Figure 5), daily requirements of about 300 kcal and 200 kcal for the two sexes, res-pectively, are indicated. 143 No precise energy values are available for foods regularly eaten by coast mink, but a reasonable approximation can be made. According to Pugsley (1942), crab meat approximates the nutritive value of "non-oily fish" at about one kcal per gram. Sculpins and rockfishes are probably in the "non-oily" category but, judging from the amount of o i l which exudes from large specimens when they.are cooked, blennoids may have higher fat content and provide more energy than the other two commonly eaten groups. The viscera of both crabs and fish are eaten, and these are probably more nutritious than the "meat" of these animals so that, on the whole, the energy value of food eaten by mink on my study areas probably averages in excess of one kcal per gram (net weight, i.e., exclusive of undigestible material). Nevertheless, this minimal value seems adequate for present purposes, even though i t may seem to inflate the figure for weight of prey required (e.g.) on the basis of information given above, non-reproductive females and non-growing males, would require 200 and 300 grams of food daily, respectively, approximately 25 per cent of body weight). It should be remembered that the energy requirement was calculated on the basis of studies of captive mink; wild animals, having more space for activity, probably require at least the weight of food indicated above. Aliev and Sanderson (1970) estimated that food consumption of feral mink in the U.S.S.R. amounted to 20 per cent or more of body weight. A requirement equivalent to 150 grams of salmon was calculated for a one kg mink by Cowan e_t al. (1957) and since Pacific salmons yield 2 kcal per gram (Geiger and Bo.rgstrom 1962), this figure is in agreement (about 300 kcal energy yield) with the others given. 144 While the information on energy requirements of wild mink i s admittedly imprecise,, that on energy intake i s even more so. Despite the fact that animals were frequently seen hunting and catching prey, i t was d i f f i c u l t to quantify t h e i r success i n a meaningful way. The size of organisms r e g u l a r l y caught va r i e d considerably so that a mink which caught several items might end up with le s s food than another catching only one. I occ a s i o n a l l y had opportunity to weigh and measure f i s h and crabs caught by observed mink, but for most observations I had to estimate prey s i z e . I did t h i s by v i s u a l l y comparing the siz e of a prey (length of f i s h ; carapace width of crabs) with the.head si z e of the mink handling i t . Rough gross weights, i n grams, were then obtained for each prey, as follows: weights of crabs estimated at over 80 mm i n width were interpolated from Figure 11 (b) assuming t h a t . a l l cancrid species have a carapace width-weight r e l a t i o n s h i p s i m i l a r to that shown for red crabs; for specimens less than 80 mm wide, a weight of one gram per millimeter of carapace width was assumed. In some cases I could not estimate a prey crab's s i z e . At such times I assumed the average si z e (as found i n middens, see Figure 10) for that species, or, i f i d e n t i f i c a t i o n was not possible, the average si z e of the most commonly eaten species, the red crab. -A net weight of two grams was recorded for most small or u n i d e n t i f i e d items eaten on the spot. The gross weights of f i s h were obtained by rough i n t e r p o l a t i o n of f i s h size data l i s t e d i n Appendix 9. On the basis of dissections of representative prey i n d i v i d u a l s , weight losses due to un-d i g e s t i b l e matter amounting to 75 per cent for kelp crabs and about 67 per cent (two-thirds) for other crabs.; were used to c a l c u l a t e net weights of crabs 145 caught. No dissections were done on fish, but a loss of 25 per cent was assumed; the actual loss may be somewhat more for sculpins with their large bony heads, and is probably less for blennioids. All weights mentioned in following pages are net weights, and are assumed to have minimal energy values of 1 kcal/g. A second variable of interest in considering energy intake is the time taken to obtain a given amount of food. As defined earlier, "hunting" refers to an entire sequence of searching, attacking, subduing, transporting and caching or eating a prey item. In following pages the term "hunting period" is used to indicate a period of time during which an animal was observed while engaged in hunting. By this definition a hunting period might constitute a portion of one hunting sequence, for example only the capture and transport phases, but could also be composed of several complete sequences. It was rare that a hunting period corresponded to the actual total time a mink spent engaged in hunting activity, since I often arrived after i t had begun, inadvertently frightened i t away before i t had finished, or observed i t only incompletely because of poor viewing conditions. When possible, I did partition hunting sequences into various components in terms of time spent at each, but following analyses use "hunting period" as the time base for . comparisons of hunting success. Hunting periods of less than 2 minutes duration are not considered. RESULTS HUNTING SUCCESS Not surprisingly, failure to catch any food items occurred primarily during hunting periods (observations) of short duration (mean =4.6 minutes, standard error = 0.5), while hunting periods during which at least one prey 146 was caught averaged 14.8 minutes (SE = 1.9). There was also a tendency for the weight of prey caught to increase with increased hunting time (Figure 17), although the considerable scatter among data points (r s. 0.504) suggests that factors other than time may influence success. Indeed, some mink obtained a large amount of food i n a short time, usually by catching large prey. One young male caught a sculpin estimated to have yielded 200 g of meat less than one minute af t e r i t began i t s hunt, spent an additi o n a l eight minutes i n the brush eating and/or caching the f i s h , then returned and hunted an add i t i o n a l four minutes but with no further success,. S t i l l , i t s . t o t a l food return for the 12 minute hunting period was the largest recorded during my observations. Most items caught by mink were smaller, however, and the average net return for 160 items which I saw caught, including the one above, was 17.7 g (+ 3.8 at 957o confidence l e v e l ) . On the basis of known prey weights and net conversion factors l i s t e d e a r l i e r , the above amount i s the equivalent of a Cancer or Telmessus crab about 52 mm wide, a 45 mm kelp crab, a 100 mm sculpin, or two 130 mm blenniods. Figure 18 depicts the hunting success of mink i n r e l a t i o n to some of the circumstances under which they hunted. The v a r i a b i l i t y of these data makes fi r m conclusions impossible, but some of the trends indicated are contrary to expectations and require explanation. I have established previously that the A p r i l to July season, with i t s r e l a t i v e l y stable weather and with several species of crabs occupying near-shore waters, i s the time of highest food abundance and a v a i l a b i l i t y , thus the d i s t i n c t l y superior hunting success indicated for the other two seasons (August-November and December-March combined) i s an apparent co n t r a d i c t i o n . Two winter hunts which yielded 17.1 and 7.8 g/minute,respectively,have i n f l a t e d the August-March r e s u l t exces-s i v e l y (no other success ratesih.excess of 5 g/minute were recorded for face page 147 Figure 17. Hunting success (net weight of prey taken per minute of hunting) for Vancouver Island mink, 1968-1972. face page 148 Figure 18. Mink hunting success in relation to several environmental variables, Vancouver Island, British Columbia. Numbers in parentheses indicate sample sizes and horizontal bars depict standard errors of the means shown. S E A S O N S : August-March Apri l - July TIME: 0400-0800 0801-1200 1201 + TIDE LEVEL (m): < 1.0 1.1-2.5 2.6 + H U N T I N G M E T H O D : Dive Poke Bird-dog Combination HABITAT: BB + RW EF + SB + ES Combination ALL OBSERVATIONS HUNTING SUCCESS (g /min) TOTAL F O O D R E T U R N (net wt., gm) HUNTING TIME (min) (18) (29) (22!) (16) 0 ) (32) (10) (5) (11) (9) (6) (20) (25) (14) (7) (47) J L 6 0 I—•—I I—•—I I • I—•—I l - » H 20 40 _ i l _ 60 80 100 0 10 20 I •—I (—•—I J L V-»-H I — • — I CO I •—I I • 1 l - » H I • 1 l -»H 149 either season, and most were below 4 g/minute). Without these two high values the mean for August-March drops to 2.1 and this does not differ significantly from the April-July value (t = 0.84, df = 43, p<0.40). The total food return for August-March remains high primarily because observations were significantly longer in that season (t = 3.3, df = 43, p<0.05), and as Figure 17 showed, amount of food caught generally increases with time. The tendency for lower hunting success at the lowest tide levels and early morning times are also unexpected since these were shown to be the preferred, therefore presumably the most profitable, times for hunting activity. Here, the duration of hunting periods appears to be the main source of discrepancy. The longest periods occurred during these "favored" times and i t should be recalled that long observations frequently included periods of prey handling (eating or caching) while many of the shorter observations terminated when an animal left the shore area with its prey. If i t did not reappear, its handling time could not be calculated and added to the total observation time. Hunting periods did not differ greatly in duration among the different hunting methods, and differences in hunting success indicated appear to be due mostly to differences in total food return. As described earlier, mink usually catch large prey when diving and small prey when poking, and this is reflected in results shown. Habitat differences are due largely to the fact that the longest observations occurred in the extensive open areas e.g., eelgrass flats; however, most captures occurring in open habitats were seen while at least some of those occurring on more protected shores, especially on boulder beaches, were probably missed. Therefore, the mean success for boulder beach hunts was probably higher than that shown. 150 THE DAILY REQUIREMENT Assuming average energy returns of one kcal/g net weight of food and energy requirements calculated on page 143, a male mink would have to catch up to 12 crabs of average or larger size and up to 100 small fish to meet its daily needs, while a female without maternal demands could get by on about one-third fewer of these items. For many reasons i t is difficult to determine how much food individuals actually obtained in a 24 hour period. Examination of middens provides only minimal values both because small prey which do not contribute residue:to middens are eaten and because remains of large prey may be left below the high water line or lost to scavengers. The largest accumulation of crab carapaces over a known period of time was 172 in (up to) 39 days. The average net weight of meat from these crabs was 21.0 T" 2.1. g each, giving a total of just 92;5 g per day or less than one-third of the presumed daily requirement. Direct observations are also deficient since, as indicated earlier, most were not complete in terms of a mink's total activity in any given session of hunting sequences. Further, it is likely that few of the observed hunting periods constituted a mink's only attempt to obtain food during the day in question. As shown earlier, some animals actually rejected prey after catching i t ; in a hunting period of 45 minutes, one male caught six crabs, ate all of one and part of another, cached three, and then abandoned the largest, voluntarily stopping its hunt after accumulating what was calculated as half its daily requirement. Despite.the limitations of observation data in providing figures for absolute amount of food obtained, expressing food return (successful hunts only) as net weight of prey per minute (on the basis of hunting period, as defined earlier) gives a measure which provides some insight into how long 151 i t may take mink to obtain the daily requirement. Most animals observed for short periods had been hunting for an.unknown time before I arrived, so that their catch per unit effort is over-estimated; observations of less than five minutes duration showed a mean success rate (12.0 g/minute) significantly higher than those of five minutes or longer (t = 4.95, df = 68, p<0.01) and, to keep estimates conservative, I have excluded these from following calculations. During ..the longer observations, mink obtained food at an average rate of 2.3 g/minute (SE = 0.4). Despite the,fact that this figure is built upon an assumed caloric return per gram which must be lower than that actually pertaining and upon a time longer than that actually taken to find and capture prey, i t indicates that males could obtain their daily rations in 200 minutes or less (Mean = 130) and a female could do so in less than 135 minutes (Mean = 87). In reality, under some conditions observed mink obtained food at rates which would require less than an hour's activity per day while others, for example a female which obtained less than 0.1 g/minute over a 43 minute observation, could not have secured the daily requirement hunting 24 hours consecutively at that rate. In terms Of actual hunting (searching) time, the effort expended is less, although i t should be noted that the main interruption of hunting is prey handling; therefore, relatively unsuccessful mink spent a larger pro-portion of their observed activity periods in searching than did successful ones. From data for 33 longer observations (Mean = 27.3 minutes), observed mink spent an average of 58.4 per cent (SE = 9.1) of their activity in hunting and were handling prey during the remainder. Application of this figure to the times required to satisfy daily needs, as given above, leads to the conclusion 152 that males couid expect to obtain all of the food they required in about 80 minutes of searching and, under the conditions of all observations providing data, would almost never require two full hours of such activity. For females the comparable figures are 54. minutes (mean) and 85 minutes (maximum). If activity is considered only in terms of time, as has been the case up to this point in this report, the above information seems contradictory to a statement.in the introduction that the high energy requirement of mink is due largely to activity. It is important, therefore, to point out that when a mink is active i t is often very active. Movements of several hundred meters during a hunting session are not uncommon, and when a midden is at some distance from a hunting spot, such movements may be made repeatedly. The energy demand of frequent divesj digging, and handling of heavy prey (recall that up to 75 per cent of the weight of crabs is "waste".material, but i t must