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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  presenting  this  an a d v a n c e d d e g r e e the I  Library  further  for  agree  in p a r t i a l  fulfilment  of  at  University  of  Columbia,  the  make  it  freely  that permission  this  representatives. thesis  for  It  financial  for  gain  Depa r t m e n t  University  of  British  2075 Wesbrook Place Vancouver, Canada V6T 1W5  Date  Columbia  for  extensive by  the  is understood  written permission.  The  British  available  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 h i s of  shall  thesis  shall  requirements  reference copying of  Head o f  that  not  the  I  agree  and  be a l l o w e d  that  study.  this  thesis  my D e p a r t m e n t  copying or  for  or  publication  without  my  ABSTRACT The mink ( M u s t e l a v i s o n evagor) which f r e q u e n t s the P a c i f i c c o a s t a l of Vancouver I s l a n d , B r i t i s h Columbia,  f o r a g e s p r i m a r i l y i n the marine  zone, f e e d i n g m o s t l y upon s m a l l c r u s t a c e a n s and f i s h . f a m i l y C a n c r i d a e are taken throughout they move i n t o i n t e r t i d a l waters most f i s h  shores  intertidal  Decapod c r a b s o f the  the y e a r , but e s p e c i a l l y i n summer when  to mate and moult.  K e l p c r a b s ( P u g e t t i a ) and  s p e c i e s appear to be most v u l n e r a b l e i n w i n t e r , when storms c r e a t e  instability  i n t h e i r near-shore h a b i t a t .  and the degree  Water depth, s u b s t r a t e p a r t i c l e  o f p r o t e c t i o n from heavy wave a c t i o n are among the most  f a c t o r s i n f l u e n c i n g the success o f mink h u n t i n g f o r these organisms. these f o o d - r i c h s h o r e s , most mink hunted p r o v i d e d t h e i r d a i l y requirements  size,  important Along  a t s u c c e s s r a t e s which would have  i n l e s s than two hours o f h u n t i n g  activity.  N e v e r t h e l e s s , o b s e r v a t i o n s o f i n d i v i d u a l s which hunted w i t h l e s s than  average  s u c c e s s , under v a r i o u s c o n d i t i o n s , i n d i c a t e t h a t a c c e s s i b i l i t y to food v a r i e s w i t h p l a c e and time, e s p e c i a l l y r e l a t i v e t o t i d e  level.  Males r e g u l a r l y outnumbered females  i n a l l areas s t u d i e d , and  the p r o p o r t i o n o f j u v e n i l e s i n study p o p u l a t i o n s was  lower than  a v e r a g i n g l e s s than two young per a d u l t female i n a l l seasons. season,  i n t h i s area, peaks d u r i n g l a t e May  expected, The  mating  and the f i r s t h a l f o f June.  D e s p i t e the f a c t t h a t t h i s i s two months or more l a t e r than has been r e c o r d e d f o r mink elsewhere,  t h e r e i s evidence t h a t the d e l a y e d i m p l a n t a t i o n  c h a r a c t e r i s t i c o f the s p e c i e s h a s b b e e n r r e t a i n e d .  Although both mating  and  p a r t u r i t i o n occur when g e n e r a l food a v a i l a b i l i t y and c l i m a t e are f a v o r a b l e , the a p p a r e n t l y s h o r t d e l a y (10 to 15 days) does n o t appear to enhance t h i s timing.  I t i s s p e c u l a t e d t h a t the d e l a y c o n s t i t u t e s a p e r i o d o f  i  convalesence  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 ii  s h o r e l i n e ) than  females (0.41 km) and, due to a few l o n g e r d i s p e r s a l move-  ments (up t o 9 km), j u v e n i l e males emerged as the most mobile c l a s s . There i s evidence  t h a t range s i z e was i n v e r s e l y r e l a t e d t o the q u a l i t y o f l o c a l  hunting h a b i t a t .  Although  much o v e r l a p o c c u r r e d , ..individuals  d i s t i n c t ranges ( t e r r i t o r i e s ) .  maintained  I n t r u s i o n s on p o r t i o n s o f a t e r r i t o r y  o c c u r r e d p r i m a r i l y when the r e s i d e n t t e r r i t o r y h o l d e r was n o t p r e s e n t t o a s s e r t h i m s e l f , although the primary counters, male.  avoidance r a t h e r than c o n f r o n t a t i o n appeared to be  method by which the animals  and o t h e r evidence,  keep, s e p a r a t e .  The r e s u l t s o f en-  i n d i c a t e t h a t mink s o c i e t y f a v o r s the a d u l t  R e l a t i v e to p o p u l a t i o n r e g u l a t i o n , the emerging h y p o t h e s i s  a l l members o f a p o p u l a t i o n are i n c o m p e t i t i o n probably  s u i t a b l e year-round  f o r a g i n g mink o f t h i s study  hunting  f o r some l i m i t e d  and, due t o the dominance o f a d u l t males, p r o d u c t i v i t y when t h i s  i s i n t e n s e , i . e . whentthe p o p u l a t i o n i s h i g h and/or when the sex  r a t i o i s h i g h to males.  A management i m p l i c a t i o n o f the h y p o t h e s i z e d  l a t i o n mechanism i s t h a t h h a r v e s t most l i k e l y  resource,  spots i n the case o f t h e l i t t o r a l  females s u f f e r i n c r e a s e d m o r t a l i t y and decreased competition  i s that  systems designed  to enhance p r o d u c t i v i t y .  iii  regu-  t o s e l e c t males a r e the  TABLE OF CONTENTS GENERAL INTRODUCTION  1 I . THE STUDY SITUATION  THE  STUDY AREA GENERAL DESCRIPTION PHYSIOGRAPHY CLIMATE VEGETATION INTERTIDAL HABITAT Tides F l o r a , Fauna, and S u b s t r a t e s SPECIFIC STUDY AREAS VARGAS ISLAND TOFINO INLET BROKEN GROUP ISLANDS  SEASONS OF STUDY THE  5 7 10 11 13 14 17 17 19  STUDY ANIMAL TAXONOMY AND PALEONTOLOGY DISTRIBUTION PHYSICAL CHARACTERISTICS SIZE PELAGE  21 22 23 25  II. FEEDING ECOLOGY FOOD HABITS INTRODUCTION METHODS AND MATERIALS ANALYSIS OF FECAL CONTENTS Field Collection L a b o r a t o r y Methods EXAMINATION OF FEEDING MIDDENS DIRECT OBSERVATIONS ANALYSIS OF DIGESTIVE TRACTS REPORTS FROM OTHER OBSERVERS RESULTS PREY TAKEN Crabs Other Crustaceans Other I n v e r t e b r a t e s Fish Birds Mammals Debris DISCUSSION COMPARISON WITH OTHER AREAS THE NATURE OF PREDATION BY MINK F a c t o r s A f f e c t i n g Prey S e l e c t i o n Preference Size o f Prey  iv  27 27 28 31 34 35 36 37 37 41 52 53 55 61 65 66 66 69 71 74  V u l n e r a b i l i t y and Abundance o f P r e y Crabs Fish Birds Mammals A v a i l a b i l i t y o f Prey : C o n c l u s i o n Effects of Predation E f f e c t s on M a r i n e Organisms E f f e c t s on S e a b i r d C o l o n i e s SUMMARY, FOOD HABITS FOOD GETTING INTRODUCTION AND METHODS HUNTING HABITATS S m a l l P a r t i c l e Beach B o u l d e r Beach Rockweed Shore Eelgrass Flats Estuary RESULTS AND DISCUSSION HUNTING METHODS Bird-dogging Poking Diving HANDLING PREY Eating Caching  ,  76 77 81 87 89 90 91 92 93 95 97 98 99 99 99 102 102 102 105 . 107 112 116 116  ACTIVITY PATTERNS INTRODUCTION METHODS RESULTS DIRECT OBSERVATIONS TELEMETRY DISCUSSION  122 122 126 134 139  ENERGY REQUIREMENTS AND INTAKE INTRODUCTION AND METHODS RESULTS HUNTING SUCCESS THE DAILY REQUIREMENT DISCUSSION  145 150 152  WATER REQUIREMENTS AND INTAKE INTRODUCTION AND METHODS RESULTS AND DISCUSSION  155 156  SIZE DIMORPHISM AS A TROPHIC STRATEGY INTRODUCTION AND METHODS RESULTS DISCUSSION  158 160 164  142  v  III. POPULATION ECOLOGY DESCRIPTION AND EVALUATION OF TECHNIQUES LIVETRAPPING AND HANDLING METHODS RESULTS Trapping Success Differential Vulnerability to Trapping Individual Response to Livetraps DISCUSSION AGE DETERMINATION METHODS RESULTS AND DISCUSSION REPRODUCTION INTRODUCTION METHODS RESULTS AND DISCUSSION SEX RATIO BREEDING SEASON MATING PRODUCTIVITY DELAYED IMPLANTATION FACTORS AFFECTING PRODUCTIVITY MORTALITY, CONDITION, AND PATHOLOGY INTRODUCTION AND METHODS RESULTS MORTALITY CONDITION PATHOLOGY Tooth Breakage and Wear Body Wounds Ectoparasites Endoparasites DISCUSSION PARALYTIC SHELLFISH POISONING SINUS WORM INFECTION TAIL WOUNDS SUMMER INANITION MORTALITY PATTERNS OF SMALL CARNIVORES POPULATIONS INTRODUCTION AND METHODS RESULTS AND DISCUSSION RESIDENTIAL STATUS MOVEMENTS AND RANGE SOCIAL BEHAVIOR General Territoriality POPULATION DENSITY AND TURNOVER POPULATION REGULATION IMPLICATIONS FOR FURBEARER MANAGEMENT vi  167 167 173 175 180 181 184 188 192 195 197 203 208 215 219 224 228 230 234 237 239 239 244 245 249 250 252 254 255 256 259 260 263 267 271 275 283 287  IV.  SUMMARY:  THE COASTAL MINK ON VANCOUVER ISLAND, BRITISH COLUMBIA  289  LITERATURE CITED  296  APPENDIX  313  vii  LIST OF TABLES TABLE "  PAGE  1  Climatic data from Tofino Weather Station, west coast of Vancouver Island, British Columbia, 1968-1972  2  Tidal extremes, Clayoquot Sound (Tofino), British Columbia, 1968-1972.  12  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  Spring and Summer 'foods of mink in Barkley Sound, Vancouver Island, British Columbia, 1971-1972, as determined by fecal analyses.  40  Foods in digestive tracts of west Vancouver Island mink, May-July 1971 and 1972.  43  Predominant foods of wild (North America) and feral. (Eurasia) mink (Mustela vison).  67  Proportional seasonal occurrence of sculpins (Cottidae) and blennioid fishes (Stichaeidae and Pholidae) in fecal samples from mink, Vancouver Island, British Columbia.  84  3  4  5 6 7  9  8  Hunting methods used by mink in various l i t t o r a l 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 freeranging 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  viii  PAGE  TABLE 14  Seasonal sex ratios observed in mink populations along the west coast of Vancouver Island, British Columbia, 19681973  198  15  Age ratios of wild caught mink.  218  16  Incidence of non-accidental trap deaths among livetrapped mink, west coast of Vancouver Island, 1968-1973.  235  17  Incidence of external pathological symptoms and ectoparasites in wild mink, Vancouver Island, British Columbia, 1968-1973.  238  18  Incidence of t a i l 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 livecapture histories, on Vancouver Island, British Columbia, 1968-1973.  262  Movements and range of mink along marine shores of Vancouver Island, British Columbia, 1968-1973.  266  Mink population numbers,composition and turnover on Vargas Island, British Columbia, 1968-1970.  277  Mink population age structure as determined from a sample of collected specimens, Vancouver Island, British Columbia, 1968-1973. '  282  21 22 23  ix  LIST OF FIGURES 1 2 3 4 5 6 7 8  9 10 11 12 13 14  Mink study areas, west coast of Vancouver Island, British Columbia.  6  Long-term climatic records, Tofino Weather Station, west coast of Vancouver Island, British Columbia.  8  Study locations in Clayoquot Sound, Vancouver Island, 1968-1973.  15  Study locations in Barkley Sound, Vancouver Island, 19681973.  