nevertheless be carried) should all be evident. . Quantification of activity in this sense was rarely possible, but in one case a juvenile male was found, by tracking, to have traveled a minimum distance of 3.3 km in a single hunting session along a Vargas Island beach, making a minimum of 29 trips back andjlrorth from the water arid presumably making one or more dives at each trip. DISCUSSION Golley et al. (1965) found that bobcats (Lynx rufus) maintained physical condition on widely varying quantities of food and generalized that "a carnivore must be adapted to a feast-or-famine regime". This does not seem to be true of the mink. Farrell (1966) conducted studies on mink during starvation and concluded that, because of the rapidity of food passage in this species, i t quickly evacuates its digestive tract and enters a catabolic state. He docu-mented mean daily weight losses of over 40 grams which, when expressed as a 153 percentage of body weight (5.3 to 1.TL) is much higher than the comparable figure for another carnivore, the dog (0.87=,). He noted further that, "unlike many other species, activity did not appear to diminish with fasting.." Indeed, Price (1971) showed with laboratory experiments that least weasels deprived of food greatly increased activity, and Lensink.et al. (1955) found that the extent of movement of marten in the field was inversely related to the availability of food. All evidence suggests that a rather continuous supply of food is necessary for the existence of these small mustelids, and their propensity to store food is probably a further response to this need. My observations indicate that many animals hunting the food-rich inter-tidal zone along the west coast of Vancouver Island may obtain their daily energy requirements in l i t t l e more than three hours of activity, supporting the short activity periods implicit in telemetry results presented earlier. This is considerably faster than the eight hours or so apparently required by Gerell's (1969) feral mink hunting along inland waterways in Sweden. Nevertheless, i t should be evident that not all coast mink enjoy the level of success listed as average, and none can obtain food at a high rate under all hunting conditions. Unfortunately, conditions favoring observations probably also favor hunting success, so that my information must be biased. Further, individual animals can be expected to be bound less by principles than by the practicality of what works in any given place or time. For example, although i t is certainly more profitable, on the average, for animals to hunt at low tide rather than at other tide levels, those animals which I saw hunting at higher tides may have been doing so because of some combination of circumstances which made i t profitable and not because they were unable to get food at other times. The failure of the comparative data in Figure 18 to show great differences 154 i n hunting success under the conditions measured i s therefore not s u r p r i s i n g . I n d i v i d u a l mink were hunting, as much as possible, at times and places which provided them the highest net energy return. In t h i s context, information from the a c t i v i t y section (mink hunted s i g n i f i c a n t l y more often at low tide than at other tide l e v e l s ) implies more about mean success rates than do the comparative hunting success data. I t i s the basis upon which I asserted that low tide i s the most p r o f i t a b l e hunting tide i n the previous paragraph and t h i s w i l l remain a basic premise. There were also conditions under which mink apparently did not often hunt and, i n most cases, probably could not hunt. Chief among these i s heavy surf, therefore large sections of the open coast o f f e r l i t t l e p o t e n t i a l as mink habitat. A mink whose range includes an i n t e r t i d a l area with some protection from wave action, e.g., i n the lee of a reef, i s l e t , or headland or even behind boulders on a boulder beach, i s c e r t a i n l y better o f f than one which does not. As shown i n Figure 18, animals which hunted eelgrass f l a t s had a higher food return than those hunting other habitats during my obser-vations, but there were many stormy days on which animals could not hunt these open f l a t s at a l l . In summary, i t seems evident that the r i c h l i t t o r a l waters along the shores of western Vancouver Island are p a r t i c u l a r l y beneficent to hunting mink. However, i t i s equally evident that these shores are not homogenous, and d i f f e r e n t areas d i f f e r i n the degree to which they provide access to food, p a r t i c u l a r l y under varying weather and water conditions. There are, i n short, good hunting places and good hunting times ( e s p e c i a l l y r e l a t i v e to the tide) and, because mink appear to be p a r t i c u l a r l y s e n s i t i v e to even temporary food deprivation, i n a b i l i t y to hunt at such places and/or times could be serious for the i n d i v i d u a l . 155 WATER REQUIREMENTS AND INTAKE INTRODUCTION AND METHODS Over most of its range the mink centers its activities along bodies of fresh water and therefore has no difficulty meeting its water requirements for maintenance;- and growth. In their studies on captive female mink, Farrell and Wood (1968c) calculated this, requirement as about 13.3 g water/ 100 g body weight/day and indicated that under the conditions pertaining during their work the average mink obtained 667» of its requirement from the water associated with the feed, 147o from fluid water and 207o from metabolic water. They point out that this ratio will vary, depending primarily upon the moisture content of an animal's food. On my study areas and presumably elsewhere along the Pacific coast, fresh water is available for much of the year in the form of pools of rain water in natural concavities. Permanent streams and seepages are common along mainland shores but on many low-lying coastal islands they are rare. During the July-August dry period there are many areas, especially among the small rocky islets of Barkley Sound, which support mink but which are apparently devoid of fresh water. On some of the more heavily vegetated islands there may have been seepages which I was not aware of, but on others I was certain that no source of fresh water was present. Mink living under such circumstances either had to rely on moisture in food, drink seawater, or travel to distant waterhples. The latter practice would result in considerable range overlap between individuals in some areas. I did not devote special attention to the problem of how coastal mink meet their water needs, but I kept alert to.opportunities to record pertinent observations throughout the period of field work. 156 RESULTS AND DISCUSSION Mink caught i n l i v e t r a p s were often very t h i r s t y and immediately sought water upon release. On no occasion was a mink seen to drink from the sea, even though t h i s was the clo s e s t source of water i n many cases. Some drank from r a i n pools which seemed somewhat brackish to my taste, but near-by tidepools were us u a l l y ignored. One animal, apparently t h i r s t y nearly to the point of stress, "tasted" several tidepools i n succession but drank very l i t t l e from them. I t was encountered minutes l a t e r drinking heavily from a r a i n pool. I did not often see free roaming animals drinking, but a l l which I saw drank fresh' water. On three occasions a known juv e n i l e male traveled i n the open . for one-fourth mile or more to drink from a stream at one end Of a Vargas Island beach. This movement was within the.animal's r e g u l a r l y used range, and I was not aware of much movement by other mink to that stream even during the dry season. I gathered no evidence that mink r e g u l a r l y enter the home ranges of other mink to obta i n water, but I suspect that some do. The animals do not "congregate" around water sources, however; as w i l l be shown, mink are d i f f i c u l t to trap during the l a t e summer and at one point I speculated that t h i s might have been due to the e f f e c t s of water shortage on mink movements. This remains a p o s s i b i l i t y , although l i v e t r a p p i n g along i s l a n d streams was no more successful than that along coast shores during the dry season, and mink sign (tracks, droppings) was never unusually abundant along these streams. Water requirement increases with increasing energy intake ( F a r r e l l and Wood 1968c), and the extent to which mink could be s a t i s f i e d by moisture i n foods r e g u l a r l y eaten on the coast i s unknown. As shown previously, 157 various species of decapod crabs predominate i n the die t during the dry time of year and, since these probably provide less body moisture than do marine fi s h e s (see Schmidt-Nielson 1964), i t at le a s t appears that the mink are not s e l e c t i n g food to maximize water intake i n that way. Probably at l e a s t some free water i s necessary. On many mornings during the summer, vegetation i s wet from condensation or deposition by fog, and mink may supplement t h e i r water intake by l i c k i n g up water at th i s time. Regardless of how they do i t , some mink which may be seen day after dry day on small waterless i s l e t s remain i n good health through the summer, thus they are apparently obtaining a l l of the nutrients, including water, which are needed. 158 SIZE DIMORPHISM AS A TROPHIC STRATEGY INTRODUCTION AND METHODS I showed e a r l i e r that males among mink from most areas average almost twice as large (weight) as females. Such dimorphism seems true of most small mustelids, e s p e c i a l l y those i n the genus Mustela, although i t i s s l i g h t i n the smallest species, the l e a s t weasel, and according to H a l l (1951) within species i t i s u s u a l l y greatest for the largest subspecies. Consistent with t h i s pattern i s the apparently minimal difference i n size between the sexes i n the small ermines (M. erminea anguinae) of Vancouver Island and M. e_. haidarum from the Queen Charlotte Islands (Cowan and Guiguet 1965) and i n a small subspecies of mink (M. vison aniakensis) from the Kuskokwim area of Alaska (Burns 1964b). Thus, whatever one concludes about the b i o l o g i c a l s i g n i f i c a n c e of si z e dimorphism i n mustelids must eventually be r e c o n c i l e d with the fact that i t i s not u n i v e r s a l , even with-i n species. There i s l i t t l e pertinent information about any mustelids i n the wild, however, and for the present exceptions w i l l have to be ignored. Rand (1952) speculated that some sexually dimorphic characters might be concerned with the reduction of inter-sex competition for food. Selander (1966) provided evidence of t h i s for several species of birds and Schoener (1967) demonstrated feeding differences c o r r e l a t e d with sex differences i n siz e i n an anoline l i z a r d . Brown and Lasiewski (1972) show that the charact-e r i s t i c elongate body shape of weasels i s "a s a c r i f i c e i n energetic e f f i c i e n c y " , and " p r e d i c t " that reduction i n intra-species competition through sexual dimorphism i n size i s one of the factors which enables them to e x i s t . The only published f i e l d information on t h i s point appears to be that of Sealander (1943), who examined a series of winter-caught mink i n Michigan. 159 In at least p a r t i a l support Of the above hypothesis, he found that males took inuskrats ( i . e . , large prey) s i g n i f i c a n t l y more often and small mammals and perhaps f i s h less often than did females. My data r e l a t i n g to possible d i f f e r e n t i a l niche u t i l i z a t i o n by the sexes are from analyses of digestive t r a c t s , examination of feeding middens, and analyses of observations, the general methods for which have been described previously. For the observation data I oc c a s i o n a l l y recognized i n d i v i d u a l s and could record t h e i r sex with confidence; more-..often. 1 :Co.uld l i s t only r e l a t i v e s i z e , i n which case the sex i s implied but not c e r t a i n . In following analyses, these two kinds of data are handled separately. Opportunities to check field-determined sizes against sex were afforded when I l a t e r livetrapped recognizable animals and, i n summers of 1971 and 1972, when I c o l l e c t e d specimens of both sexes. Animals l i s t e d as "small" were almost always females although i n some cases (fewer than ten per cent) they were unusually small j u v e n i l e males. A lean appearance, e s p e c i a l l y i n the neck, and a "hyperactive" behavior pattern were^as c h a r a c t e r i s t i c of animals considered small as was s i z e . The si z e c l a s s i f i c a t i o n "medium" was the one most subject to error (about f i f t e e n per cent). In some cases i t was the category used when I was uncertain, although i t was l i s t e d i n a p o s i t i v e sense most of the time. Most medium animals were juvenile males although both lean (often a i l i n g ) adult males and unusually heavy-bodied females (as during pregnancy) were l i s t e d i n t h i s category at times. "Large" animals, a l l thick-necked, r e l a t i v e l y slow-moving adult males, were r a r e l y confused for anything els e . RESULTS 160 Table 11 l i s t s frequencies at which the two sexes, (and sizes) of mink were seen hunting under d i f f e r e n t conditions, provides a comparison of the frequency with which each caught (or f a i l e d to catch) prey and, for ob-servations of successful hunts, compares the extent to which they caught d i f f e r e n t kinds of prey. I had the impression that females often hunted longer a f t e r the slack low tide than did males, but the data shown do not in d i c a t e that e i t h e r sex r e g u l a r l y reduced competition by e x p l o i t i n g a higher t i d e l e v e l . I observed both sexes hunting i n protected, rocky habitats far more often than ;on open, low p r o f i l e shores, but there was a strong tendency for large animals ( i . e . , mostly males) to hunt the open areas more often than did small ones. This i s r e f l e c t e d further i n a comparison of hunting methods observed, which shows that females (and small animals) were hunting c h i e f l y by poking during more than h a l f of the observations while males dove and bird-dogged for most of t h e i r prey. Comparing the r e s u l t s for the sexes and t h e i r respective sizes i n Table 11, there i s just one case i n which the general trend of frequencies l i s t e d do not agree. During observations of known females, f i s h were caught more often than crabs, a r e s u l t which would be expected i f these animals were indeed hunting mostly by poking i n rocky habi t a t s . However, "small" animals, presumably mostly females, caught crabs more often. I have no explanation for t h i s discrepancy. Other evidence indicates, however, that there i s considerable overlap i n foods eaten by the two sexes, at least during some times of the year. Examination of digestive t r a c t s from animals c o l l e c t e d i n summer revealed prey occurrences as follows: 161 Table 11. Evidence of differential niche utilization by the sexes, Vancouver Island mink, from observations of free-ranging animals. Sex of Mink Female lo Male # % Size of Mink  Small Otherb # % # % TIDE Low Low Other0 33 4 89 11 44 6 88 12 <0.90 118 18 87 13 167 24 87 13 <0.99 HUNTING HABITATS BB + RW 27 79 30 70 EF + SB + ES 7 21 13 30 <0.90 115 9 93 7 125 42 75 25 <0.001 HUNTING METHOD Diving Poking Bird-dogging HUNTING SUCCESS Caught 0 Prey Caught Prey e PREY CAUGHT Crabs Fish Unident. 5 25 19 . 48 29. 39 50 43 11 55 10 26 39 52 38 33 4 20 10 26 <0.10 7 9 27 24 <0.01 2 29 4 17 11 41 11 21 5 71 20 83 <:0.50 16 59 42 79 <0.10 7 28 30 61 22 43 64 54 13 52 8 17 13 25 16 13 5 20 11 22 <0.01 16 32 39 33 < 0.20 a 2 Probability of X. value as large as that obtained with data shown. Other mink sizes = medium + large 'Other tides: High Low, Low High and High High dHabitats: BB (Boulder Beach) + RW (Rockweed Shore) = relatively protected; EF (Eelgrass Flats) + SB (Small Particle Beach) + ES (Estuary) = open (relatively exposed). eCaught one or more prey during period of observation. 162 PREY PRESENT Frequency (per cent) Crabs Fish Other Female (N=43) 49 42 9 Male (N=39) 56 28 16 X =1.9, df=2, p < 0.50 PREY DOMINANT Crabs Fish Other (N=30) (N=29) 50 40 10 72 21 7 X =3.2, df=2, p< 0.30 The {tendency for males to take more crabs and fewer fish than females do is again apparent, although the degree of overlap prevails, in the statistical comparison. Opportunities to determine relative sizes of prey caught by the sexes were rare; data on estimated size of prey caught by observed mink, as used in the energy requirements section, could not be meaningfully compared for the sexes because the sample for known females is too small. However, small, medium and large sized mink caught prey with an average estimated net weight of 15.2, 18.1 and 19.3 g, respectively. A statistical comparison of these results indicates that prey taken by small (i.e., mostly female) and those taken by large (male) animals differed l i t t l e in size (t= 1.03, df = 107, p<.0.30) but this appears to be due largely to the considerable variability among the data for small mink. A few small animals took very large prey (up to 150 g) thereby inflating both mean and variance. Although the largest prey listed for large mink was 80 g, these animals caught large prey more often and small prey less often than did small mink. This is shown clearly in the following table: 163 Prey Size: No. (?„) of Observations Mink Size 0 - 10 g 11 - 25 g 26+ g Small 28 (61) 13 (28) 5 (11) 2 xMl.5, df=2, p<0.01 Large 18 (29) 30 (48) 15 (23) During the course of my wanderings over the Vargas Island study area, I was aware, at a subjective level, that crab carapaces accumulated more slowly in middens andar.eas traveled by females than in those used by males, and this seems to further support the idea that females subsist largely on smaller prey, especially fish. However, i t also appears to be true that females are less prone to eat items near shore and i t may be that in most cases the residue from their feeding was left in secure, often-underground middens which I did not find. Comparing sizes of prey items in middens belonging to males versus those of females is risky, both for the above reason and because usually onlyXlargeiitems appear in middens. In April 1970, a male and female "shared" a small, treeless islet near Vargas Island, but they maintained separate middens and latrines. Both were clearly subsisting almost entirely on crabs caught in the same waters, and a size comparison of these appeared valid. On the basis of net weights converted from carapace measurements of the three species involved, the male's prey (mean = 25.1 g, range = 7-90) were only slightly larger than those of the female (mean = 23.5, range = 8-52; t = 0.69, df = 152, p<0.50). This result indicates that there may be, at times, almost complete overlap in prey taken by individuals of different sexes, but i t does not necessarily mean that females are capable of handling any prey which can be handled by 164 by a male. At times I saw males exerting themselves considerably to remove, from the water, large crabs which were c l i n g i n g with t h e i r chelae to marine vegetation, and i t seemed u n l i k e l y that females could have managed. Two males which I saw get severely pinched by large red crabs p e r s i s t e d a f t e r they broke away and subdued the crabs involved, while a female under the same circumstances f l e d when the crab f i n a l l y released her. An adult female mink which I found f r e s h l y dead at the edge of a Barkley Sound boulder beach was autopsied and appeared to have drowned. I speculated that some i n t e r t i d a l animal, perhaps a large red crab, had held her under water; other circumstances which might have led to t h i s drowning were not apparent. Duririggthe -observations, hunting success (catching prey versus not catching prey, regardless of observation duration) was somewhat greater for males than for females, but observations of known females were too few for r e s u l t s to be meaningful s t a t i s t i c a l l y . Small mink were unsuccessful i n hunts much more often than were larger ones., (both medium and l a r g e ) , and the proportion of successful hunts for large animals alone i s 85 per cent, s i g n i f i c a n t l y higher than the 59 per cent success indicated for small mink 2 X = 4.20, df = 1, p-i-0.05). Comparing longer hunting periods (>5 minutes) which yi e l d e d prey, hunting success (expressed as net weight of prey caught per minute of observation) d i f f e r e d l i t t l e for small mink (2.2 g; SE = 0.6) as compared to large ones (2.1 g; SE = 0.4) DISCUSSION The data r e l a t i n g to size dimorphism i n the coast mink suggest that there are indeed some differences i n feeding niche between the two sexes. 165 Most observations were made under the most favorable feeding conditions, which happen also to be the best for observation, and i t i s l i k e l y that differences at r e l a t i v e l y unfavorable times such as during storms or neap tides would be of greatest s i g n i f i c a n c e to the animals. I t i s also probable that the degree of overlap i n feeding niche varies with population density, greater overlap occurring at r e l a t i v e l y low population l e v e l s such as those p r e v a i l i n g during much of the study period, and less occurring as competition increases. The tendency for females to hunt more often than males by poking among rocky shore habitats remains with me as a strong subjective impression and i t i s supported rather well by the data given. Females, being smaller, can hunt such habitats more e f f i c e n t l y since they can e x p l o i t cracks and crevices which would exclude the big-headed, thick-necked males. Further, the fact that poking produces smaller prey than other hunting methods i s probably less c r i t i c a l to a female, whose t o t a l d a i l y food requirement during most of the year i s less than that of a male. Even though males frequently hunted boulder beaches and rockweed shores (Table 11), they usually hunted by methods ( e s p e c i a l l y diving) which y i e l d larger prey.. While the questions pertinent to the subject of dimporphism are l a r g e l y q u a l i t a t i v e i n nature, centering on the degree of overlap i n feeding habits between the two sexes, the question of whether one sex r e g u l a r l y hunts more su c c e s s f u l l y (catches more prey or catches prey more e f f i c i e n t l y ) than the other i s also of i n t e r e s t . A l l of the foregoing presupposes competition between the sexes and, since the data indicate that at times there may indeed be considerable overlap i n foods taken by the two sexes, one might expect 166 the less competitive sex.to be forced, on the average, to hunt i n less p r o f i t a b l e s i t u a t i o n s . The fact that large mink among my observations .did enjoy a higher proportion of successful hunts that did small mink i s suggestive. Intensive observations of a few known animals would probably be more i n s t r u c t i v e on t h i s point than my series of extensive records has been. 167 I I I . POPULATION ECOLOGY DESCRIPTION AND EVALUATION OF TECHNIQUES Material presented i n foregoing pages was based l a r g e l y upon observ-ations of free-ranging animals and th e i r "sign". To obtain c e r t a i n kinds of data on i n d i v i d u a l s (movements, range, reproductive condition, pathology) and populations (composition, numbers, turn-over) i t was also necessary to examine animals i n hand. A v a r i e t y of techniques was employed i n these studies, and most w i l l be described where appropriate i n following sections. However, capture and handling of l i v e animals provided the basis for most of the r e s u l t s presented l a t e r , and i t i s e s s e n t i a l to o u t l i n e , i n d e t a i l , the capture techniques used i n terms of the extent of e f f o r t and success, the d i s p o s i t i o n of specimens a f t e r capture, and the biases and l i m i t a t i o n s associated with these capture a c t i v i t i e s . Also basic to many of the con-clusions drawn i n subsequent pages i s the r e l i a b i l i t y of my methods for determining age of specimens, therefore s p e c i a l attention to t h i s subject i s also warranted i n t h i s opening section on population processes. LIVETRAPPING AND HANDLING METHODS Two models of c o l l a p s i b l e wire-mesh l i v e t r a p s from the National Live-trap Company, Tomahawk, Wisconsin, were used. The smaller, single door trap (15 x 15 x 48 cm) was more convenient to use than the double-door (15 x 15 x 60 cm) model because i t was l i g h t e r and more compact, and could be set i n locations at which the larger trap would not f i t . The small trap was somewhat less e f f i c i e n t i n that animals, e s p e c i a l l y experienced ones were occa s i o n a l l y able to remove b a i t without a c t i v a t i n g the treadle. 168 The double-door feature of the large trap i s to f a c i l i t a t e runway sets, but since such sets were not e f f e c t i v e i n my areas, the rear door was usually l e f t closed. Sixty-three per cent of a l l captures were made with the larger trap. Most sets involved the use of b a i t , both because mink were attracted to i t and because i t was advisable to provide food to help sustain those which entered the traps i n poor condition. The most convenient and ef-f e c t i v e b a i t was f i s h , and more than 90 per cent of a l l sets involved i t s use; b i r d remains, when they were a v a i l a b l e , also worked w e l l , and a few captures resulted from use of mollusks and crustaceans. In summer i t was necessary to change baits almost d a i l y because of spoilage, but they could be l e f t up to several days i n winter. In some areas i t was common for 50 per cent or more of the traps out i n any given night to be incapacitated or b a i t l e s s as a r e s u l t of interference from other animals, e s p e c i a l l y deer mice and land slugs (Limacidae, Arionidae). Where raccoons occurred i n numbers, successful l i v e t r a p p i n g for mink could be almost impossible because of t h e i r interference, and i t was necessary to avoid such areas for most trapping operations. Most traps were set well above the current high water mark, occasi-n a l l y among d r i f t debris, but most often along runways and i n natural c a v i t i e s (hollow logs, small caves, exposed root systems) above the beach-l i n e . E a r l y experience indicated that mink r a r e l y traveled more than about 50 m from the beach, and that ..trapping beyond that distance was l a r g e l y wasted e f f o r t . Traps set too near current high water were subject to flooding by storm-heightened t i d a l surges. I o c c a s i o n a l l y put traps out on low t i d e beaches i f I could be sure that I would return before inun-dation, but success rates for such e f f o r t was no greater than that for 169 traps set as described above. In a l l cases, traps were well covered with l o c a l materials, e s p e c i a l l y driftwood, to reduce exposure to r a i n and sun. Traps were usually checked i n the morning, since most captures occurred at night. The objective of l i v e t r a p p i n g operations was to enable c l a s s i f i c a t i o n and examination of as many animals using a given section of shoreline as possible. Trap d i s t r i b u t i o n , therefore, was based on the d i s t r i b u t i o n and abundance of "sign" and the known locations of animals rather than on a regular g r i d or transect system. For instances i n which a mink was known to be present but was d i f f i c u l t to catch, I might concentrate ten or more traps within 100m of shoreline; for long sections of shore with no sign and l i t t l e p o t e n t i a l as mink hunting habitat, adjacent traps might be separated by up to 500 m. However, an attempt was made to keep several traps within the range of any given mink, to enable documentation of i t s movements. On Vargas Island, traps were d i s t r i b u t e d i n an average density of about 10 per kilometer of shoreline, while the corresponding figure along the more densely populated shores of Barkley Sound was 17 per k i l o -meter. Up to 55 traps were tended during a single trapping session, a l -though the average was about 35. In i n t e n s i v e l y studied areas, e s p e c i a l l y Vargas Island and Barkley Sound, trapping was conducted once each month i f p ossible. Traps were usually l e f t set i n one place for four days. During the f i r s t year of study "trapping sessions" of up to 10 days were attempted, but the number of new animals caught a f t e r four to f i v e days was not worth the extra stress imposed upon those animals which might be caught repeatedly. Occasionally trapping sessions had to be terminated at two or 170 three days because of l o g i s t i c a l problems (equipment f a i l u r e ; i n a b i l i t y to procure b a i t ) and/or very severe weather. When a i n d i v i d u a l animal was caught i n the same trap two days i n succession, that trap was not r e s e t during the remainder of the s e s s i o n , and i n some cases where such a r e -capture of an i n d i v i d u a l was p r e d i c t a b l e on the b a s i s of past experience, the trap was l e f t c l o s e d a f t e r the f i r s t capture. This procedure helped to reduce s t r e s s on i n d i v i d u a l s and f a c i l i t a t e d i n f o r m a t i o n on movements. On the f i r s t capture during a given s e s s i o n , a mink was forced from the l i v e t r a p i n t o a wire handling cone (see P l a t e 4) where i t could be s a f e l y examined. During the f i r s t year of study animals were anesthetized w i t h ether as described by L o c k i e and Day (1964), but t h i s technique was inconvenient and i t o b v i o u s l y caused at l e a s t temporary i l l n e s s i n some animals. Once i n the handling cone, the animal was examined f o r gross a b n o r m a l i t i e s , general p h y s i o l o g i c a l and reproductive c o n d i t i o n , and age, then was marked, weighed and released at the capture s i t e . The s p e c i f i c c r i t e r i a f o r determination of reproductive status and c o n d i t i o n are described i n f o l l o w i n g s e c t i o n s (pp. 195 and 228, r e s p e c t i v e l y ) } t e c h n i q u e s f o r determination of age are d e t a i l e d at the end of t h i s s e c t i o n . A numbered f i n g e r l i n g tag (S i z e Number 1) from the N a t i o n a l Band and Tag Company, Newport, Kentucky, was placed on each ear, and these tags con-s t i t u t e d the primary means f o r i d e n t i f y i n g i n d i v i d u a l s . Because tag r e t e n t i o n was poor e a r l y i n the study, each ear was al s o tattooed w i t h a s i n g l e a l p h a b e t i c or numeric character (Tattoo K i t No. 101C from Ketchum Manufacturing Sales L t d . , Ottawa, O n t a r i o ) . However, I learned that tags face page 171 Plate 4: Steps i n the handling of a livetrapped mink a & b) Chasing the animal from the l i v e t r a p into a wire handling cone. c) Examining mink so confined. d) Attaching an ear tag. e) Weighing. f) Release. 