16  Mean weights of livetrapped mink (all seasons) on the west coast of Vancouver Island, 1968-1973.  24  Incidence of unusually light-colored pelage among mink, west coast of Vancouver Island, 1968-1973.  26  Seasonal food habits, as determined by fecal analyses, of mink in Clayoquot Sound, Vancouver Island, 1968-1973.  38  Prey animals observed being captured by mink in l i t t o r a l habitats along the west coast of Vancouver Island, British Columbia, 1968-1972.  42  Annual and bimonthly variations in midden occurrences of crabs eaten by mink, Vancouver Island, 1968-1972.  44  Histograms of crab size classes eaten by mink, Vancouver Island, 1968-1972.  50  Crabs commonly eaten by mink, Vancouver Island, British Columbia.  51  Duration of 264 underwater dives by hunting mink, Vancouver Island, British Columbia.  109  Incidence of hunting, traveling, and sleeping of mink in relation to time of day and the time of the lowest tide.  129  Results of 84 census transects north from Rassier Point, Vargas Island, 1968-1969.  133  15 16  17 18 19 20  21 22 23 24  Activity of radio-tagged mink in relation to levels of the tide and time of day.  135  Activity of radio-tagged male mink along the waterfront in Tofino, British Columbia, February.1972, as determined by continuous monitoring.  137  Hunting success (net weight of prey taken per minute of hunting) for Vancouver Island mink, 1968-1972.  147  Mink hunting success in relation to several environmental variables, Vancouver Island, British Columbia.  148  Seasonal variation in livetrapping success at Chalk Island, Barkley Sound, April 1972- May 1973.  176  Proportional incidence of the sexes among livetrapped mink during the seasons of study, Vancouver Island, British Columbia, 1968-1973.  179  Monthly reproductive condition of livetrapped mink, Vancouver Island, British Columbia, 1968-1973.  204  Monthly evidence of reproductive activity in wild mink, Vancouver Island, British Columbia, 1968-1973.  206  Water crossings by male mink during t?he mating season, Barkley Sound, British,Columbia, 1971-1972.  214  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 Frontispiece  Page 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 t a i l wounds in Vancouver Island mink  242  8  More examples of t a i l wounds  243  9  Aspects of condition in Vancouver Island mink  248  xii  ACKNOWLEDGMENTS As i s always t h e case a t t h i s stage i n t h e " g e s t a t i o n " o f a m a n u s c r i p t , one becomes a c u t e l y aware o f t h e a s s i s t a n c e p r o v i d e d by o t h e r p e o p l e a l o n g the way.  I have, b e f o r e me, a l i s t o f more than 40 names, and gnawing a t  my i n n a r d s i s t h e c e r t a i n t y t h a t I have p r o b a b l y f o r g o t t e n some and, i n any event, c a n ' t mention  them a l l .  So, f i r s t ,  t o those I do n o t l i s t below, I  n e v e r t h e l e s s extend my g r a t i t u d e . Among those who can n o t be f o r g o t t e n , Dr. I a n McTaggart Cowan i s a t the t o p o f t h e l i s t .  He was always a v a i l a b l e t o d i r e c t and a s s i s t , when  d i r e c t i o n and a s s i s t a n c e were needed, b u t he p a t i e n t l y gave me t h e r e i n s at  other times.  One c o u l d n o t ask more o f a s u p e r v i s o r . H i s 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 t o u n d e r s t a n d , l e t alone emulate, was a s t i m u l a n t , and t h e b r e a d t h o f h i s e x p e r i e n c e a c o n s t a n t i n s p i r a t i o n . B u t , perhaps most i m p o r t a n t was t h e r e v e l a t i o n : once, as I f l o u n d e r e d on a s i d e t r a c k o f my l i f e , I d i s c o v e r e d , i n Dr. Cowan, a warm, u n d e r s t a n d i n g human being. My f a m i l y h a s done w e l l c o n s i d e r i n g t h a t , m o s t l y , i t has done w i t h o u t . My w i f e , Mary E t t a , l i v e d i n s h a c k s , t r a p p e d , s k i n n e d , nursemaided, weathered to  cooked,  storms, c o m p i l e d d a t a , e d i t e d , and through i t a l l c h a l l e n g e d me  s t a y human.  I needed t h a t .  S p e c i a l thanks a r e a l s o due: M i c h a e l M i l e s and James B i g g a r , f o r c o n s c i e n t i o u s and competent h e l p 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, K a t h l e e n S t e w a r t , and Dr. P.  Zuk f o r i d e n t i f i c a t i o n o f 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 o f  xiii  h i s t o l o g i c a l m a t e r i a l ; D e l o r e s L a u r i e n t e , f o r a s s i s t a n c e i n some o f the d a t a a n a l y s e s ; f e l l o w s t u d e n t s , G. C a l e f , R.W. others, for occasional f i e l d  Campbell,  S.R.  Johnson,  a s s i s t a n c e , r e g u l a r encouragement, and  and  fine  companionship; west c o a s t r e s i d e n t s , e s p e c i a l l y the A r n e t s , B a r r s , Gibsons, Palms, P a r l e e s , S a r l u n d s ; Seymours, Peg W h i t t i n g t o n , and Joe f o r continuous moral, for  and o f t e n l o g i s t i c a l  T a y l o r , H.D.  the manuscript;  F i s h e r , and W.D.  Kitts, for their c r i t i c a l  F i n a l l y , d e s p i t e my  so. basic b e l i e f  i n the i n n a t e p e r v e r s i t y o f  o b j e c t s , I must here acknowledge "Minkboat", my with l i f e .  Her  readings  Mrs. H o l l y L i n d e n , f o r t y p i n g the f i n a l copy, and f o r  her p a t i e n c e w h i l e doing  was  support; Jack Todd,  i n f o r m a t i o n and mink specimens from s o u t h e a s t e r n Vancouver I s l a n d ;  Drs. J.M. of  fraternal,  Wilkowski  inanimate  t w e l v e - f o o t , pneumatic,  link  dogged d e t e r m i n a t i o n to s t a y at the top of the water column  the u l t i m a t e key  to the completion o f t h i s  xiv  project.  "My daddy s t u d i e s the mink...;. poor o l e mink."  Marec'a H a t l e r , 1972  age 2  FRONTISPIECE : A male mink on a Vancouver I s l a n d i n t e r t i d a l  XV  shore  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 i t s 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 f i r s t , 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 d i f f i c u l t 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. map,  this can provide more pertinent detail.  1  Like a large-scale  2  Relevant (1969, 1970)  r e c e n t s t u d i e s on c a r n i v o r e s i n c l u d e work by H o r n o c k e r  on the mountain l i o n ( F e l i s c o n c o l o r ) , Mech (1970) on  w o l f ( C a n i s l u p u s ) , J o n k e l and Cowan (1971) on the b l a c k bear americanus),  S c h a l l e r (1972) on the A f r i c a n l i o n ( P a n t h e r a  (Ursus  leo)  Kruuk (1972) on the s p o t t e d hyena ( C r o c u t a c r o c u t a ) . Among the  and few  i n t e n s i v e f i e l d s t u d i e s o f m u s t e l i d p o p u l a t i o n s are work on marten americana) i n Montana (Newby and Hawley, 1954;  the  Hawley and Newby,  (Martes 1957;  Weckwerth and Hawley, 1962), w e a s e l s ( M u s t e l a 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 s o u t h e r n Sweden ( G e r e l l ,  1967a, b; 1968;  1969;  1970;  1971).  A l l o f these p a p e r s , and  p r o v i d e d bases f o r comparison w i t h my the g e n e r a l i t y and The mink was  others,  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  s i g n i f i c a n c e o f observed p a t t e r n s . 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 among t h i s group, the m u s t e l i d s appeared t o be the l e a s t w e l l known.  and, Study  o f 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 t o 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  findings.  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 s t u d i e d at a l l , and q u e s t i o n s r e l a t i n g to i t s a d a p t a t i o n to t h i s environment were o f s p e c i a l i n t e r e s t . 2. ) Economic - S m a l l 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 l o n g been i m p o r t a n t understanding  resources  o f these animals  i n the f u r t r a d e ; b e t t e r b i o l o g i c a l  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 o f 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 i t s 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 f e a t u r e s o f the c o a s t environment are o f p a r t i c u l a r importance to mink? - what foods are used, and under what c o n d i t i o n s ? How do mink o b t a i n these foods? - what f a c t o r s a f f e c t the h e a l t h o f i n d i v i d u a l s ? - what are the main i n f l u e n c e s on numbers i n p o p u l a t i o n s ? - how do mink d i v i d e up h a b i t a t among themselves and, s i n c e they are p r i m a r i l y s o l i t a r y i n h a b i t , how do they keep separate? These q u e s t i o n s might a l l be summarized i n t o a s i n g l e o b j e c t i v e : To gather  i n f o r m a t i o n on a l l aspects  of the l i f e h i s t o r y of c o a s t mink  which r e l a t e to the w e l f a r e o f both the i n d i v i d u a l and The  first  step was  Along the way, philosphy", tained  and  population.  This i s natural history.  i n f o l l o w i n g pages, I a l s o have i n d u l g e d  i n "natural  i . e . , a c o n s i d e r a t i o n o f the meaning o f the r e c o r d I  (see Sears, Most o f my  p o i n t out  to observe and r e c o r d .  the  ob-  1944). d a t a have been analyzed  t h a t I p r e f e r to use  statistics  statistically,  but I w i l l  here  as a guide r a t h e r than a c r u t c h .  Hence, I have not chosen an a r b i t r a r y l e v e l o f p r o b a b i l i t y upon which to basemy/cconclusions. p r o b a b i l i t y obtained accept my  conclusions  Rather, i n most cases I have simply and  i n v i t e the r e a d e r  to decide whether he  at the observed l e v e l .  r a t h e r than s t a t i s t i c a l  stated  Biological  s i g n i f i c a n c e alone, has  been my  the will  significance, 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 on the west coast of Vancouver Island, British Columbia.  1964)  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 r a i n f a l l , 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 i t s 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 i t s length, although irregular h i l l s 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 h i l l s described above rarely exceed heights above 100 m.  The exposed coastline is irregular and rugged  as a result of differential resistance to wave erosion, and is characterized 1  f a c e page 6  Figure  1.  