172 placed through the thickest part of the ear, on the upper, anterior edge near the head, were seldom lost and tattooing was discontinued after two years. One animal retained both tags, correctly placed, for 27 months, and another lost only one in 34 months. As a further precaution:, against loss of identification, the ventral white pattern of each animal was diagrammed at first capture (see McCabe 1949; Gerell 1971). On the six occasions in which animals had lost both tags and 'either bore no tattoos or tattoos which were illegible, I was able to confirm identification from distinctive white patterns on all but one. To facilitate field identification of individuals, a small strip (about 5 x 15 mm) of nylon-coated vinyl "Saflag" (The Safety Flag.Co. of America, Pawtucket, Rhode Island) was attached with one of the ear tags. Addition of the Saflag material often resulted in loss of the tag, so application to both ears was not advisable; I had six colors" which could be distinguished in the field and these enabled distinction of up to. 12 animals with just one color on one ear. This was ample for Vargas Island and Tofino Inlet work, but in Barkley Sound i t was necessary to combine two colors on a single ear in a few cases. The maximum known retention of the flags was just under five months, although most mink shed them in less than two months. While confined in the handling cone (Plate 4e), animals were weighed to the nearest 25 grams with a Pesola spring scale of 2500 grams capacity (Pesola Scales, Waagon Strasse, Basel, Switzerland). In some cases animals became very wet in the livetraps, and in such cases weights were not taken. Minkcwere usually handled in the cone on only the first capture of a given session. On recaptures during that session they were 173 simply identified and released from the traps unless further examination was necessary (e.g., reweighing or observation of a new injury). RESULTS Trapping Success Table 12 summarizes livetrapping success over the years of study. An average effort of almost 20 trap nights was required for each of the 521 captures recorded, although effort per capture varied annually, locally, and seasonally. The differences in success between years and between areas are mostly reflective of the numbers of animals present. Low success during the first year (1968-69) resulted largely from my experi-mentation with different baits and sets at locations (e.g., far inland from the beach) which I did not expect to be productive, but for which confirmation of my suspicions seemed necessary. The year of poorest success, 1970-71, was the year of a dramatic decline of mink on Clayoquot Sound study areas, especially on Vargas Island, and the almost three-fold increase in success the following year was due to my moving operations to Barkley Sound, where the highest densities of mink were encountered. Seasonal differences were pronounced and consistent, with the best success in spring and early summer, the poorest in late summer and early f a l l , and with moderate to good success in winter. To clearly document the pattern from spring through f a l l , I trapped Chalk Island in Barkley Sound once monthly for 9 months between April 1972 and May 1973. Twenty-five traps were distributed around this island a week before the first trapping and they were left in place continuously for the next 14 months, thereby allowing mink to remain accustomed to them. They were baited only with fish and during the July, August, and September sessions, fresh 174 Table 12. Summary of mink livetrapping results, west coast of Vancouver Island, British Columbia, 1968-1973. TRAP NIGHTS CAPTURES TRAP NIGHTS/CAPTURE MONTHS Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 695 642 945 738 1911 1328 990 257 573 854 911 143 11 50 73 43 157 65 42 4 2 33 34 7 63.2 12.8 12 17 12 20 23 64.3 286.5 25.9 26.8 20.4 YEARS May Jul Jul Jul Jul ' 68-Jun '69-Jun ' 70-Jun ' 71-Jun '72-Jun '69 '70 '71 '72 '73 3058 1928 1628 2161 1212 104 106 46 161 104 29.4 18.2 35.4 13.4 11.7 AREAS Vargas Island Tofino Inlet Barkley Sound Other 3911 845 2970 2261 137 73 237 74 28.5 11.6 12.5 30.6 ALL 9987 521 19.2 175. bait was provided daily. As shown in Figure 19, success gradually decreased from April to June, declined abruptly and steadily from July through September, then dramatically increased again in October. As will be discussed, these seasonal differences in trapping success appear to be related to differences in the mink's seasonal demand for, and ability to obtain, food. It was evident that trapping success was poorest in calm weather, when wave action.on beaches was minimal, and during spring tides when intertidal exposure was greatest, and after the first two years of study I tried to avoid trapping under such conditions, especially during summer when both good weather and extreme tides often occur together. Differential Vulnerability to Trapping Of 206 different mink taken in livetraps during the years of study, 148 were released at regularly trapped locations within the three main study areas. Sixty-two of these (42 per cent) were never seen again, but the remaining animals were recaptured from 1 to 16 times each (Mean = 3.4). Appendix 15 lists the distribution of recaptures among the various sex and age classes in the three main areas of study. Generally, juvenile males constituted the class most often recaptured and males were retaken at least once after the initial capture more often than were females, especially in Barkley Sound, but differences were not large in either case (p<0.20). The fact that there were differences between areas suggests that these results should be interpreted with caution. The low recapture frequency at Tofino Inlet in comparison with that at the other two areas, for instance, probably reflects the fact that many of the animals involved were first caught along the Tofino Village Waterfront, where chances for mortality from recreational trapping and domestic dogs is high. That is face page 176 Figure 19. Seasonal variation in livetrapping success at Chalk Island, Barkley Sound, April 1972 - May 1973. 176 n C> 3. «° tj CO o r " CO Lf) in C CO 3 Tf 6 "2 CL ° < W O CO o LO o eo o CM CL -C o co £ Z 177 probably many were not recaptured because they were no longer a l i v e . The data of Appendix 15 represent varying periods of contact with i n d i v i d u a l s , and under widely varying circumstances, and probably do not accurately portray differences i n v u l n e r a b i l i t y to trapping among,the sex and age classes which could be i d e n t i f i e d . I t i s necessary to consider the p o s s i b i l i t y that t h i s d i f f e r e n t i a l v u l n e r a b i l i t y e x i s t s , since i t could bias trapping r e s u l t s , i . e . , some classes might be caught out of pro-portion to t h e i r occurrence i n the population. Comparisons devised to detect bias i n trap response u t i l i z e r e s u l t s of trapping sessions which, as described e a r l i e r , are short periods of time (usually 4.days) during which the status of any given i n d i v i d u a l should have remained r e l a t i v e l y uniform. The v a l i d i t y of the comparisons depends upon the v a l i d i t y of the following assumptions: 1. Animals caught early i n a trapping session are more vulnerable than those caught l a t e r . 2. Animals recaptured i n a given session are more vulnerable than those which are not, and an increasing number of recaptures per session indicates increasing v u l n e r a b i l i t y . A condit ion of both assumptions i s that animals of a l l classes were equally exposed each day to the opportunity to be trapped, and I believe that my trap placement made i t l i k e l y that t h i s condition was met i n most cases. C l e a r l y the above comparisons deal only with the trappable population, that i s , animals trapped at least once. There were some animals which were seen frequently, were apparently resident on my study areas for periods of up to 10 months, but which never entered l i v e t r a p s . Of seven such animals for which I had evidence of sex, four were females and three were large males, thus a difference between the sexes i n t h i s regard i s not apparent. 178 There was no statistically significant tendency for any one sex or age class to enter traps consistently early or late in a trapping session, and this was true both within and between areas. However, females appeare in traps on the first day of a session more than three times as frequently during the mating season (May and June) as during other times of the year 2 (X = 5.3, df = 1, p<0.05). This suggests that females may be especially vulnerable at that time, although i t should not be interpreted to mean that males are not. Although they did not enter traps proportionately earlier in summer, as shown in Figure 20, they were caught four times as often as were females in that season. The proportion of females in the trapped sample did not increase appreciably until the August-November season, when adults were lactating and juveniles first became susceptible to livetrapping. For any one sex and age class, differences in recaptures per session were not evident between areas so data have been lumped. As shown in Appendix 16, juvenile males were caught more than once per session far more often than were adult males outside the mating season (p<0.05). Within the mating season, adult males were caught in such multiple situations more often than at other times of year, and males generally were recaptured more often than were females (p<0.10 for both comparisons) In their tendency to enter traps more than once per session, adult males differed l i t t l e from females of either age class during most of the year, and were almost identical to juvenile males in this regard during the mating season. Among juveniles, the sexes responded to traps similarly throughout the year; adults were apparently as vulnerable as juveniles during the mating season, but were significantly less so during the rest face page 179 Figure 20. P r o p o r t i o n a l incidence of the sexes among l i v e t r a p p e d mink during the seasons of study, Vancouver I s l a n d , B r i t i s h Columbia, 1968-1973. 100 80 x2=7.45,df = 2,p<0.05 60 male 40 female 20 DJFM AMJJ n = 87 n=193 A S O N n:53 SEASONS 180 of the year (p<0.01). There is also evidence that tendency to enter traps repeatedly is related to physiological condition. Of 253 sessions in which individuals were recaptured once or less, just 12 per cent involved animals rated in poor or fair condition, while almost 30 per cent of 47 animals recaptured more than once were so rated (X2 = 6.3, df = 1, p<0.02). Individual Response to Livetraps As implied above, individual mink varied widely in their apparent vulnerability to livetrapping, some entering traps repeatedly in any one session, others entering them only one to a few times over a period of months, and s t i l l others refusing to enter them at a l l . I was able to watch mink responding to baited traps on several occasions. Behavior observed at these times ranged from panic-flight in one female encountering a trap for the first time, to complete disinterest in the case of a very large male which I knew well but never succeeded in catching. The latter animal regularly used one of the traps as a springboard from which i t leaped to a nearby log on the route to its midden. Some animals which had been caught previously, especially those in poor condition, entered traps unhesitatingly and immediately began eating the bait with no regard for the clang of the closing trap door. In one case an animal moved ahead of me after release and was found in three successive traps. Most.-experienced animals responded to traps with ambiv-alence, and attempted to get at the bait by means other than through the entrance. Most commonly they attempted to dig under the trap from the rear or side, and i f they succeeded i t was often possible for them to grab 181 the b a i t through the wire mesh. The pieces of wood and stone I used to cover traps (usually placed both along the sides and on top) were designed to prevent other approaches to the b a i t as well as'..-to reduce exposure to weather, but mink were sometimes able to remove these materials. Animals which f a i l e d to f i n d an alternate route to the b a i t always returned to the entrance and, i n the.instances of my d i r e c t observations, eventually entered the trap; t h i s , too, was done with much ambivalence. C h a r a c t e r i s t i c a l l y they greatly elongated themselves, stretching the head forward into the trap but taking - as few steps forward as pos s i b l e . In one case a young male succeeded i n taking the b a i t from a short, s i n g l e -door trap without touching the treadle. Six others f a i l e d and were caught, and a l l but one of these immediately forgot the b a i t and began attempts to escape. DISCUSSION Stokes and Balph (1965) have shown that sample biases i n w i l d l i f e studies are frequently mediated by behavioral t r a i t s of i n d i v i d u a l s or classes of i n d i v i d u a l s . They found seasonal differences i n trap response between male and female Uinta ground s q u i r r e l s as a r e s u l t of increased male a c t i v i t y during the mating season and apparently increased s o c i a l dominance of females thereafter. Chapman and Trethewey (1972) recorded seasonal differences i n capture rates of ea:S;t.eijn ;cottiDntaills and r e l a t e d these to weather conditions. There were no apparent seasonal differences between sex and age classes, but i n terms of tendency to be recaptured, adults appeared to be more d i f f i c u l t to trap than were juve n i l e s , and males were more so than females. Similar r e s u l t s obtained for cotton rats by Summerlin and Wolfe (1973) were shown to be r e l a t e d to s o c i a l dominance, 182 the highest ranking animals emerging as more trap prone than subordinates. In this case the attractant used was the scent of cotton rats, not a food bait. Pack e_t al. (1967) showed that dominant gray squirrels, again mostly adult males, did not allow subordinates access to artificial feeders and, even when dominant animals were not nearby, the lower ranking individuals used feeders very cautiously and with much anxiety. This result suggests that dominant individuals might be more likely to enter baited traps than would subordinates, but as Balph (1968) points out, although eating bait is rewarding, capture is punishing. He noted that experienced ground squirrels reacted to baited traps in the same way that the experienced mink of my observations did, that is, with considerable ambivalence. Further, as I indicated earlier, captured mink often lost interest in the bait when the trap door closed so that they received only the punishment of capture and handling, but no reward at a l l . This was particularly true of large, vigorous individuals; juveniles usually ate a l l of the bait and animals in poor condition always did so. A few in poor condition actually accepted bits of fish which I offered them while they were s t i l l in the trap. Since I used baited traps almost exclusively, the response to traps I have reported is in part a response to an extra food source. I have shown previously that mink catch and eat a variety of small animals in the areas I studied, and that they also scavenge food items at times. Finding bait in a trap most closely represents scavenging, and an animal might refuse such food i f i t were adequately meeting its nutritional needs in other ways, i f i t found the bait food less palatablei(e.g.,, not as fresh) 183 than other foods i t could obtain, i f i t had been punished by previous capture or by a nearby dominant animal, or for combinations of the above reasons. Acceptance of the b a i t , on the other hand, could r e s u l t from the converse of any of the. above reasons but i s most l i k e l y r e l a t e d to an animal.!>s need, for food beyond what i t has recently caught. The fact that capture success was better i n stormy, weather than during periods of calm and better during neap tides than during spring,tides i s almost c e r t a i n l y due to the r e l a t i v e d i f f i c u l t y which animals had i n obtaining natural foods at such times. As I have shown, adult.males entered traps most r e a d i l y during the mating season, a time when they are expending considerable energy i n search of females and probably have t h e i r highest food requirement. G e r e l l (1971) also found that adult male mink were more vulnerable to trapping during the mating season and, as did I, found that j u v e n i l e s , e s p e c i a l l y males, constituted the classes most prone to repeated capture i n one trapping session. He also f e l t that females were generally easier to capture than were adult males; my r e s u l t s i n -dicated that adult females were most trap-prone when they were l a c t a t i n g , again a period of maximum food requirement, but that they were probably neither more nor less vulnerable to trapping than were adult males during the r e s t of the year. Results both from Sweden and from my study areas indicate that the cla s s of mink most l i k e l y to be taken out of proportion to i t s occurrence i n the population i s the j u v e n i l e male, while adult males may be under-represented i n livetrapped samples during most of the year. Differences 184 in trap response appear to be slight during the mating season but other factors, especially increased activity among males, may nevertheless lead to biased trapping results at that time. Overall trapping success is greatest from October through April, when weather conditons are generally poorest and natural foods such as crabs are least available; trapped samples probably most accurately represent true population composition during that period, especially in the later half when the animals are coming into reproductive condition. AGE DETERMINATION METHODS To determine the age of livetrapped mink in the field, I employed a combination of criteria used in studies of small mustelids elsewhere. The young of both sexes were recognizable by body size alone up to the age of about four months; after that time juvenile females were difficult to distinguish on this basis although most males did not attain full adult stature until they were more than ten months old. The degree of wear on the upper canines was a useful indicator of age when used in conjunction with other characteristics, but could not be used reliably alone. A number of authors (see Greer 1957) have demonstrated the accuracy with which juvenile males may be distinguished from adults on the basis of size and conformation of the baculum. These differences between adult and juvenile stages of development are detectable by palpation on most specimens, and most recent mustelid studies, including mine, have used this technique with good success. For females, signs of past reprod-uctive activity such as conspicuous teats (Newby and Hawley 1954) or 185 evidence of neck i n j u r y from mating ( M i t c h e l l 1961) provided the only means of r e c o g n i z i n g " a d u l t " females among otherwise f u l l y - g r o w n i n d i v i -duals. The term ad u l t was defined as an animal which had a t t a i n e d an age of one year, that i s , one which had survived to J u l y of the year f o l l o w i n g i t s b i r t h (see reproduction s e c t i o n ) . Since most i n d i v i d u a l s of both sexes were r e p r o d u c t i v e l y a c t i v e to at l e a s t some degree at an age of 10 to 11 months, t h i s c u t - o f f date seems b i o l o g i c a l l y j u s t i f i a b l e . F u r t h e r , although some males had not developed the t h i c k necks, f u l l weights, and bolder behavior of o l d e r animals, and were t h e r e f o r e r e c o g n i z a b l e as y e a r l i n g s , many young males and v i r t u a l l y a l l young females, except i n the cases of tagged animals w i t h known h i s t o r i e s , were i n d i s t i n g u i s h a b l e from o l d e r animals by J u l y . I n p r a c t i c e , using the above d e f i n i t i o n , a l l animals of ad u l t s i z e taken from J u l y through November could be i d e n t i f i e d w i t h confidence, as a d u l t s . From December through about March, males could be r e a d i l y separated i n t o a d u l t and j u v e n i l e c l a s s e s by baculum morphology, and i n many cases body s i z e d i f f e r e n c e s alone were s u f f i c i e n t . A d u l t females which had bred i n the past year remained r e c o g n i z a b l e during the w i n t e r , although r a t h e r complete h e a l i n g of wounds i n c u r r e d during mating and gradual atrophy- of mammae l e d to i n c r e a s i n g chances f o r e r r o r as t h i s season progressed. For both the winter season and the p e r i o d from A p r i l to June the terms "known" or "suspected" were used to modify age determinations made. Animals w i t h c h a r a c t e r i s t i c s p r i m a r i l y of one age c l a s s , but w i t h one or more of these c h a r a c t e r i s t i c s c o n f l i c t i n were suspected to be i n that age c l a s s (e.g., a very l a r g e male w i t h worn 186 canines, but with a small baculum, would be considered a suspected adult). Known age animals were tagged animals with known histories, especially those tagged as small young, and those for which all criteria for a given age class were met. To check the reliability of field assigned age classes, to provide more precise age data for correlation with reproductive and pathology information, and to assess the longevity of animals on my study areas, I counted layers in the periostial zone of a dentary bone from each of 117 usable specimens obtained during the study. Klevezal' and Kleinen-berg (1969) used this technique for a series of known-age ranch mink and found that the periostial layers corresponded in number to age in years for about 80 per cent of the animals they examined. The remaining 20 per cent had one extra layer as a result of a supplementary "adhesion line" (the boundary between layers) which had formed during the second year. The above authors found that the most well-defined annuli could be found in the section of jaw in the vicinity of the last premolar and first molar teeth, and all of my sections were cut from that area.. On practice specimens, sections of two thicknesses (70 and 140 microns) were taken and samples of these were either decalcified and stained with hematoxylin prior to mounting on slides, or were mounted with no further preparation. The thinner sections provided the best detail and were used on all sub-sequent specimens. These were mounted immediately after cutting since the decalcifying and staining did,not materially improve readability. The details for preparation of sections are therefore as follows: 187 1. Skeletal material was cleaned by boiling in water, by boiling in a weak KOH solution, or by use of a dermestid beetle colony, and were then stored dry. Specimens boiled in KOH, especially the younger ones, became very fragile and prepared jaw sections appeared to be subject to fracture along adhesion lines. Klevezal' and Kleinenberg (1969) reported that whitening agents such as hydrogen peroxide also caused damage to the periostial zone of their material. 2. One dentary, usually the left i f i t were intact, was de-hydrated in an alcohol series and then mounted in plastic resin. 3. Three transverse sections 70 microns thick were cut from near the center of the jaw, as described above, and were then put into temporary storage in a weak formalin solution. The cutting implement was a Bronwill Thin Sectioning Machine, from Bronwill Scientific Co., P.O. Box 277, Rochester, New York. 4. Sections were examined wet under a binocular dissecting microscope at high power, and the best of the three for each specimen was then set aside for mounting. In some cases all three sections were equally readable and only one was required on a permanent mount, but in most cases at least two and sometimes all three were necessary to minimize reading error. In a few cases, jaws which had yielded poor sections were recut. 5. Sections which had been selected in the previous step were removed from the storage solution, blotted partly dry, then mounted on clean glass slides with glycerin jelly. A cover slip was applied and the slide was then turned over to dry, the weight of the slide being used in this case to hold the section flat. In some cases a small additional weight was necessary. 6. Mounted specimens were examined under a Leitz Gmbh stereo dissecting microscope usually at medium power (about 16x), and using the combination of transmitted and reflected light which best illum-inated or shadowed adhesion lines. Colored filters, especially green, enhanced reading by providing contrast. 188 7 . R e a d i n g i n v o l v e d l o c a t i n g t h e s i n u o u s " r e s o r p t i o n l i n e " a t t he b o u n d a r y o f t h e p e r i o s t i u m and t he m e s o s t i u m , t h e n c o u n t i n g t h e a d h e s i o n l i n e s d e l i m i t i n g l a y e r s f r o m t h a t p o i n t t o t h e o u t e r edge o f t he b o n e . RESULTS AND DISCUSSION S i n c e w i l d m i n k a r e p r o b a b l y on l e s s u n i f o r m f e e d i n g r e g i m e s t h a n t h o s e k e p t i n f u r f a r m s , i t i s l i k e l y t h a t b o t h g r o w t h r a t e s and r a t e s o f bone d e p o s i t i o n a r e more v a r i a b l e . K l e v e z a l ' and K l e i n e n b e r g (1969) r e p o r t e d good ag reemen t be tween age i n y e a r s and t h e number o f a d h e s i o n l i n e s i n t he p e r i o s t i u m o f f a r m m i n k ; t h e y i n d i c a t e t h a t t h e f i r s t a d h e s i o n l i n e fo rms i n abou t O c t o b e r , i . e . . , r o u g h l y s i x months a f t e r b i r t h , t h e r e f o r e an a n i m a l w i t h one l i n e , by t h e i r m e t h o d , i s f r o m o n e -h a l f t o abou t one and o n e - h a l f y e a r s o l d . Among my s p e c i m e n s were 39 a n i m a l s w h i c h , a s s u m i n g a J u l y b i r t h f o r a l l , were known o r s t r o n g l y s u s p e c t e d to have been f r o m two t o t w e l v e months o l d a t d e a t h . None o f s e v e n a n i m a l s f o u r months o l d o r y o u n g e r had y e t f o rmed an a d h e s i o n l i n e , a l t h o u g h s i x o r s e v e n h a d s i n g l e l i n e s a t s i x m o n t h s . A l l bu t one o f t he 25 a n i m a l s be tween s i x arid t w e l v e months o l d had a t l e a s t one l i n e , and f o u r o f t h e s e (one o f f i v e 1 1 - m o n t h - o l d mink and t h r e e o f t e n a t 12 months) h a d two l i n e s . The s e c o n d l i n e s p r o b a b l y r e p r e s e n t s u p p l e m e n t a r y a d h e s i o n l i n e s , as d e s c r i b e d e a r l i e r ; K l e v e z a l ' and K l e i n e n b e r g (1969) s u g g e s t t h a t t h e y a r e c a u s e d by an abno rma l i n t e r r u p t i o n i n g r o w t h d u r i n g d e p - . o s i t i o n o f a p e r i o s t i a l l a y e r . , The above r e s u l t s i n d i c a t e t h a t a n n u a l a d h e s i o n l i n e s f o r m i n t he p e r i o s t i u m o f bones f r o m c o a s t m ink by abou t J a n u a r y , so t h a t a l l a n i m a l s h a v i n g no s u c h l i n e s and a n i m a l s c o l l e c t e d be tween J a n u a r y and June h a v i n g one l i n e a r e j u v e n i l e s as I have p r e v i o u s l y d e f i n e d them i . e . , a n i m a l s 189 less than 12 months old. Acknowledging that some error probably occurs as a result of formation of supplementary lines, animals collected between July and December which have one line and most animals showing two or more lines are adults. Unfortunately I had no known age specimens between the ages of 13 and 22 months and therefore cannot certify that the second annual line actually forms at about 18 months, as I would predict. However, two specimens known to have been 23 and 24 months old,respectively,each bore the expected two lines. A female suspected to have been 24 months old had three lines, but the error in this case may have been due either to the presencetof. arsuppdementary line or to faulty age determination at first capture. I had only two older specimens whose age was known within one year. One male known to have been 36 months old at death had three lines, as expected; another was believed to have been adult when first caught in April 1969 (i.e., at least 21 months old) and i f this were the case, was 55 months old (5 lines expected) when captured in February 1972. In fact, only four lines were present but there was substantial deposition beyond the fourth line and i t is possible that the fifth line was being formed. Although more known age wild specimens are needed to assess the extent of the error resulting from supplementary adhesion lines and to confirm the timing of formation of annual lines, this age determination technique shows considerable promise. It is evident that the number of adhesion lines in the periostium closely approximates age in years, and throughout the remainder of this paper the term "laboratory age" will refer to the number of such lines counted in a given specimen or series of specimens. 