Mink study areas, west c o a s t o f Vancouver I s l a n d , B r i t i s h Columbia. The a r e a covered i s i n d i c a t e d i n the i n s e t . The main study areas are shown i n g r e a t e r d e t a i l i n F i g u r e s 3 and 4.  7 by g r e a t expanses o f s m a l l p a r t i c l e beaches and headlands.  Numerous i n l e t s , bays,  and  s m a l l to e x t e n s i v e bare  s m a l l a r c h i p e l a g o s p r o v i d e some  p r o t e c t i o n from the open sea and, w i t h beaches o f b o u l d e r s and  tideflats  s u p p o r t i n g marine v e g e t a t i o n , o f f e r s h o r e l i n e s a t t r a c t i v e to mink. o f the i n l e t s extend  rock  Many  i n l a n d f o r 15 km or more and p e n e t r a t e the Vancouver  I s l a n d Mountains which, r i s i n g a b r u p t l y to e l e v a t i o n s o f 1600-2000 m, the i n l a n d border o f the c o a s t a l  form  plain.  CLIMATE Data gathered at the T o f i n o M e t e r o l o g i c a l S t a t i o n , approximately midway  between Clayoquot  Sound and B a r k l e y Sound on the E s o w i s t a P e n i n s u l a ,  are d e p i c t e d i n F i g u r e 2 i n the form o f long-term averages. f o r the y e a r s o f study are l i s t e d h i g h , a v e r a g i n g over 325  cm,  i n T a b l e 1.  Specific  data  T o t a l annual p r e c i p i t a t i o n i s  and at l e a s t some can be expected on more  than h a l f the days i n any g i v e n y e a r .  Summers however, e s p e c i a l l y d u r i n g  J u l y and August, are c h a r a c t e r i s t i c a l l y d r y .  Record r a i n f a l l s  f o r a 24-hour  p e r i o d have exceeded 10 cm i n e i g h t months o f the y e a r , w i t h a maximum o f 17.5  cm once i n January.  Snowfall i s t y p i c a l l y l i g h t ,  a n n u a l l y over a 17 year p e r i o d . p i t a t i o n was  a v e r a g i n g j u s t 42  D u r i n g the y e a r s o f study (1968-72),  a p p r o x i m a t e l y 25 per cent above the average  cent below i t i n 1970,  but was  heavy s n o w f a l l s (185 cm and 130  near  average  i n 1968  preci-  and 25 per  i n the ..other t h r e e y e a r s . U n u s u a l l y  cm r e s p e c t i v e l y ) o c c u r r e d i n 1969  Temperatures are t y p i c a l l y m i l d and u n i f o r m and,  Even d u r i n g December and January,  averages  the c o l d e s t months,  below f r e e z i n g o c c u r on l e s s than h a l f o f the days.  and  1971.  although extremes o f  from -1.5° C to 33° C have b een r e c o r d e d , the annual range 20 degrees.  cm  about temperatures  The r e c o r d minimum, g i v e n  above, o c c u r r e d d u r i n g t h i s study (January 1969); although i t was c o l d e s t w i n t e r s on r e c o r d , the mean annual temperatures  one o f the  f o r t h a t year and f o r  f a c e page 8  F i g u r e 2.  Long-term c l i m a t i c r e c o r d s , T o f i n o Weather S t a t i o n , west c o a s t o f Vancouver I s l a n d , 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 o f winds from f o u r 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 a x i s denote the mean p r o p o r t i o n (per c e n t ) o f 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) Mean  4  6  6  7  10  13  14  14  13  10  7  5  9  1968 1969 1970 1971 1972  4 -1 4 3 2  6 4 7 3 4  7 6 7 4 6  6 7 7 7 6  . 11 11 9 9 11  12 14 13 11 12  15 14 13 14 15  14 13 14 15 15  13 13 12 12 11  9 10 9 9 8  7 8 7 6 7  2 7 3 2 3  9 9 9 8 8  3  ecipitation (cm) Mean  41  32>.  31  26  10  9  8  8  15  .",37.  42  45  306  1968 1969 1970 1971 1972  61 30 24 56 32  24 27 14 33 39  42 54 22 47 70  23 41 32 24 33  10 14 13 8 6  15 3 3 19 11  7 5 9 5 18  17 14 4 11 3  26 38 21 21 15  72 29 20 50 7  46 54 29 50 39  39 36 47 31 49  381 322 238 354 323  a  a  -  17 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, i s probably the most important climatic feature for organisms which hunt at the edge of the sea. in Figure 2, westerly winds predominate are lowest.  As shown  i n summer, when average v e l o c i t i e s  Those from the northwest usually accompany f a i r , dry weather  and on most summmer days do not begin blowing u n t i l late morning or early . afternoon.  Fog.is common during the calm mornings of that season.  The storm  winds i n the area are e a s t e r l i e s , especially those from the southeast, and these are the predominant winds from September through A p r i l .  Southeasterlies,  which commonly attain sustained v e l o c i t i e s of 50 km per hour, are almost always accompanied by p r e c i p i t a t i o n . 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 c h a r a c t e r i s t i c of exposed mainland shores south of Tofino and the islands of Barkley Sound have been produced by B e l l and Harcombe (1972, 1973).  The forest fringing the shore i n this region i s t y p i c a l l y composed  of large trees with a dense shrub understory, the combination excluding l i g h t , retaining moisture, and thus providing a dark, humid environment  for mink  at ground l e v e l . . The main forest community i s that dominated by western red cedar (Thuja p l i c a t a ) and western hemlock (Tsuga heterophylla) and these species, together with Sitka spruce (Picea sitchensis) i n many locations, support mink dens, runways, l a t r i n e s and middens among exposed root systems and other natural c a v i t i e s . The primary shrubs i n forested areas are salal(Gaultheria shallon), salmpnberry (Rubus s p e c t a b i l i s ) , and several species of Vaccinium.  Salal  grows to h e i g h t s o f 10 f e e t o r more at many l o c a t i o n s and u s u a l l y so t h a t i t a c t s as an almost impenetrable  b a r r i e r to a l l except  Mink u s i n g the runways which commonly p a r a l l e l  small  rocky  dug  shrub  predators,  Where s o i l i s poor and/or s h a l l o w such as on  i s l e t s , c o n i f e r s grow o n l y s p a r s e l y i f at a l l , and  u s u a l l y dominate.  animals.  the beach through these  zones are d o u b t l e s s l y a f f o r d e d p r o t e c t i o n a g a i n s t most p o t e n t i a l especially raptors.  densely  small  shrub communities  I n such cases burrows used f o r dens and middens may  be  amid the r o o t s o f shrub clumps; however, the r o o t systems o f t r e e s  ap-  pear to be more commonly used. On  i s l e t s o r headlands where heavy shrub cover  ways and middens are commonly found  i s absent,  mink r u n -  i n other p l a n t formations,  o f which  clumps o f a f e r n , Polypodium s c o u l e r i , and patches o f w i l d r o s e  (Rosa  nutkana) or beach r y e  f e r n has  (Elymus m o l l i s ) are the most common.  The  no common name, but I i n f o r m a l l y r e f e r r e d to i t as "mink f e r n " s i n c e i t was of  r a r e l y found  f r e e o f mink s i g n .  beach r y e , o t h e r ,  smaller grass  The  l e a v e s o f t h i s f e r n and  s p e c i e s , mosses and  (Honkenya p e p l o i d e s ) were p l a n t s i d e n t i f i e d In summary, the wet,  communities, o f which those i n t h i s context  seabeach sandwort  i n n e s t s prepared  m i l d c o a s t a l c l i m a t e supports f r i n g i n g the ocean shore  pieces  by mink.  a v a r i e t y of plant  are o f s p e c i a l  interest  i n t h a t they p r o v i d e mink w i t h p r o t e c t i o n from exposure  and  predation. INTERTIDAL HABITAT Tides  with  The  t i d a l c y c l e i n the r e g i o n of s t u d y . i s the s e m i - d i u r n a l mixed  two  low  Dohler  t i d e s and  two  h i g h t i d e s o f unequal h e i g h t o c c u r i n g d a i l y  1966). Throughout the remainder o f t h i s r e p o r t , the f o u r t i d e s  be r e f e r r e d to as the lower low  (LL), higher  low  (HL), lower h i g h  (LH)  type, (see will 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/Year During Study  3  Month  Mean  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  May  3.61  (3.54-3.75) -0.09  (-0.21-0.06)  4  3  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)  Dec  4.05  (3.93-4.17) 0.11  ( 0.00-0.24)  Range  Mean  Range  '68  '69  '70  '71  '72  3  2  2  4  ^ o r month and year (1968-1972) indicated, the number of days on which tide f e l l below sea level datum (0.00).  1  ^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. tides typically occur in winter.  The highest  In relation to time of day, the annual d i s t r i -  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 California have many common features, especially i n 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 a l l shore zones.  As is evident int.all of the above cited papers, different  14 organisms, t y p i c a l l y Most o f  those r e g u l a r l y  "demersal" the  occur at d i f f e r e n t  levels  i n the  intertidal  e a t e n by mink o c c u r i n the " l o w e r  z o n e s o f R i g g and M i l l e r  (1949),  horizon.  intertidal"  i n a s s o c i a t i o n w i t h some o f  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  (Laminaria)  and r e d  (Gigartina, Iridaea)  These are exposed to 1 m, a r e s t i l l  a l g a e , and e e l g r a s s  shore hunting predators  P r o b a b l y more i m p o r t a n t  than t i d e  R i g g and M i l l e r  "where the r e l a t i v e l y an a b u n d a n c e o f  (1949)  sheltered,  tide level  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 shoreline.  at  tide  brown  (Zostera).  l e v e l s below  moderately a c c e s s i b l e from t h a t depth to  be r e a c h e d o n l y b y deep d i v e s a t h i g h e r  and  about  about  2 m, b u t  can  levels. i n d e t e r m i n i n g what  situation  is  species  the n a t u r e o f  f o u n d t h e g r e a t e s t number o f  the  species  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  t i d e p o o l s provides the g r e a t e s t v a r i e t y  More exposed s i t e s " o b v i o u s l y c o n s t i t u t e  of  a more r e s t r i c t e d  and  habitats". environment".  S P E C I F I C STUDY AREAS Most 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 C l a y o q u o t Sound ( F i g u r e although,  3)  as t h e s e f i g u r e s  w e r e c o n d u c t e d i n two  areas  of  and a s i n g l e a r e a i n B a r k l e y S o u n d ( F i g u r e show, o b s e r v a t i o n s o f  n a t u r e w e r e made a t many a d j a c e n t may be c h a r a c t e r i z e d b r i e f l y  as  locations.  an a t  4)  least occasional  The t h r e e m a i n s t u d y  areas  follows:  VARGAS I S L A N D  2 This  is  a large  (about  C l a y o q u o t Sound ( F i g u r e outcrops  are the predominant  near-shore water to  3).  depths  30 km ) ,  low-lying  i s l a n d at  t h e mouth  E x t e n s i v e s a n d b e a c h e s and n u m e r o u s shore types  are t y p i c a l l y  the open P a c i f i c Ocean, w h i l e  around i t s  shallow.  