190 Given this more precise indicator of age, i t is of interest to assess the reliability of age determined for livetrapped specimens by field criteria (hereafter referred to as "field age"). The specimens for which laboratory age was obtained were given a field age prior to autopsy, and comparison of the two methods gives the results shown in Table 13. Of the 117 specimens examined, 106 (91.7%) were ,p,l aced in the same age class by both techniques. It appears that the most common age determination errors in the field were in classing some juvenile females as adults, and some adult males as juveniles. The former error occurred only during and after the mating season, a period in which almost a l l females show signs of reproductive activity and are therefore difficult to distinguish. The apparent misidentification of adult males as juveniles may involve animals with unusually poor development, although some of these may not be field errors at a l l , but are laboratory errors due to the presence of supple-mentary adhesion lines. Juvenile males were occasionally called adults, by field criteria, but adult females were never mistaken for juveniles. The above results indicate that field ages applied to livetrapped animals are quite reliable and that, given proper consideration for biases which arise in the latter months of an animal's first year, these field ages are useful for analyses of age-related phenomena among trapping results. 191 Table 13. Comparison of age determinations from field and laboratory criteria, wild mink from Vancouver Island, British Columbia. Field Age Sex laboratory Age No. Adults No. Juveniles Agreement (per cent) Adult Male Female Both 45 28 73 3 5 8 93.3 84.9 90.1 Juvenile Male Female Both 3 0 3 19 14 33 86.4 100.0 91.7 Field Age: Adult-animal known or suspected to be 12 months old or older; Juvenile-animal known or suspected to be less than 12 months old. Laboratory Age: "Agreement: Adult-animal with two or more lines in periostium, or, having just one line when collected between the months of July and December; Juvenile-animal with no lines in the periostium, or, having one line when collected between January and June (see text). Proportion of animals placed in same age class by both methods. 192 REPRODUCTION INTRODUCTION Interest in enhancing production among fur farm animals has stimulated considerable research on the breeding biology of the mink, and the literature holds a wealth of information on this subject. The term "breeding" as used here, refers to the entire reproductive process, from "mating" (copulation) through parturition. Summarizing from the major reviews of Enders (1952), Hansson (1947) and Venge (1973) and the work of Pearson and Enders (1943), Shackelford (1952), Enders and Enders (1963), and Onstad (1967), the general pattern of mink reproduction may be synthesized as follows: Sexual activity, which involves animals in their first year (about 10 months old) as well as older animals, is limited to a single, short mating season each year, commencing in February or March in most areas. Responding to the stimulus of increasing daylight, the testes of males undergo up to a five-fold increase in size by the peak of the mating season, while the gonads of females approximately double in weight by that time. Both sexes, but especially the males, become much more active and excitable as the mating season approaches. In the ranch situation, the receptivity of a female is usually detected by testing her with a male. In order to mate, even i f the female is receptive, the male must first overpower her and grasp her by the nape of the neck with his teeth, a process often more closely resembling fighting than courtship. Ovulation is induced, and i t follows copulation and/or the stimulus of the courtship battle by approximately 42-52 hours. Both superfecundation and superfetation appear to be normal, 193 i.e., ova from a single ovulation may be fertilized by successive matings within a day or two of the ovulatory stimulus and, even after the form-ation of corpora lutea, further ovulations and conceptions commonly occur. The duration of estrus for individual females averages about three weeks (maximum about five weeks) and the occurrence or non-occurrence of copulation, ovulation and/or pregnancy do not appear to alter the duration. iEailingtovulatiori',-:. ,v a female enters at once into anestrum. If ovu-lation has occurred, pseudopregnancy (rarely) or pregnancy follow. Obser-vations from many hundreds of pregnancies indicate that the mean length of gestation in the ranch mink is about 51 days;, but, with a range of about 40 to 75 days, this is characterized as perhaps the most variable mammalian gestation known (Pearson and Enders 1943). Actual gestation from implan-tation is about 30 days, and a delayed implantation of from about 10 to 45 days accounts for the observed variability. Enders (1952:724) stated that "Kits born on the forty-first day following copulation appear to be slightly immature and, on our limited experience, are not likely to survive". He implies that delayed implantation always occurs, and that young arising from pregnancies with the shortest knownodel.ays^about 10 days) are the least viable. The longest gestations appear to occur in those females which mate earliest, while the females which breed late in the season have shorter pregnancies. Thus, the period of parturition at any one fur farm is shorter than the period during which copulation occurred; the young are born early in May in most areas. Litters as large as 17 have been reported, but few exceed 10 and the average varies up to about 4.5. The mink involved in fur farm studies are hybrids and mutations arising from several of the known wild subspecies. According to Enders (1952), 194 Mustela vison vison from eastern Canada and the northeastern United States, M. v. mink ranging somewhat south of the previously mentioned subspecies, and M. v. ingens from northern Alaska are the three main subspecies involved, although three other far north subspecies, lacustris, lowii, and melampeplus, and others farther south including letifera of the central United States and energumenos of western Canada and the northwestern United States have probably contributed to some herds. Apparently many ranchers s t i l l intro-duce wild stock into their herds from time to time. It is likely, there-fore, that the farm findings summarized above are applicable, in at least a general way, to animals in the wild. Indeed, afterr.an intensive study of factors affecting reproductive success in farm mink, Gilbert (1968) concluded that these animals were l i t t l e removed from the wild state. Actual comparative field information from wild population is almost non-existent. The commercial fur-trapping season in most areas is over before breeding, so specimens from this source have been of no assistance in providing direct reproductive data. Further, since placental scars disappear and genitals decrease greatly in size shortly after parturition (Elder 1952), winter caught specimens do not even provide indirect evidence of the breeding history of individuals. Some information has accrued from observations of gonadal development and activity patterns of wild indivi-duals, (Mitchell 1961; Gerell 1971; Harbo 1958), observations of pregnant females which had been livetrapped and brought into the laboratory (Svihla 1931; Gerell 1971), and observations of litters at dens (Harbo 1958; Burns 1964). However, virtually the only measure of productivity in wild pop-ulations has been the proportion of juveniles in trapped samples, and much furbearer research has been directed toward the development and perfection 195 of techniques for distinguishing fully grown juveniles from adults (Petrides 1950; Elder 1951a; Lechleitner 1954; Greer 1957; Birney and Fleharty 1968). During this study, I had unprecedented opportunities to contact mink during the breeding season. The following pages compare my field obser-vations with the published laboratory findings summarized above, and consider how these broader aspects of mink reproduction may be related to the welfare of individuals and the status of populations. Histological data obtained furing the course of this work will be presented elsewhere. METHODS Techniques for the study of reproduction in my coast mink populations were similar to, and sometimes coincidental with, procedures for studying other aspects of li f e history. Early morning observations at low tide during the summer months produced information on the increased activity of males during that season, and provided documented descriptions of mating behavior. Radio telemetry yielded some data on movements of individuals during the mating period and assisted in the location of two natal den sites. Examination of livetrapped animals throughout the year provided data on the sex and age composition of populations and gave a continuous record of gonadal development and timing of b'ther reproduction-related events. The testes of livetrapped males were examined by palpation and then given a relative size rating based on objects of known size: small (barley to pea size), medium (larger than a pea, but smaller than a marble), and large (marble size or larger). The vulva of females was rated as "swollen" or "not swollen" and, after blowing into the fur in the inguinal area, mammary development was indicated as "conspicuous" or "inconspicuous". Conspicuous teats were usually measured to the nearest mm with a small straight rule. In addition 196 to the above, livetrapping also provided information on changes of repro-ductive status and condition of individuals during and after the mating s eason. In 1968, 1969 and 1970, I did l i t t l e trapping on the main study areas during July and August in order, to prevent additional stress on pregnant and lactating females, and thereby minimize my influence on the composition of populations. By 1971 and 1972 it was evident that I needed more data from that time period, so I continued to trap intensively and I also em-ployed a programme of collecting females and selected males along shores adjacent to the main study areas. In 1973 a few known animals were col-lected within the Barkley Sound and Tofino Inlet study areas. As described in the food habits section, collection was usually done with a 12 gauge shotgun, although whenever possible animals were livetrapped and then dis-patched with an overdose of Nembutal administered intraperitoneally. Freshly killed female specimens were examined for mammary development and reproductive status (inactive, pregnant, lactating) and the repro-ductive tracts were removed and preserved in A.F.A. for later study. Data gathered for this report include counts and assessment (viable conceptus or resorption site) of uterine swellings and size measurements of fetuses. Total length (in mm, nose tip to tail tip) was obtained by laying a string around the curve of the spine and then measuring the string with dial calipers. For small embryos, the calipers were used directly to measure crown-rump length. Each conceptus was then separated from placental tissue and weighed to the nearest 0.1 g on a portable field balance. The gonads of males were trimmed of connective tissue, leaving epididymides attached, were measured (length and width) with dial calipers, and were 197 then weighed together on the balance mentioned above. RESULTS AND DISCUSSION SEX RATIO The composition of populations as determined from my livetrapping results is difficult to interpret. For most areas, theppppulationccontacted for any given year runs high to males, often by a ratio of 3:1 or more, and if only adults are considered male-dominated ratios of more than 4:1 were common. However, these annual figures do not accurately portray ratios present at any one time, and when the data are broken down into shorter time periods such as seasons, one is faced with a series of figures which may not be comparable. For example, since the different sex and age classes were shown to differ seasonally in their apparent susceptibility to live-trapping, i t is probably not valid to make inter-seasonal comparisons. Comparing the same seasons between years is less riskyj however the fact that my trapping effort varied from area to area between years may have resulted in my biasing results by sampling different proportions of existing populations. Contingencies (weather conditions, interference from vandals) and the occasional necessity for exploiting new data sources elsewhere often dictated the amount of trapping effort I could undertake at any given time. Table 14 lists seasonal sex ratios observed on the main areas studied. On the basis of seasonal differences in vulnerability to trapping, as out-lined previously (pp. 175-180) and assuming a population with a stable sex ratio of 1:1, I would expect the following results for the three seasons of study: April through July - preponderance of males as a result of their 198 Table 14. Seasonal sex ratios observed in mink populations along"the west coast of Vancouver Island, British Columbia, 1968 -1973. Location 1968-69 1969-70 1970-71 1971-72 1972-73 a b e Season n ratio n ratio n ratio n ratio n ratio Vargas Island AMJJ 13 3.3 11 0.8 19 1.7 ASON 10 2.3 7 0.8 4 1.0 DJFM 9 1.3 14 1.3 Tofino Inlet AMJJ ASON DJFM 6.0 2.5 6.0 4.0 16 3.0 1.7 Barkley Sound April-May June-July ASON March+May 25 2.1 12 2.0 24 2.0 17 1.4 28 3.7 Seasons: AMJJ = April - July; ASON = August - November; DJFM = December -March. 'n = sample size ratio = number of males per female Vargas Island data includes known animals not trapped; all other areas livetrapping results only. 199 greater activity prior to and during the mating season; August through November - preponderance of females due to their increased energy demands during lactation and the reduced activity of males during this period; December through March - the least biased results, but perhaps slightly skewed in favor of females due to their having less secure home ranges than do adult males (see pp.261-267 )• With these expectations in mind, the data in Table 14 are summarized and qualified in following paragraphs. Vargas Island - Of the areas studied, this had the poorest mink habitat, therefore, the least dense population, and individuals (some of which I never trapped) were much easier to keep track of than was the case on the other study areas. I had not yet learned a l l of the Vargas Island mink hunting and denning spots during the f i r s t season of study (AMJJ, 1968-69), and females are certainly under-represented in the sex ratio shown in Table 14. Data for the following season (ASON) are probably more reflective of actual population composition, while the winter data probably under-represent males, as expected. The information from April through November in 1969 is biased by my focussing activities in the middle one-third of the study area to the extent that several mink ranges were not trapped or observed during that time period. In other seasons and other years, most of these other ranges were occupied by males, so I am certain that males s t i l l predominated, and data for seasons following support this view. The AMJJ season in 1970 was the period during which a severe die-off occurred (p.231), and a surprising influx of animals during that season, perhaps representing a stress movement in response to whatever caused the die-off, was again high to males. After the die-off, the sex ratio of the four known remaining animals was 1:1. 