those on the  of  rocky  35 km p e r i m e t e r ,  The w e s t  and  shores are exposed  east side are protected  from  f a c e 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 tideflats) . 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  F i g u r e 4.  Study l o c a t i o n s i n B a r k l e y Sound, Vancouver I s l a n d , 1968-1973. E v e r y i s l a n d shown e x c e p t Cree I s l a n d i s known to have h a r b o r e d a t l e a s t one mink a t some time d u r i n g the s t u d y . I s l a n d s o f most i n t e n s i v e o b s e r v a t i o n s are shaded. The p r i m a r y s t u d y a r e a ( 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 t h e i n s e t ; d o t s 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 sites.  17  most summer s u r f , but very  face d i r e c t l y i n t o southeast  storm winds and may  i n h o s p i t a b l e , e s p e c i a l l y i n w i n t e r . Most study  I s l a n d were c o n c e n t r a t e d southeast-facing  shore,  along  an approximately  8.5  be  a c t i v i t i e s on Vargas km  section of  this  a view o f which i s shown i n P l a t e 1 ( a ) .  TOFINO INLET P r o t e c t e d r o c k beaches and  t i d a l mudflats  on the Vancouver I s l a n d  mainland shore i n the immediate v i c i n i t y o f the v i l l a g e o f T o f i n o and adjacent  islets  t h i s area. I n l e t and  to the n o r t h  ( F i g u r e 2 ) , were the most s t u d i e d h a b i t a t s i n  Shallow waters and i s l e t s are s m a l l  s t r o n g t i d a l c u r r e n t s are the r u l e i n T o f i n o  (most l e s s than 2 km^  l i n e studied, i n t e n s i v e l y was  on  somewhat over 2.5  i n a r e a ) ; the t o t a l  shore-  km.  BROKEN GROUP ISLANDS T h i s a r c h i p e l a g o o f more than 80 52 km  i s l a n d s , i s l e t s and  south o f the Clayoquot Sound study  areas, o c c u p i e s  reefs, lying an a r e a o f  about  approxi-  2 mately 100 Sound.  The  km  ( o f which about 15 per cent  t y p i c a l l y rocky  shores  i s land) i n c e n t r a l B a r k l e y  i n t h i s area are rugged, o f t e n sheer  much d i s s e c t e d where exposed to heavy waves, although and  r e e f s serve as breakwaters f o r o t h e r s  many o f the  and  islets  so t h a t r e l a t i v e l y calm waters  and p r o t e c t e d b o u l d e r  beaches are common.  such p r o t e c t e d  at f i v e named i s l a n d s ( T u r t l e , W i l l i s , Dodd, Walsh,  shores  and C h a l k ) , h e r e a f t e r c a l l e d  Most work was  c a r r i e d out  the " T u r t l e I s l a n d Group", i n the  on  north-  c e n t r a l p o r t i o n o f the Broken Group ( F i g u r e 4 ) . 2  The  sub-group i s about 5.7  o f s h o r e l i n e were r e g u l a r l y  covered  km  , and  j u s t under 10 km  d u r i n g o p e r a t i o n s w i t h i n t h a t a r e a . An  the Broken Group i s shown i n P l a t e 1 ( c ) .  area o f t h i s  island  a e r i a l view o f t h i s p a r t o f  face page 18  P l a t e 1:  Study Areas a) E a s t Vargas I s l a n d , showing a p p r o x i m a t e l y 3 km o f the study area s h o r e l i n e . I s l e t s 1 and 2, r e s 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 f i g u r e 3. Note the long expanse of sand beach. b)  More o f the E a s t Vargas s h o r e l i n e , l o o k i n g n o r t h . The i s l e t s shown are the same as those i n " a " .  c)  Broken Group I s l a n d s , w i t h p o r t i o n s o f the T u r t l e I s l a n d Group i n the foreground (1-Dodd I . , 2W i l l i s I . , 3- T u r t l e I . , 4 - T u r r e t I.) and w i t h the E f f i n g h a m Group i n the background.  d)  A c l o s e r view o f the N. T u r t l e I s l a n d shore, showing i t s r o c k y n a t u r e and the c l o s e n e s s o f the f o r e s t 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 o f o p e r a t i o n s d u r i n g work i n the B a r k l e y Sound study a r e a .  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 f i r s t 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 l i f e 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 i n previous period. Documentation for the b i o l o g i c a l trends indicated above i s provided i n 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 f i r s t taxonomic review of this semiaquatic 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, i t s e l f , 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 a l l of the United States and the provinces and territories of Canada, from Nova Scotia (Northcott ejt a l . 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. north extend almost to the Arctic coast (Cowan 1948).  Records in the  Although.it is  indigenous to North America, escapes from fur farms have resulted in establishment 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 a l l 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 i t s 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 l i t t o r a l 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 l i s t s 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 livetrapping 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 a l l areas (P>0.30 for a l l 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 o f l i v e t r a p p e d mink ( a l l seasons) on the west c o a s t o f Vancouver I s l a n d , 1968-1973. Age determined by f i e l d c r i t e r i a ; numbers under sex and age c l a s s e s are sample s i z e s f o r the f o u r p e r t i n e n t d a t a p o i n t s , r e s p e c t i v e l y , and v e r t i c a l b a r s a r e 5% confidence l i m i t s .  Adult T  Vargas Island  Tofino Inlet  Males  (39-25-78-147)  J  Juvenile  J-  (31-24-31-98)  Males  T  Adult Females  1  (21-5-19-47)  T  0 Juvenile Females  1  1  Barkley Sound  A l l Areas  (16-7-12-36)  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 f i r s t year of l i f e .  When they f i r s t 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, w i l l 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 f i r s t  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;Xv 6.5, df = 1, p<0.02). t=  There was l i t t l e difference in incidence between areas or between age classes within sexes.  face page 26  F i g u r e 6.  I n c i d e n c e o f u n u s u a l l y l i g h t - c o l o r e d p e l a g e among mink, west c o a s t o f Vancouver I s l a n d , 1968-1973.  60 Female  c « 0) CD  _g  Male  50  40  a)  Q_  O)  w  x  u c  1  30  20  M  J  J  A  28  27  64  36  23  1  15  10  13  9  10  1  F  M  sample CT 9  23 3  <J  2  1 A  J size  {  10  T—r  •  "I—r  o  N  D  1  16  14  5  1  9  10  1  s  -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 l i f e :  Evidence of the habitats i t hunts, the  times of day i t is active and, therefore, the degree to which i t may be exposed to various environmental contingencies, the extent of i t s seasonal movements, the degree to which i t might limit or be limited by i t s 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 i t s economic importance, and even i t s temperament may often be inferred from the nature of i t s diet.  No other single aspect of l i f e history provides a  better preview to an understanding of where an animal f i t s 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. of remains in dozens of feeding middens.  Examination  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 a l l 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 v i s i t 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 represent 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 v i s i t 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 w i l l 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 d i f f i c u l t y 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 individual 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 a l l feces in the pile appeared fresh or i f a l l  were known to have been deposited since a previous v i s i t , the pile was left intact.  (4) Thorough mixing of a l l 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 a l l 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 a l l 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 a l l 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 A l l 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 laboratory study.  Mink chew their food thoroughly, a fact noted previously for  32 this (Korschgen 1958) and other Mustela a l l items as precisely as possible.  species (Day 1966).  I identified  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 diagnostic 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. However, 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 occurrence", 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 a l l 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 o f these animals when they o c c u r r e d i n the same s c a t s w i t h more d i g e s t i b l e animals such as f i s h .  T h i s b i a s 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 r e s i s t a n c e to weathering  i n the f i e l d .  S c a t s c o n t a i n i n g f i s h o r b i r d m a t e r i a l were f a r more c o h e s i v e than c o n t a i n i n g arthropods and were l e s s l i k e l y  those  to be d r i v e n a p a r t by heavy  r a i n o r wind.  F u r t h e r , a s c a t c o n t a i n i n g c r a b remains may take on an  old  i n l e s s than t h r e e days under dry c o n d i t i o n s , w h i l e a b i r d  or  appearance  f i s h b e a r i n g scat, under the same c o n d i t i o n s , w i l l remain  a week o r more.  Thus, I was more l i k e l y  and t h e r e may be a tendency  fresh-looking for  to p i c k up the f i s h and b i r d  scats  toward o v e r - r e p r e s e n t i n g the frequency o f these  items i n the mink d i e t .  D e s p i t e these problems,  s c a t d a t a are s u f f i c i e n t  to g i v e a g e n e r a l p i c t u r e ; o f the range o f foods  used by mink i n the c o a s t environment, relative  I am c o n f i d e n t t h a t my  and p r o v i d e good evidence as to the  importance o f each i n the d i f f e r e n t  seasons  and areas I have  studied. EXAMINATION  OF FEEDING MIDDENS  Accumulations termed  o f p a r t l y eaten o r i n e d i b l e remains  o f prey,  here  f e e d i n g middens, may be found i n and near den s i t e s and a t 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 h u n t i n g s p o t s , a l o n g the shore.  These middens a f f o r d e d o p p o r t u n i t i e s f o r 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  p r e y items than c o u l d u s u a l l y be made from s c a t remains, p a r t i c u l a r l y i n the case o f f i s h and b i r d s .  