200 During the winters of 1968-69 and 1969-70, temporary periods of snow cover made i t possible for me to record activity along the entire shore-line of the Vargas Island study area. Although i t appeared that there were at least a few animals in the population which I had not contacted, track sizes suggested that most of these were males. In fact, some may have been known males which traveled farther than I had previously known them to. From the data of Table 14 and from hundreds of hours spent walking and boating along the Vargas Island shoreline, I can state with confidence that the sex ratio there was high to males, probably by a factor of about two to one, from my arrival in May 1968 until the die-off of 1970. Tofino Inlet - Livetrapping on the waterfront and islets of Tofino Inlet was designed primarily to provide information about individuals. Traps were often placed at some distance apart at locations appearing to be, or knownitoobe, favored by hunting mink, and such areas were most often occupied by males. Hence, the data in Table 14 are biased. The winter (DJFM) data for 1971-72 resulted from the most intensive trapping effort applied in this study area. Traps were placed along the entire Tofino waterfront without regard for known hunting spots, and were tended for sessions of from four to ten days. The sex ratio obtained, at 3:1 in favor of males, may be higher than that actually present, but this result and casual observations suggest that males did indeed predominate. Barkley Sound - For practical reasons, the rugged shores of the Broken Group Islands were worked primarily during the summer months. As shown previously, effort in terms of trap placement and time spent in obser-vation was intensive although, because the animals in this area appear to be better fed and more secretive than in other areas studied, it is possible that less than half of those present were caught. Judging from 201 the predominance of males during the season of apparently greatest female v u l n e r a b i l i t y (ASON), the figures shown for the other seasons appear re-a l i s t i c . I have no other evidence which would indicate that females might have been more abundant than the l i v e t r a p p i n g data ind i c a t e , but high mink numbers and densely vegetated, rugged habitat made attempts at such assessments d i f f i c u l t . There i s l i t t l e among the published l i t e r a t u r e with which one may com- .. pare the above r e s u l t s . Harbo (1958) livetrapped i n summer on an i n t e r i o r Alaskan study area exposed to commercial fur trapping i n winter. Males outnumbered females i n the commercial catch by approximately 2.5:1, but the livetrapped sample was dominated by females (2.1:1) i n one summer and was 1:1 i n two others. Adult females outnumbered adult males throughout these l i v e t r a p p i n g operations. G e r e l l (1971) recorded a predominance of females (average about 1.5:1) i n one exploited population of f e r a l mink i n Sweden. Another population which he livetrapped averaged s l i g h t l y high to females (1.2:1) during the f i r s t year immediately following heavy ex-p l o i t a t i o n by fur trappers, but under complete protection subsequently, the male segment of the population again predominated (average 1.3:1 during the second year of protection and 1.4:1 during spring of the following year). I t should be pointed out that none of the sex r a t i o s l i s t e d by G e r e l l and only a few from my study areas (Table 14) depart s i g n i f i c a n t l y from 1:1 at even the 0.10 l e v e l (binomial tests, chi-square t e s t s ) . I t w i l l take more sophisticated methods and larger sample sizes to provide conclusive evidence; however, trends i n r e s u l t s given above suggest that mink pop-ulations exposed to regular "predation" are l i k e l y to have nearly even sex r a t i o s . On the other hand, those s u f f e r i n g l i t t l e mortality from 202 extrinsic sources, for example those on my study areas where there was virtually no predation and l i t t l e , i f any, commercial trapping pressure (see mortality section), may build up a preponderance of males. This also appears to be true for marten (Quick 1956b). Other authors have related high male:female ratios in mustelids to high population levels and/or declining food supplies. Elder (1951b) noted that increasing populations of weasels (subgenus Mustela) contained about 407o females, while those in the decline phase were down to about 20% female. Lockie (1966) demonstrated that on one study area female weasels (M. nivalis) disappeared completely following a population crash of their small rodent prey, and Weckwerth and Hawley (1962) showed that the decline of a marten population following a small mammal crash was precipitated by loss of females. These observations suggest that the build-up of males in an unexploited population with a relatively stable environment (including food supply) may be a natural consequence of competition be-tween the sexes, especially considering the female's smaller size and added burden of maternal responsibilities. The '"problem" of excess males in samples of mustelids has troubled biologists for years. It has usually been assumed that the ratio should be 1:1, and the greater boldness, larger cruising radius, and perhaps social dominance of males have all been used to explain away observed sex ratio disparities. Indeed, such factors probably do have the effect of biasing results in favor of males, for Greer (1956a) and Quick (1956b) have shown that among mink and marten,respectively, males greatly out-number females in commercial harvests early in a trapping season, but as the season progresses, the cumulative sex ratio increasingly approaches 203 1:1. On the basis of observations made earlier, trapped populations would be expected to have approximately equal sex ratios, so this result is not surprising. However, i t does indicate that errors of interpre-tation can result i f sex ratios are calculated at the wrong time. Whether this applies equally to removal trapping and livetrapping is not known. The important point to be made here, and i t was also emphasized by Lockie (1966), is that^under.some circumstances a mustelid population can be genuinely low in females. . I am convinced that such circumstances prevailed on my Vancouver Island study areas during this study. BREEDING SEASON As shown in Figure 21, testicular recrudescence in some individuals began as early as February, but full-sized testes were not common until late April. In May and June almost all animals had large testes, but by July regression had begun and from October through January, a l l animals encountered had small testes. As measured on specimens taken throughout the year, testes rated "small" weighed less than 2.0 g (weight of pair) with the minimum about 0.4 g for juvenile males taken in October and November. Paired weight for testes of adult, males in the resting state was about 1.0-1.5 g. Medium ratings involved testes weighing about 2.0-4.0 g, while large testes weighed about 4.5 g or more, up to a maximum of 7.1 g recorded for one adult male taken on 31 May 1971. Mean weight of paired testes for 21 animals collected.between mid-August and late March was 1.2 g (Standard Error = 0.2), while for 35 animals taken be-tween mid-April and the end of July the corresponding figures were 4.6 g (SE =0.3). Among females, some degree of vulvar swelling was recorded in each of the months April through July. The precise nature of this swelling is face page 204 Figure 21. Monthly reproductive condition of livetrapped mink, Vancouver Island, B r i t i s h Columbia, 1968-1973. (Explanations i n text) ° • N • D 1 15 Testis S ize [H small medium large Vulvar swelling H swollen • not O 4* ^ H 8 M a m m a r y Development conspicuous • not 205 not known, but i t was presumably r e l a t e d to estrus i n at least A p r i l through June, and seems to have accompanied the l a t e r stages of pregnancy, including p a r t u r i t i o n , i n July. Conspicuous, palpable teats, i n d i c a t i n g past or current l a c t a t i o n , were present throughout the year, but decreasing frequencies from January through A p r i l suggest that either formerly developed mammae tended to regress and become inconspicuous or that adult females were under-represented i n samples during that time. Frequencies increased i n May and June, as animals became pregnant, and.all females taken i n July and August had conspicuous teats. Decreasing frequencies af t e r that time are due to the appearance of juveniles i n the trapped sample. Figure 22 depicts evidence of mating a c t i v i t y as derived from be-h a v i o r a l observations and examination of livetrapped and c o l l e c t e d specimens. I saw a copulating p a i r on 18 June 1969, and l o c a l residents R. McLeod, D. Arnet and J . Wilkowski (personal communication) witnessed copulations on 17 June 1969, 3 and 9 June 1972,respectively. In addition, I saw pa i r s involved i n what was c l e a r l y postcopulatory grooming on 31 May 1969 and 29 May 1971. Including these s i x observations, four unsuccessful mating chases and two instances i n which mating sounds were heard but the animals involved were not seen, a l l recorded mating a c t i v i t y occurred between la t e May and 11 July. Open necfc~wounds.voccurred only, i n late-May and June, while many of the wounds observed i n the l a s t h a l f of June and a l l of those seen i n July were i n various stages of healing. Females pregnant with measureable embryos ( t o t a l or crown-rump length 5 mm, or greater) were taken between 26 June and 19 July and, as face page 206 Figure 22. Monthly evidence of reproductive activity in wild mink, Vancouver Island, British Columbia, 1968-1973. Explanation of Symbols: A) Mating Activity Open triangles - copulation Open circles - post-copulatory grooming Closed circles - unsuccessful mating chases x - apparent mating activity; detected by sound, but not confirmed by sight. B) Neck Wounds x - wound present, but severity unknown Open circles - raw, open wounds Verticallyc.divided circles - scabby, healing wounds Closed circles - wounds mostly.rlhealed, with new hair growing in C) State of Pregnancy Closed circles - at least one fetus present; mean size as indicated on vertical scale Triangle - newborn kit (about two days old). Squares - animals with neck wounds, but not yet showing uterine swellings, x - implantation apparently occurring l J i F | M | A i M , J , J 1 A i S | O | N | D | M A T I N G ACTIVITY I I I j A A A A | •> • • I oo " | j (12 observations) 1 24 May- 18 June N E C K i H\ 3. ! • OoOO 1 Oo1 o 1 • | (42 observations) W O U N D S c 1 78 May - 77 July mm) 100 i ! 1 A FETUS g> 80 i j 1 • • ! SIZE * 60 i 1 A ' • • i rump 40 : ! 1 w • • ! crown -20 ; 1 # l • • • • • o 207 shown in Figure 22, the trend was for increasing size of fetuses over this time period. Four females collected earlier, on 31 May, and 5, 11 and 15 June, all bore neck wounds, but did not yet show uterine swellings, although the May animal did show evidence of luteal activity in one ovary. Lactating females were seen only in July (5), August (1) and September (1), with the earliest record on 23 July. A nearly hairless kit, 95 mm long and certainly less than a week old (see Enders 1952:731) was found near the entrance to a Vargas Island den on 2 August 1969, (Plate 6d) and another kit of about the same size was apparently left by its mother on:a trail near Willis Island on 27 July 1971. All of the above evidence indicates that, on my Vancouver Island study areas, mating begins during the last half of May and terminates around the middle of June, and most litters are born in.rmi'd-.to late July. This is by far the latest documented breeding season in this species. As indicated by Enders (1952) and others, fur farm animals throughout North America mate mostly in February, March and occasionally into April, and both farm and feral animals in Europe have retained this schedule (Gerell 1971; Hewson 1971; Aliev and Sanderson 1970). Svihla (1931) kept a pair of wild Louisiana mink which copulated in late January (young born in mid-April), but in New York (Coues 1877), Michigan (Marshall 1936), Iowa (Waller 1962), and Montana (Mitchell 1961), February and March are again the main months of mating. There is some evidence that reproductive activity is somewhat later in west-central Alaska. On the basis of personal communication information, and by back-dating from June births using Enders' (1952) average gestation period of 51 days, Harbo (1958) and Burns (1964a) inferred that their Alaskan mink mated in mid-to late April. 