On a number o f o c c a s i o n s I found f r e s h ,  specimens o f these i n s t o r a g e at den middens.  Most middens c o n s i s t e d  l a r g e l y , o f t e n e n t i r e l y , o f the carapaces and o t h e r e x o s k e l e t a l of  crabs.  intact  fragments  I n such cases I u s u a l l y d i d l i t t l e more than t o note the  s p e c i e s p r e s e n t and to i n d i c a t e which predominated. more thorough examinations  P e r i o d i c a l l y I made  o f accumulations o f c r a b remains  (76 middens  35  over the y e a r s o f s t u d y ) . meter, each specimen  p r e s e n t and summarized the degree  dages had been u t i l i z e d . of intact  At these times I measured, to the n e a r e s t m i l l i -  The  l a t t e r was  to which the appen-  accomplished by c o u n t i n g the number  and p a r t i a l w a l k i n g l e g s and c h e l i p e d s p r e s e n t .  examination o f a c r a b midden was  Whether the  s u p e r f i c i a l o r thorough,  i n a l l cases I  broke a l l carapaces p r e s e n t to i n s u r e t h a t I would not measure them a g a i n and to g i v e a b a s i s  f o r d a t i n g remains  found i n subsequent  visits.  Whenever remains o f f i s h were found, they were examined c a r e f u l l y identified  to s p e c i e s i f p o s s i b l e .  measured and o c c a s i o n a l l y weighed.  I n t a c t or n e a r l y i n t a c t  and  specimens were  B i r d remains were i d e n t i f i e d  examined f o r c o n d i t i o n s which might have c o n t r i b u t e d to t h e i r  and  vulnerability.  I n those cases i n which f i s h or b i r d s had been p a r t l y eaten, I took notes on the manner i n which the e a t i n g was  done.  E x a m i n a t i o n o f middens, alone, c o u l d not p r o v i d e an a c c u r a t e p i c t u r e o f the c o a s t a l mink's d i e t because  o f the heavy b i a s toward  those p r e y  items  which have a l a r g e p o r p o r t i o n o f u n d i g e s t i b l e m a t e r i a l , those which are l a r g e , o r both.  These examinations were v a l u a b l e , however, when used i n  c o n j u n c t i o n w i t h the o t h e r .methods o f food study.-  Indeed, most o f  my  i d e n t i f i c a t i o n s o f f i s h and b i r d s eaten by mink were f i r s t made i n middens. DIRECT OBSERVATIONS The number o f o p p o r t u n i t i e s I had p r e y i s unprecedented.  to a c t u a l l y watch mink s e c u r i n g  I saw mink h u n t i n g on 452 o c c a s i o n s , and  i n c l u d e d the h a n d l i n g o f 222 d i f f e r e n t p r e y items. I u s u a l l y  those  observed  h u n t i n g animals from a s m a l l boat, at d i s t a n c e s o f from about 5 m up to 100 m o r more, but o c c a s i o n a l l y I was land.  Since s p e c i f i c  a b l e to approach  and observe  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 o f f i s h , were  from  often  i m p o s s i b l e , these o b s e r v a t i o n s were o f l i m i t e d v a l u e i n p r o v i d i n g d a t a  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 subsequent identification of these led to additions on the known food species list.  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 d i f f i c u l t to handle.  These, plus identification of  the kinds of habitats and circumstances in which various kinds of prey were caught, provided insight into the r e l i a b i l i t y 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 i n 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 i n i t i a l volumetric determination 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 f u l l 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 i s , i t s e l f , 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  face  F i g u r e 7.  Seasonal food h a b i t s , as determined by f e c a l a n a l y s e s , o f mink i n C l a y o q u o t Sound, Vancouver I s l a n d , 1968-1972. The t h r e e seasons i n d i c a t e d are described.^in the t e x t . The numbers i n parentheses p r e c e d i n g the bars i n the "Crabs" s e c t i o n r e f e r to sample s i z e s (number o f s c a t s ) f o r each time p e r i o d , j a n d t t h e s e p e r t a i n to the f o l l o w i n g food c a t e g o r i e s ( f i s h , b i r d s , and o t h e r ) as w e l l . The h o r i z o n t a l span o f each bar i n d i c a t e s the frequency o f o c c u r r e n c e f o r a g i v e n food d u r i n g the time p e r i o d i n d i c a t e d , and t h i s may be r e a d from the percentage, s c a l e above. The e x t e n t o f shading o f each bar d e p i c t s the average volume a t t a i n e d by the i t e m i n i t s o c c u r r e n c e s d u r i n g t h a t p e r i o d . For these volume d e s i g n a t i o n s , each bar i s d i v i d e d v e r t i c a l l y i n t o f i v e s e c t i o n s and the number o f these which are b l a c k e n e d i n d i c a t e s the mean volume as f o l l o w s : one (mean v o l J = 1-5%; two (6-25%); t h r e e (26-50%); f o u r (51-75%); f i v e (76-100%).  page  38  38  Clayoquot Sound C R A B S  Dec -Mar  Apr -Jul  Aug -Nov  , 9 6 » (0) 1969 (132) 1970 (is), 1971 (133): 1972(275)  FISH  BIRDS  1968 1969 1970 1971 1972 1968 1969 1970 1971 1972  OTHERi 1968 1969 1970 1971 1972 ALL YEARS CRABS FISH BIRDS OTHER  s D B I N =588  H U  N= 504  i i  N = 324  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.  b Frequency (7.) and Volume Category (Vol.) a  Dec-Mar Vargas (n=298) 7. Vol. c  Food Items  Apr-Jul Tofino (n=290) % Vol.  Vargas (n=363) % Vol.  MOLLUSKS 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  71 1 16 T 5 1 11 48 3  (5) (5) (4) (5) (5) (5) (3) (4) (4) (3)  61 1 25  (5) (2) (5)  89 2 35  (5) (4) (5)  13  (5)  1  (5)  T T 22  (5) (1) (5)  1 47 13 1  (3) (5) (5) (2)  10  (4)  4 T  3  (3)  1  (4) (5)  (2)  T  (3)  Aug-Nov Tofino (n=141) 7. Vol. T  (3)  76 T 64  (5) (5) (5)  49 41 •6 6  (4) (3) (4) (4)  50 18 5 27  (5) (4) (5) (5)  30 27 7 3  (4) (4) (4) (3)  BIRDS Anatidae Seabirds Laridae Passeriformes unidentified birds  15 9 1  (4) (4) (4)  7 3  (4) (4)  2 2  (4) (5)  T T 2  (5) (5) (4)  d  e  5  (4)  MAMMALS Microtus townsendi Peromycus maniculatus Mustela vison Procyon lotor unidentified mammals  1 T  (3) (4)  T 1  (5) (2)  DEBRIS  1  (3)  (4) (1) (4)  55  (5)  47  (5)  3  (5)  2  (5)  1  (1)  5  (4)  1  (2)  (5) (3)  1 22 21 1  (1) (4) (4) (4)  3 1  (4) (2)  1 1  (3) (3)  2  (2)  51 42 10 8  (4) (3) (4) (4)  55 39 5 16  (5). (4) (5) (5)  12 5 5  (4) (4) (5)  6  (5)  5  (5)  32 16 3 17  (5) (4) (4) (5)  (2)  5  (3)  1  (3)  4  (2)  (5) (2)  3  (2)  (1)  1  (1)  5  (3)  T T  T 5  77 1 33  1  ^. v  (3)  Tofino (n=U2) 7. vol.  14 9  INSECTS FISH Small fish Medium fish Large fish  Vargas (n=212) 7. Vol.  3  (4)  5  Frequency of occurrence, expressed as percentage (nearest whole per cent) of scats i n 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.. a  The number of scats in sample. ^Fish size categories defined in text. Seabirds - Gaviidae, Podicipedidae, Phalacrocoracidae and Alcidae.  c  e  (2)  Table 4.  Spring and Summer Foods of Mink in Barkley Sound, Vancouver Island, B r i t i s h Columbia, 1971 - 1972, as determined by Fecal Analyses.  Frequency (7.) Mar - Apr (n=37) 7. V o l .  1971 May-Jun  c  Food Items  Crustaceans Decapoda Hemigrapsus sp. Cancer productus Pugettia producta Petrolisthes sp. Unident. crabs Isopoda Amphipoda  89 76 22 62  30  Fish Small Fish Medium Fish Large Fish  22 8 11 5  d  (3) (4)  (1) (5)  (5)  8  (3)  Birds Passeriformes Unident. Birds  Debris  80 22  (5) (4)  3  (1)  18 8  (3)  17 15  (4)  13  (3)  3  (5)  3 3  (2) (2) 3 3  Mammals . - Microtus townsendl 22  Mar-Apr (n-3) 7. Vol  33 33  92 92  92 92 3 92  (4) (2) (3)  Jul-Sep (n=33) % Vol  1  1972 May-Jun (n=100) % Vol  Jul-Oct (n=81) 7. V o l .  (3)  (2)  Mollusks  and Volume Category ( V o l . )  a  33  (5)  33 33 33  (5) (5)  93 93  91 61 4 53 1 2 1 28 3  (3) (5) (5) (2) (5) (5) (2)  22 14 4 5  (4) (4) (5)  2  (3)  2  (3)  93  (5)  9 9  (5)  (5) (5)  (4)  Frequency 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 1%); 1=1-57.; 2=6-257.; 3=26-507.; 4=51-757.; 5=76-1007.. The number of scats in sample. ^Fish size categories defined in text. a  b  c  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 i t s occurrence over a l l 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 fluctuation 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 a l l occurrences over most years. A l l 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 a l l 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  f a c e page 42  F i g u r e 8.  Prey animals observed b e i n g c a p t u r e d by mink i n l i t t o r a l h a b i t a t s along the west c o a s t of Vancouver I s l a n d , B r i t i s h Columbia, 1968-1972. R e l a t i v e f r e q u e n c i e s of o c c u r r e n c e f o r each i t e m can be i n t e r p o l a t e d from the percentage s c a l e s at the top. Numbers i n p a r e n t h e s i s are sample s i z e s (number o f c a p t u r e s observed) f o r the bar charts, below. Items C and F ( o t h e r c r a b s ; o t h e r f i s h ) i n c l u d e c r a b s (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 s p e c i e s o t h e r than those l i s t e d . U n i d e n t i f i e d prey may have been e i t h e r f i s h or c r a b s i n some c a s e s , but mostly i n v o l v e d v e r y s m a l l animals which were eaten immediately a f t e r c a p t u r e .  Dec-Mar I  0  1 — — i  1  20  40  60  Crabs a - red  crab  b - helmet c - other  Apr-Jul 1  1  80 100  mm  crab (including  unidentified)  I  1  0  20  1 — — i  40  Fish  1  1  80 100  I  0  1  20  40  Unidentified  d -  blennioids  e -  sculpins  f  60  Aug-Nov  -ofher  Prey  —  i  60  1  1  80 100  43  Table 5. Foods in digestive tracts of west Vancouver Island mink, May-July 1971 and 1972.  Food Items  Stomachs Freq' Vol.  Intestines Freq.' Vol.  CLAYOQUOT SOUND (16 Stomachs, 15 Intestines) CRABS Cancer productus Pugettia producta Telmessus cheiragonus Cancer gracilis Hemigrapsus nudus  37.5 6.2 43.8 12.5 6.2  51.8 25.0 64.1 55.0 10.0  OTHER INVERTEBRATES Isopoda Amphipoda Polychaeta Mytilus sp.  6.2 6.2 6.2  1.0 95.0 5.0  VERTEBRATES Small fish Rattus norvegicus  37.5 6.2  BARKLEY SOUND (13 Stomachs, 12 Intestines) CRABS Cancer productus 38.5 Pugettia producta 7.7 Hemigrapsus nudus 7.7 VERTEBRATES Small fish  84.6  33.0 20.0 66.7 13.3  50.0 93.3 94.5 97.5  6.7 6.7  5.0 1.0  84.2 100.0  20.0 13.3  8.3 50.5  79.0 '60.0 25.0  58.3 8.3  86.4 100.0  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.  f a c e page  F i g u r e 9.  Annual (a) and bimonthly (b) v a r i a t i o n s i n midden o c c u r r e n c e s o f crabs eaten by mink, Vancouver I s l a n d , 1968-1972. Each crab s p e c i e s was g i v e n an " o c c u r r e n c e s c o r e " of two when i t was the p r e dominant item i n a midden, or a s c o r e o f one i f i t o c c u r r e d i n numbers equal to or l e s s than o t h e r species. The numbers i n parentheses below the h o r i z o n t a l s c a l e are the t o t a l o c c u r r e n c e scores o b t a i n e d d u r i n g the seasons i n d i c a t e d .  44  44  Clayoquot  Sound  Barkley S d .  Red Crab (80.4)  All Cancer (51.0)  Red Crab  (36.4)  Kelp Crab  (34.4)  O t h e r Cancer (l4.6) Helmet C r a b  Other Crabs (l.6)  1968  1969  1970  1971  (29)  (44)  (89)  (115)  1972-1973 (3l)  X All Years (308)  Kelp Crab (15.4)  (l3.4) O t h e r Crabs (3.9)  X All Years (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 a l l 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 f a 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 ( C J . 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 presumed 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 i t s low occurrence among  mink foods there, the Dungeness crab was common in Clayoquot Sound throughout the study, especially i n 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 f i r s t 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 predominated 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 i t 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  lower.  48 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 i t s 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 a l l 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. r i c 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 l i 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  f a c e 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  CARAPACE  c. Helmet Crab [n =161)  WIDTH  (mm)  f a c e page 51  F i g u r e 11.  Crabs commonly eaten by mink, Vancouver I s l a n d , B r i t i s h Columbia. a) b)  S i z e r e l a t i o n s h i p s o f f o u r s p e c i e s w i t h the same carapace measurement (M). Carapace w i d t h - g r o s s weight r e l a t i o n s h i p o f r e d crabs and k e l p c r a b s .  51  (a)  Red  Crab  Helmet  (b)  Crab  Kelp  Crab  500r  400  300  Red  Crabs  •  Kelp  Crabs  A  O) 200  100  50  I  I  70  90 carapace  I  I  110 width  I  I  130 (mm)  I  I  150  B a r k l e y Sound,  Such e v i d e n c e , from middens and from o b s e r v a t i o n s o f crabs  seen elsewhere,  i n d i c a t e t h a t mink are a b l e to prey upon i n d i v i d u a l s up to  the maximum s i z e f o r a l l s p e c i e s d e t e c t e d i n food samples w i t h the p r o b a b l e e x c e p t i o n o f the Dungeness c r a b .  A t 900 g o r more, many mature c r a b s o f  t h i s s p e c i e s outweigh most female mink and approach By comparison,  the second  t h e weight o f many males.  l a r g e s t s p e c i e s ( r e d crab) r a r e l y a t t a i n s a weight o f  500 g and most i n d i v i d u a l s o f the s i z e r e g u l a r l y taken by mink are h a l f or l e s s  (Figure 11).  Comparisons (Appendix r e d c r a b s caught  7) between seasons  i n w i n t e r averaged.a  at both T o f i n o I n l e t  and Vargas  and between areas show t h a t  b i t l a r g e r than those from o t h e r  differences probably r e f l e c t habitat differences o f B a r k l e y Sound p r o v i d i n g b e t t e r h a b i t a t  The area.,  (the r o c k y nearshore  f o r r e d c r a b s ) r a t h e r than  differ-  Crustaceans  Some mink, e s p e c i a l l y those i s o l a t e d on s m a l l r o c k y i s l e t s , on a l a r g e i s o p o d , the s e a s l a t e r  h a r d e r to o b t a i n t h e r e d u r i n g t h a t season. are fewer  fed regularly  ( L i g i a p a l l a s i i ) . H i g h e s t use was r e c o r d e d  i n w i n t e r a t Clayoquot Sound ( T a b l e 3 ) , perhaps  because  o t h e r foods are  I n B a r k l e y Sound, where t h e r e  a l t e r n a t i v e foods and/or h i g h e r p o p u l a t i o n s o f s l a t e r s ,  eaten r e l a t i v e l y o f t e n i n s p r i n g and summer.(Table 4 ) .  they were  A c c o r d i n g to Care-  (1973) these animals emerge at n i g h t from c r a c k s and c r e v i c e s among  b o u l d e r s above the h i g h t i d e l i n e , feed.  areas  s e l e c t i o n by h u n t i n g mink.  Other  foot  seasons  I s l a n d , and t h a t those from middens i n B a r k l e y  Sound were s u b s t a n t i a l l y l a r g e r than those i n Clayoquot Sound.  ential  that,  and move i n t o the i n t e r t i d a l  zone to  A t such times they may be taken s i n g l y by mink, but they are p r o b a b l y  most v u l n e r a b l e when t i d e s are h i g h at n i g h t .  Under these c o n d i t i o n s 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 contained 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 examination 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 i s 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 i t s 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 i t s 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 i n v e r t e b r a t e s were used l i t t l e . o f t e n appeared on b a i t  A species of beetle  i n my l i v e t r a p s and on items i n middens, and a l s o  o c c a s i o n a l l y showed up i n f e c a l samples. advertently,  (Carabidae)  a l t h o u g h on one o c c a s i o n  Most may have been eaten i n -  several beetles occurred  i n a single  scat. Fish F i s h was  second o n l y to crab  mink ( F i g u r e 7 ) .  among foods used by Clayoquot Sound  Both a t Vargas I s l a n d and at T o f i n o ,  f i s h occurred i n  h a l f o f the s c a t s c o l l e c t e d i n the August-November and December-March periods  (i.e. fall  and w i n t e r ) but i n the A p r i l - J u l y p e r i o d  summer) frequency o f o c c u r r e n c e f e l l a l s o the number two r a n k i n g somewhat l e s s o f t e n t h e r e , i n the p e r i o d s  o f study.  to 30 per cent  item among B a r k l e y  (Table  3).  F i s h was  Sound mink, but i t o c c u r r e d  at a frequency o f 20 p e r cent o r l e s s (Table However, T a b l e  5 shows t h a t some B a r k l e y  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 t i d e b o u l d e r h e a v i l y upon s m a l l  ( s p r i n g and  4)  Sound  beaches, were  preying  fish.  F i s h remains found i n f e c e s were n o t o f t e n i d e n t i f i a b l e to taxonomic c a t e g o r i e s , but a l l were l i s t e d under one o r more o f t h r e e (1) s m a l l  (centra of largest vertebrae  medium ( c e n t r a o f l a r g e s t v e r t e b r a e (centra of vertebrae vertebrae).  2-5 mm  l a r g e r than 5 mm,  As the data  i n Tables  were i n .the " s m a l l " c a t e g o r y .  i n diameter),  o r , coarse  Appendix 9 l i s t s  (2)  and (3) l a r g e  f i s h bones p r e s e n t ,  but no  occurrences  the l e n g t h , weight and cen-  and s u b t i d a l f i s h  into perspective.  categories:  i n diameter),  3-5 show, by f a r most f i s h  trum diameters f o r s e v e r a l i n t e r t i d a l meaning o f the s i z e c a t e g o r i e s  l e s s than 2 mm  size  to b r i n g the a c t u a l  As these f i g u r e s 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 proportional 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 have no scales at a l l .  Xiphister atropurpureus ,  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 intertidal 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 important 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 minkcaught specimens at middens and den-sites, and the following paragraphs summarize these specific findings.  A l l 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 i t is a large species and i t s easily identifiable head was conspicuous in middens. Occurrences of this species, recorded in a l l 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 i n ^ a l l 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 a l l areas,  contained tiny green bones suspected to have come from small sculpins.  In a  few cases these included preopecular bones bearing b i f i d spines and were a l 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 a l l study areas, and both were eaten by mink. Two specimens 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 successfully 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 coloration, 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. H i l l i e r , 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. A l l 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 spawnedout 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 shrubbery 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 a l l 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 a l l bird occurrences 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 a l l 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. Representatives 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, i t s midden  contained the remains of two young gulls and fragments of some gull eggshells.  The 12 scats in i t s 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 k i l l i n g 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). record of this species as mink food.  This was my only  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 i t s 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 o c c u r r e n c e s marine.  listed  as d e b r i s i n v o l v e d v e g e t a t i o n , both  I b e l i e v e t h a t most was  food b e i n g eaten,  dogs are seen to do,  f o r t h e i r food v a l u e .  a dark, m i n e r a l m a t e r i a l which may glands o f  but I do not  and animal  composed  I t i s l i k e l y t h a t mink o c c a s i o n a l l y chew and  p l a n t s much as domestic p l a n t s are eaten  taken a c c i d e n t a l l y as i t adhered to  a l t h o u g h on a few o c c a s i o n s an e n t i r e s c a t was  o f v e g e t a b l e matter.  land  swallow  think that  any  A few d e b r i s o c c u r r e n c e s were o f  have been the remains from the d i g e s t i v e  mollusks.  DISCUSSION COMPARISON WITH OTHER AREAS The at  establishment  o f v i a b l e p o p u l a t i o n s o f the North American mink  s e v e r a l l o c a t i o n s on o t h e r c o n t i n e n t s , f o r example S i b e r i a  (Benkovsky  1971), o t h e r p a r t s o f the U.S.S.R. ( A l i e v and Sanderson 1969), Sweden {(jGerell 1967a), Norway (Wildhagen 1956)  and p o r t i o n s of England,  and Wales (Thompson 1968), a t t e s t s to the a d a p t a b i l i t y o f t h i s C e r t a i n l y one o f the f e a t u r e s o f i t s l i f e to  i n c l u d e mostly s i z e o f hares Hamilton  o f a l l prey animals  s m a l l e r than the mink, although mammals up  (Lepus sp.)  to the  and b i r d s o f cormorant s i z e o r l a r g e r are  b i a n s , t h i r t e e n genera o f mammals and  two  taken.  