208 From laboratory studies on farm mink (Pearson and Enders 1943; Khronopulos and Drozdova 1957; Bowness 1968; Duby and Travis 1972), i t is apparent that manipulation of photoperiod can hasten or retard both the mating season and the timing of implantation. Some aspect of photoperiod therefore serves as the proximate factor controlling these processes, but it is likely that, seasonal favorability for survival of mother and off-spring serves as the ultimate factor (see Lloyd 1970). In a comprehensive review of the subject, Sadleir (1969a) has documented the vulnerability of mother and young during the period from late pregnancy through lactation and, has shown that especially in temperate regions where a "fixed optimal season" occurs within the annual cycle, a natural selection has set the mating season at one gestation length ahead of this optimal period. He points out, however, that for animals with delayed implantation (such as the mink), the reason why the mating season occurs when i t does is less evident. Since the entire reproductive cycle of my coastal animals differs substantially in timing from that known over the rest of the species' geographical range, a comparison of environmental factors pertain-ing during this cycle seems warranted and will be presented in the section on delayed implantation. MATING Of at least equal interest to the subject of when the mating season occurs is a consideration of what i t entails. All available evidence indicates that copulation in mustelids is strenuous and vigorous, --"often furious" (Enders 1952). My observation of 18 June 1969, summarized from 209 Appendix 17A-2, w i l l serve to i l l u s t r a t e : I had been watching a small female mink hunting on a tide-exposed reef, but she had temporarily disappeared behind i t . I was attacted by sudden squealing from that d i r e c t i o n and, as I watched, a known, tagged adult male appeared, dragging the female into a small protected hollow at the top of the reef. She struggled and squealed vigorously, but he held the back of her neck f i r m l y i n h i s teeth and she was un-able to escape. During the i n i t i a l excitement I was able to move to within 20 feet of the animals, and I watched the mating from that distance. For the f i r s t f i v e minutes the female struggled and vocalized frequently, but did so only o c c a s i o n a l l y as time went on. I t was cl e a r that each time the female struggled, the male tightened h i s grip on her neck, and at times he v i s i b l y chewed. He maintained the neck hold for the duration of the mating, which was 1 hour and 50 minutes. Both animals showed signs of fatigue a f t e r -ward; the male, i n fa c t , appeared to have a temporary para-l y s i s i n the hindquarters, and he collapsed when he attempted to jump over a rock to follow the female. Of i n t e r e s t r e l a t i v e to the above mating are i t s duration and i t s v i o -] sncfc lence. Consulting the d e f i n i t i v e work on ranch mink reproduction (Enders 1952), i t i s apparent that t h i s was an average performance. He states that copulation i s t y p i c a l l y prolonged, with intromission l a s t i n g up to 3 hours i n some cases. The mean duration for 227 matings, as recorded by Hansson (1947) was 64 minutes. Regarding the violence of the act, Enders has recorded incidents i n which males have k i l l e d females when, tak an improper neck grip during mating, t h e i r canines have penetrated the base of the s k u l l to the brai n . He states that "... t h e . l i k e l i h o o d of accidental death i s increased by resistance on the part of the female". As mentioned e a r l i e r , both superfetation arid superfecundation occur in the mink, and the duration of estrus i s not altered by ovulation, copu-l a t i o n or f e r t i l i z a t i o n . A consequence of th i s feature i s that females may be subjected to the violence of the mating process on several oc-casions over a period of weeks. About t h i s Enders (1952) writes "..death 210 from exhaustion i s not uncommon when females are exposed without r e s t to a series of vigorous males". Another summary of a seri e s of observations on my Vargas Island study area i s i n s t r u c t i v e here: On 31 May 1969, I v i s i t e d a small i s l e t to check on a tagged female which had taken up residence i n a hollow log there. She was present and, at my approach, she jumped down from the log and ran into a thick patch of vegetation. Seconds l a t e r I found her i n the grip of a large, tagged male from the adjacent Vargas Island mainland. In less than a minute he released her, and h i s subsequent behavior l e f t l i t t l e doubt that he had mated just p r i o r to my a r r i v a l . Upon her release, the female ran to her den. A short time l a t e r another male appeared and, a f t e r a chase, cornered the female near the den log. Following a b r i e f s c u f f l e , accompanied by much squealing, the female broke away and escaped into the log. In the two hours which followed, t h i s male (a tagged juvenile from a nearby i s l e t ) p e r s i s t e d i n h i s attempt to catch the female and nearly succeeded in one other b i t i n g , squealing encounter. He was s t i l l there when I l e f t . The next day I again saw a male on the i s l e t and I saw mink, presu-mably males, swimming there from Vargas Island on two of the following three days. On 5 June I again v i s i t e d the i s l e t and found the female curled up as though asleep, i n a spot where I had seen her sleeping previously, but she was dead. . As indicated, i t was apparent that the female had mated with the f i r s t male on 31 May (see d e t a i l e d notes i n Appendix 17A-5), and i f she was not bred again i n subsequent days, she at least was v i s i t e d and probably pursued frequently. To sustain her frequent defenses she doubtlessly had an i n -creased c a l o r i c requirement at t h i s time, but with one or more males maintaining s u r v e i l l a n c e nearby i t i s l i k e l y that she had d i f f i c u l t y hunting. Autopsy showed three punctures and numerous bruises on the back of her neck, but these were not serious enough to have caused death. Her dige s t i v e t r a c t , except for the extreme d i s t a l portion of the i n t e s t i n e , 211 was empty and she had no subcutaneous fat. I believe she died primarily as a result of continued harassment by the males. Other animals have showed signs of declining health at this time, and some have disappeared (see mortality section, p. 237). The extent to which neck wounds may have contributed to mortality of females is unknown, but as shown in Plate 5, these wounds were often extensive and in many cases they had become infected. Foott (1970) recorded open wounds and occasional infections on the faces of female sea otters, again wounds incurred during mating, and documented one death attributable to a "large nasal ulcer" apparently obtained in this way. Nearly every female mink I contacted from about mid-June through July showed evidence of neck wounds 30 mm or more in diameter. Even i f these wounds did not directly affect condition, they serve as an indication of the pressures to which females on my study areas were subjected during this season. Females on fur farms, which are usually bred twice over a period of weeks (Enders 1952), apparently do not develop such wounds. Assuming that farm males are not gentler than wild males, the implication is that most of my wild females were bred repeatedly. As indicated by Onstad (1967), frequent matings and other associated activity also places a strain on males during the mating season. Enders (1952) noted that one male "in rut" traveled a calculated 32 km on an exercise wheel overnight, and i t is possible that many wild males duplicate this in their movements back and forth in search of females. Gerell (1969) documented mating season movements of 5 and 11 km for one radio-tagged male, and indicated that male movements into new areas was common during that time. face page 212 Plate 5: Female neck wounds incurred during mating a) A raw, open wound b) P a r t i a l l y healed wound. c) P a r t i a l l y healed wound, only scab present. 212 213 Observations and recaptures of tagged animals and monitoring of three instrumented with radio c o l l a r s during May and June also provided evidence of expanded male movements on my study areas, although one-way distances involved were less than those recorded above. Resident males were found up to 1 km away from t h e i r usual ranges, although there appeared to be a tendency for them to return each day. The radio-tagged males, at l e a s t , always spent the bulk of t h e i r d a i l y rest periods within t h e i r known ranges, and were found elsewhere only when active. Unfortunately, these animals could not be monitored during the hours of darkness, and i t i s possible that both distances moved and time spent away were greater during that period. Animals whose home ranges were not known also appeared on study areas during the mating season, but I was not able to determine how far they had come. The greatest movement I detected was that of one juvenile male which was contacted on four of the f i v e major islands and two adjacent i s l e t s i n the T u r t l e Island Group between 17 May and 5 June 1972, covering a minimum c i r c u i t of about 5 km i n that period (see Figure 23). Perhaps a better measure of increased male: a c t i v i t y during the mating season i s the frequency with which they were seen " t r a v e l i n g " ( i . e . , moving from one l o c a t i o n to another without stopping to hunt or engage i n other a c t i v i t y --see d e f i n i t i o n s , page 127) during that period. Of 335 mink observations recorded during May and June, 80 (23.97o) were of t r a v e l i n g animals, while only 33 of 456 observations (7.2%) during the r e s t of the year involved t r a v e l i n g (X = 43.3, df = 1, p<0.01). Forty-three of these " t r a v e l i n g " incidents were water crossings, of which 38 (88.47=) occurred during May and June. Figure 23 i l l u s t r a t e s implied and observed water-crossings face page 214 Figure 23. Water crossings by male mink during the mating season, Barkley Sound, B r i t i s h Columbia, 1971-1972. 214 A C O C K C H A N N E L •••..TINY H A L K * . ^bS? 1 t I I I 0 Observed Movements Inferred Movements Movements of Juv. O* No. 380, 4 M a y - 2 2 June 1972 0 Km L 1/2 = 1 — 1 215 documented in the Barkley Sound area during the mating seasons of 1971 and 1972. One effect of increased movements among males would be the l i k e l i -hood of increased contacts between them and several aggressive encounters were observed during this period (see Appendix 17B). There was no in-dication that any injuries resulted from the agonistic behav-ior observed but further drain on energy reserves, especially in the case of sub-ordinate animals, is indicated. Interestingly, there is evidence that males do not fight over the possession of females (Appendix 17A-1, 5). Indeed, since more than one male may contribute to a litter conceived during one or more ovulations, there would be l i t t l e to gain in such a fight and, considering the likely consequences of injury to an animal which must actively seek its prey, there might be much to lose. The above observations suggest that, like the females, males have an increased energy demand during the mating season. Weight losses during this time are well known even among regularly fed ranch mink (Onstad 1967) and were also apparent among my wild specimens. A following section of this report, dealing with mortality, documents an annual "summer inanition" syndrome, involving weight losses, deteriorating condition, unusually high incidence of trap deaths, and disappearance of known animals. It seems evident that this syndrome is at least partly related to the rigors facing both sexes during the mating season. PRODUCTIVITY Twelve females taken during various stages of pregnancy carried apparently viable embryos and fetuses in the following frequencies: 216 in utero Litter Size Frequency 2 3 4 5 6 7 8 2 1 4 1 2 1 1 Mean = 4.6 Gerell (1971) recorded a mean of 3.8 young (range 3-6) for 10 litters from feral mink in Sweden, although i t is not clear at what stage of develop-ment these counts were made. I am aware of no other published information that might be construed as relating to primary litter size in wild caught mink, although means for large samples of ranch animals, up to 4.5 as given by Enders (1952), indicate that on my study areas, production at this level was about average for the species. As Harbo (1958) points out, i t is difficult to determine the number of young mink actually weaned by wild females but, on the basis of trapping results near dens and the assumption that there is a relationship between the post-nursing number of enlarged mammae and number of surviving young, he inferred that females on his interior Alaska study area raised an average of 2.6 young in 1956. I had no opportunities to count young in natal dens without causing severe disruption, and obtained counts of only three family groups (mother plus 1, 3 and 4 respectively) after the young had emerged from the den. As indicated earlier, assessment of mink productivity for fur manage-ment purposes has largely involved calculation of age ratios among winter-trapped samples. Because growth characteristics of the baculum have en-abled the best separation between the age classes (Elder 1951; Greer 1957) female specimens have often been ignored and ratios have commonly been expressed in terms of the number of juvenile males per adult male. Other 217 authors, e s p e c i a l l y those engaged i n l i v e t r a p p i n g , have attempted to de-termine age of females and have expressed p r o d u c t i v i t y as juveniles per adult female. My data and those of G e r e l l (1971) have been summarized and converted to these r a t i o s , and are.compared with r e s u l t s from other areas i n Table 15. Much of what was written about biases associated with my sex r a t i o information (p.197) also applies here, and these data should be examined for trend rather than d e t a i l . I have shown e a r l i e r that, i n response to l i v e t r a p s , juveniles are more vulnerable than are adults, and t h i s also appears to be true for capture techniques used by commercial fur trappers (Greer 1956b). Hence, i t i s l i k e l y that the juvenile pro-portions indicated i n Table 15 are somewhat higher than those a c t u a l l y present i n the populations studied. Since many of the figures i n Table 15 are averages over a period of years and, i n some cases, for two or more locations, some general comments about the i n d i v i d u a l , values composing them are i n order. For any given year, there i s a tendency for the highest proportion of juveniles to appear i n the catch during the period during and shortly a f t e r they emerge from natal dens, while the lowest proportion occurs i n the following mating season at which time the cohort i n question has been exposed to m o r t a l i t y factors for almost a f u l l year. This i s an expected r e s u l t , and the fact that i t appeared i n the data gives some cause for b e l i e v i n g that biases present may not be serious. Comparing l o c a l areas, Vargas Island and Barkr ley Sound d i f f e r e d l i t t l e i n terms of age r a t i o s among males, but number of young per female was greater i n Barkley Sound, perhaps because con-d i t i o n s for r a i s i n g young are better there. Table 15. Age r a t i o s of wild caught mink. No. Juv. Males No. Juveniles Per Ad. Male Per Ad. Female Mean Range Mean Range Source Vancouver Island, 1968-73 (L) AMJJ (Mating Season) 0.8 0.3 - 1.5 ASON (Young Emerge) 0.9 0.4 - 1.3 DJFM (Young Independent) 0.5 0.4 - 0.7 Southern Sweden, 1967-70 (L) Spring (Young mostly den-bound?) 0.9 0.4 - 1.2 Summer (Young Emerge) 2.7 0.8 - 4.8 Autumn (Young Independent) 1.5 1.4 - 1.5 Winter (Mating Season) 0.9 0.3 - 2.0 Southeastern Alaska Winter, 1956-57 ( C ) c 0.9 Int e r i o r Alaska Summer, 1956 (L) Southeastern Alaska (C) Winter, 1960 1.2 North Dakota Winters, 1953-55 (C) 4.5 4.0 - 5.2 Winters, 1953-63 3.1 1.0 - 5.2 Summers, 1963-66 (L) »' 2.4 0.4 - 4.5 Montana, State Sample (C) Winters, 1953-55 2.8 2.4 - 3.2 Madison R., Montana Winters, 1955-58(C) Summer & F a l l , 1955-58 (L) Winter & Spring, 1957-58 (L) Red Rock Lakes, Montana Summer & F a l l , 1957-58(L) This Study 1.9 0.3 - 3.0 1.9 1.0 - 3.0 1.8 0.7 - 3.0 G e r e l l (1971) 5.8 3.0 -11.0 4.2 1.5 - 5.5 2.9 1.0 - 4.7 3.1 1.5 - 7.0 Harbo (1958) 2.3 Harbo (1958) 1.4 Croxton (1960) 2.7 Hibbard (1957) Adams (1964) 15.9 2.0 -2