t h r e e o f amphi-  of c r u s t a c e a n s , e i g h t o r d e r s o f  a s s o r t e d b i r d s , r e p t i l e s , mollusks  a s i n g l e s e r i e s o f mink stomachs from New of  in i t s  s p e c i e s known would be v e r y large.,, and would  (1959), a l o n e , i d e n t i f i e d n i n e f a m i l i e s o f f i s h and  i n s e c t s and  animal.  h i s t o r y which c o n t r i b u t e s much  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  feeding. A l i s t  Scotland  York.  and o t h e r i n v e r t e b r a t e s i n Table 6 presents  results  the major mink food h a b i t s s t u d i e s o f which I am aware, showing the  Table 6.  Predominant foods of wild (North America) and feral (Eurasia) mink (Mustela vison). The f i r s t and second ranking classes of food animals in each area and season are l i s t e d (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).  Fall S O N  Winter D J F  New York  several?  M,F  New York  marsh  Michigan  several  M, A  Sealander (1943)  Pennsylvania  several  F.C  Guilday  North Carolina  marsh  F,M  Wilson (1954)  Missouri  several  I.A  Korschgen (1958)  Interior Alaska New York  rivermarsh several  F,M  Iowa  marsh  M,B  Western Alaska  river delta  Sweden (nine areas)  Sweden  - marsh lake-marsh river stream river stream river river-lake sea coast river  M.F  sea coast  F,M  Hamilton (1959)  M,B  M.B  Waller (1962)  M,B  F.C  Burns (1964)  F.M F.C F.C  M,B C,B C.F  C,F  C,F  F.C  M,F M,F F.C  C,F C.F&M M,F M,F F.C  M.B M.A F,M CF C,F C,M M.F M.F F.C  F.B  F.B&M  F.B  F.M  Gerell (1968)  Erlinge  (1969)  Aliev & Sanderson (1969)  M,F C.F  (1949)  M,F  F,M F.M F,M  C,F  Hamilton (1936)  Harbo (1958)  M,F  B.M  SOURCE  Hamilton (1940)  M.F  F.I  Russia B r i t i s h Columbia  Spring M A M  Summer J J A  HABITAT  AREA  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 f i r s t 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 i s 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 a l l those areas for which there is no listing of mammals in Table 6, i.e., where they were not f i r s t or second in occurrence 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 w a t e r temperatures which reduced t h e a c t i v i t y and a g i l i t y o f f i s h , making them e a s i e r t o c a t c h .  E r l i n g e (1969) a l s o r e l a t e d s e a s o n a l  differences i n  f i s h caught by b o t h r i v e r o t t e r s and mink t o t h i s . e f f e c t o f water t e m p e r a t u r e , and  showed i n an e a r l i e r paper ( E r l i n g e 1968a) t h a t 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 t h e  f a s t e r ones appeared t o be p r e f e r r e d .  H i s c o n c l u s i o n s were based on f e e d i n g  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 t h a n were i n t a c t f i s h o f t h e same s p e c i e s , and t h e slower o f two s p e c i e s f e l l p r e y t o t h e o t t e r s more f r e q u e n t l y when b o t h were i n t r o duced t o g e t h e r .  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 p r o c e s s e s a t lowered water t e m p e r a t u r e s i s e v i d e n t from d i s c u s s i o n s 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 i n s h o r e water temperatures v a r y l i t t l e o v e r t h e y e a r , and t h e r e i s e v i d e n c e t h a t o t h e r f a c t o r s such as t h e e f f e c t o f wave a c t i o n on r o c k y shore c o v e r p r o b a b l y f i s h on my study  c o n t r i b u t e d more t o t h e i n c r e a s e d w i n t e r use o f  areas.  THE NATURE OF PREDATION BY MINK Factors A f f e c t i n g Prey S e l e c t i o n Because t h e r e i s some c h a r a c t e r i s t i c t e r m i n o l o g y feeding ecology  of predators,  a b r i e f review  a s s o c i a t e d w i t h the  i s i n order.  I n her exhaustive  r e v i e w o f s t u d i e s f r o m a l l over the w o r l d , Ewer (1973) c o n c l u d e d t h a t most carnivores are " o p p o r t u n i s t i c " i n  their f e e d i n g .  She c i t e s t h r e e 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 e x p e r i e n c e  w h i c h combine t o determine  what a g i v e n c a r n i v o r e w i l l e a t i n any l o c a l c o n t e x t , and p o i n t s o u t t h a t little  i s known o f t h e l a s t two i n n a t u r e .  predator,  Experience,  i s an i n d i v i d u a l m a t t e r and i s p r o b a b l y  a property o f the  of l i t t l e  importance i n  the c o n s i d e r a t i o n o f a s p e c i e s ' 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 sometimes be involved.  Some authors apply the term "preference" whenever a  prey species is shown to be eaten out of proportion to i t s 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 a l l 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 i s 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 "availability" 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, a l l 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 crayfish 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 t h a t I d i d not examine each more than once.  I can g e n e r a l i z e , though  From t h i s e x p e r i e n c e  s u b j e c t i v e l y , about r e l a t i v e toughness  s k e l e t o n s o f s p e c i e s commonly eaten.  I n hardness  o f the  exo-  and t h i c k n e s s , those o f  k e l p c r a b s are by f a r the.ascendants, w h i l e those o f the r e d crab are second.  Carapaces  o f s l e n d e r c r a b s are moderately hard, but are u s u a l l y  and easy to break,  and those o f helmet  c r a b s , though  tough,  r a t h e r than h a r d and are p r o b a b l y the e a s i e s t to chew.  thin  are l e a t h e r y  Finally,  for a l l of  these s p e c i e s , carapace t h i c k n e s s i n c r e a s e s w i t h i n c r e a s i n g s i z e . few p e r t i n e n t o b s e r v a t i o n s o f h u n t i n g mink are c o n s i s t e n t , w i t h  A g a i n , the  animals  s e l e c t i n g a s o f t e r c r a b ( w i t h i n o r between s p e c i e s ) when two were e q u a l l y available. Perhaps  a more o b j e c t i v e measure o f the a t t r a c t i v e n e s s o f v a r i o u s 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 d e s c r i b e d the manner i n which d i f f e r e n t p r e y are eaten and,  f o r c r a b s , have compared use o f t h r e e s p e c i e s d i s c u s s e d above.  summarize, mink w i l l  almost always  eat a l l o f the body meat and  from a c r a b , r e g a r d l e s s o f s p e c i e s ; they r a r e l y touch the hard,  To  viscera small-  diameter, t u b u l a r appendages o f the k e l p c r a b , but w i l l o f t e n consume at least  the p r o x i m a l p o r t i o n s o f r e d crab appendages, e s p e c i a l l y the t h i c k ,  meaty c h e l i p e d s .  The c o m p a r a t i v e l y s o f t appendages o f the helmet  however, are r e g u l a r l y eaten.  F u r t h e r p o s t - c a p t u r e evidence o f  mink p r e f e r e n c e s i n c l u d e s i n c i d e n t s i n which mink caught c r a b s (removed appendages), l a r g e r than 100 mm) but then l e f t  but then l e f t  crab,  apparent  and m u t i l a t e d  them near shore (mostly r e d crabs  and i n c i d e n t s i n which crabs were taken to middens  to s p o i l  Helmet c r a b s were never  ( o c c a s i o n a l l y r e d c r a b s , but most o f t e n k e l p c r a b s ) . so t r e a t e d .  74 Certainly the degree to which a mink may exercise choice depends upon the state of i t s 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. A l l 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 a l l 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 i t s 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. appears to be mostly size which limits use of the latter species.  It  Crustaceans  eaten in other areas (Guilday 1949; Gerell 1968; Burns 1964), mostly fresh water crayfishes, aire within the size range given above. A l l 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 Cormorants 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 w i l l (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 p a l l a s i i , 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 sinceV d6zens of species (invertebrates, fish, birds, mammals) of the t  appropriate size at maturity frequent l i t t o r a l 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 l i 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 l i f e 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 welldefined movements in from deeper waters (see MacKay 1942). time that they are most accessible to beach predators.  It i s at this  I have established  earlier that the Dungeness crab is used l i t t l e by mink, apparently because of i t s 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' l i f e history as i t may relate to i t s 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 (NovemberDecember) 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 l i t t o r a l foraging 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 southeastern 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 l i f e  history has not been recorded, but the above observations suggest that, like the Cancer crabs to which i t is related in the family Cancridae, i t moves onshore in summer to complete i t s 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 i t in use there.  I have already speculated that where i t occurs, i t is  preferred, both because of i t s relatively soft carapace and because i t s rather small chelae do not enable i t to defend i t s e l f to the extent that species of Cancer and Pugettia are able to. The kelp crab is another species whose l i f 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. mink,  It  is relatively  during  this  early winter into It  at  As Knudsen (1964a)  storms have removed most o f  intertidal  is  time.  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  that  It there  w a t e r s w h e r e t h e y may become t e m p o r a r i l y  time  t h a t mink p r e d a t i o n on the  e a t e n by mink throughout  i s h o t known w h e t h e r  would i n c r e a s e t h i s  species'  bearing females during  Sound.  It  and I  brown a l g a e )  crabs May,  to mink.  I  it  which I  that  the primary  still  a distinct  significant among  winters. mating  season,  and w h e t h e r  a s s o c i a t e d w i t h mating to mink.  which  Knudsen has found egg-  the  a b o v e - m e n t i o n e d movement  in  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 factor  use of  affecting this  10 t o  of  stands  the v u l n e r a b i l i t y  crab f a l l s waters  off  of  kelp  c o n s i d e r a b l y by  i n numbers a t  that  20 a n i m a l s i n e e l g r a s s b e d s and o n  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 .  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 In  following  addition  of  to b e i n g hard to  the eat,  Failure  preferences the  kelp  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  species  6).  The l i f e that of  move  carnivorous.  s p e c i e s becomes  three  occupies i n t e r t i d a l  suggested e a r l i e r .  (Appendix  to  more  and  h a v e 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  saw c o n c e n t r a t i o n s o f  o f mink to e x p l o i t  fall  m o n t h s e x c e p t M a y , S e p t e m b e r and O c t o b e r  Interestingly,  e x p o s e d mud b o t t o m s  crab  is  two o f  vulnerability  changes ( i . e . , is  even though  time.  all  appears l i k e l y  seasonal habitat of  there  late  Kelp crabs predominated  a r e any movement o r b e h a v i o r p a t t e r n s  Puget Sound,  after  to  t h e s e a l g a e many i n d i v i d u a l s  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 ) . crustaceans  indicates,  also  cycle of  the  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  the r e d crab (Knudsen 1964a).  by mink d i d n o t  parallel  that  for  The f a c t  t h a t use of  r e d c r a b s was a p p a r e n t l y  identical  the  species  due m a i n l y  to  80  d i f f e r e n c e s i n abundance between the two the s l e n d e r crab was  rarely  species.  seen d u r i n g the f i r s t  f r e e - l i v i n g o r among mink food items, but unusual as i n d i c a t e d by unprecedented f o l l o w e d by a dramatic The  As  indicated  two  earlier,  y e a r s o f study,  abundance i n w i n t e r  o b s e r v a t i o n s o f numbers o f l i v e c r a b s ,  " f u n c t i o n a l response"  ( H o l l i n g 1959)  either 1971,  was  by l o c a l mink.  exact n a t u r e o f mink p r e d a t i o n on the s l e n d e r crab i s not d i s c e r n a b l e  from my  o b s e r v a t i o n s ; i t i s e v i d e n t t h a t mink w i l l prey h e a v i l y upon the  s p e c i e s when they can, but p r e d i c t i o n o f time and c i r c u m s t a n c e s  i s not pos-  s i b l e to the e x t e n t t h a t i t i s f o r s p e c i e s d i s c u s s e d p r e v i o u s l y . Most o f the o t h e r crab s p e c i e s 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 c o n t i n u o u s l y abundant under i n t e r t i d a l b o u l d e r s over study areas. o f up to 500  Low  (1971) r e p o r t s t h a t these c r a b s may  be found  both  i n densities  animals per square meter i n good h a b i t a t , and good h a b i t a t  p e r t a i n e d on many beaches i n both B a r k l e y and Clayoquot p o p u l a t i o n o f raccoons  i n Clayoquot  75 per cent o f 330 raccoon  Sounds.  A large  Sound s u b s i s t e d l a r g e l y on Hemigrapsus;  s c a t s c o l l e c t e d t h e r e throughout  dominated by t h i s genus ( H a t l e r , u n p u b l i s h e d ) .  The  1971  were  f a c t t h a t mink d i d not  o f t e n eat shore c r a b s , even though they hunted on the same beaches w i t h the raccoons, was  a p p a r e n t l y due  to t h e i r i n a b i l i t y  to c a t c h them r e g u l a r l y .  Mink a p p a r e n t l y can not o v e r t u r n most b o u l d e r s which s h e l t e r shore c r a b s , or at l e a s t they do not t r y to do so o f t e n . u s u a l l y caught back up  i n the mouth.  The  Prey items beneath b o u l d e r s  are  d p r s o - v e n t r a l l y f l a t t e n e d c r a b s , which  i n t o the t i g h t e s t c r e v i c e s , are d i f f i c u l t  to grab, i n t h i s manner.  Raccoons, on the o t h e r hand, remove crabs from t i g h t  spots w i t h t h e i r  paws, and o f t e n move s m a l l e r b o u l d e r s to expose prey beneath.  fore-  Most mink  81  p r e d a t i o n upon shore c r a b s i n v o l v e d l a r g e specimens,  m o s t l y males.  was p r o b a b l y due to d i f f e r e n t i a l b e h a v i o r between the sexes  This  since,as  Knudsen (1964a) p o i n t s . o u t , m a l e s o f t e n f o r a g e out on the tops o f i n t e r t i d a l b o u l d e r s , w h i l e females  are "more t i m i d and remain  along the lower  s i d e s o f r o c k s where they can r e a c h a h i d i n g p l a c e i n case o f emergency". F i s h - Due to the p a u c i t y o f l i f e h i s t o r y i n f o r m a t i o n on most s p e c i e s eaten by mink on my study areas, i t i s d i f f i c u l t relative vulnerability.  C l e a r l y some f i s h  fish  t o determine  their  such as the salmons o c c u r i n  areas a c c e s s i b l e t o mink o n l y s e a s o n a l l y , d u r i n g a s m a l l p a r t o f the y e a r . O t h e r s , such as s m a l l r o c k f i s h e s and g r e e n l i n g s , may be p r e s e n t throughout  the y e a r , but a r e p r o b a b l y a v a i l a b l e , t o mink o n l y under  circumstances underwater. matically.  subtidally uncommon  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 q u a r t e r s I t i s d o u b t f u l t h a t such free-swimming f i s h are hunted  syste-  S i n c e evidence has i n d i c a t e d t h a t the p r i c k l e b a c k - g u n n e l group  (suborder B l e n n i o i d e i ) and the s c u l p i n s , e s p e c i a l l y i n t e r t i d a l c o n s t i t u t e the most important food f i s h i n my areas, I w i l l  forms,  limit  further  d i s c u s s i o n to these two groups. Abundance among i n t e r t i d a l number o f organisms  fish  i s most d i f f i c u l t  to a s s e s s .  The  one might see on a beach i s i n f l u e n c e d by many v a r i a b l e s  i n c l u d i n g s l o p e and exposure  o f the beach, t i d e l e v e l ,  amount and n a t u r e o f  a v a i l a b l e c o v e r , n a t u r e o f s u b s t r a t e , d i s t r i b u t i o n o f food items, and perhaps w'eather c o n d i t i o n s , season,  and time o f day.  must add some p u r e l y p r a c t i c a l ones: the f i r s t  To these c o n s i d e r a t i o n s one  B l e n n i o i d s are hard t o count.  r o c k has been turned, the animals a r e moving and counts  a d j a c e n t rocks w i l l  inevitably include duplications.  cannot be made without  After  from under  Counts by removal  severe d i s r u p t i o n o f the h a b i t a t and maiming o f the  82 fish.  Most f i s h o c c u r under r o c k s o n l y i n the lower r e g i o n s o f the  tidal  zone (lower m i d - t i d e and  1962), thus they can be counted  low t i d e h o r i z o n s o f R i c k e t t s and  E x a c t replacement  p o s s i b l e , thus f e a t u r e s o f the h a b i t a t o f importance inevitably  o f r o c k s i s seldom  to h u n t i n g mink are  i t more p r o f i t a b l e f o r me  spend low t i d e p e r i o d s l o o k i n g f o r mink r a t h e r than f o r f i s h , not apparent  t h a t abundance per se was  governing " a v a i l a b i l i t y " appraisal.  less  altered.  F o r the above reasons, because I f e l t  i t was  Calvin  only during a short period, u s u a l l y  than an hour, and o n l y on some days.  inter-  and  the most important  i n t h i s case, I s e t t l e d  because factor  for a less objective  I o f t e n t u r n e d over b o u l d e r s , i n d i f f e r e n t areas and  f e r e n t times o f y e a r , to see r o u g h l y what was  to  present.  at d i f -  Appendix 11  lists  some r e s u l t s o f these o b s e r v a t i o n s , and a l l o w s the f o l l o w i n g g e n e r a l i z a t i o n s . I n the h a b i t a t b e i n g d i s c u s s e d , i . e . , low t i d e beaches o f l o o s e r o c k s boulders  (which I w i l l  show l a t e r appears  to be p r e f e r r e d mink h u n t i n g  h a b i t a t ) , b l e n n i o i d s always predominated among f i s h , o c c u r r e d more commonly than d i d s c u l p i n s . t i d a l habitats;  in classical  o f seawater are l e f t  and  clingfishes  There are o t h e r k i n d s o f  " t i d e p o o l " s i t u a t i o n s , i . e . , where  puddles  (or at l e a s t were the most c o n s p i c u o u s ) .  The  o b s e r v a t i o n s r a r e l y hunted such h a b i t a t s , however they o f t e n  dove i n waters a d j a c e n t to low t i d e beaches o f a l l k i n d s , and s i t u a t i o n s I a l s o saw The  inter-  i n d e p r e s s i o n s a f t e r the t i d e has f a l l e n , : . s c u l p i n s  were the most abundant f i s h mink o f my  and  i n these  s c u l p i n s more o f t e n than I saw b l e n n i o i d s .  f a c t t h a t s c u l p i n s were i d e n t i f i e d  i n the f e c a l samples more than  twice as o f t e n as were b l e n n i o i d s r e q u i r e s some d i s c u s s i o n . impression that s c u l p i n s , having  I t was  my  s e v e r a l d i a g n o s t i c bones, are more e a s i l y  83  identified and  from s k e l e t a l m a t e r i a l  i t c o u l d be  this fact.  f o r c e o f t h i s argument i s reduced by  dominate i n a l l seasons.  As  n i o i d s were i d e n t i f i e d more o f t e n i n the July),  and  groups I have d e a l t  sculpins p r e v a i l e d during  r e l a t i v e abundance (Appendix 11)  spring-summer p e r i o d  the r e s t o f the y e a r .  do not permit any  n i o i d s there  are changes i n numbers p a r a l l e l to the  o c c u r r e n c e of s c u l p i n s , but  My  shown by  the  seasonal  abundance r a t i n g "many" (from Appendix 11)  Observations  No. "Many"  regarding  superlative  t a b l e below:  All  Areas  No. Observations  No "Many"  Dec-Mar  3  0  4  1  Apr-Jul  4  2  7  5  Aug-Nov  3  0  4  1  totals  15  10  F i v e o f seven o b s e r v a t i o n s period,  and  beaches and  on  the A p r i l - J u l y  d i s t r i b u t i o n o f my i n the  data  through  apparent changes i n  Vargas I s l a n d No.  that  suggest t h a t f o r b l e n -  o c c u r r e n c e i n the d i e t , i . e . , the maximum i s a t t a i n e d d u r i n g i s best  fact  (April  conclusions  seasonal  This  the  the d a t a i n T a b l e 7 show, b l e n -  proportional  period.  with,  argued t h a t t h e i r apparent predominance i s r e f l e c t i v e only, of  However, the  they d i d not  than are the o t h e r  at the "many" l e v e l were i n the  at Vargas I s l a n d where o b s e r v a t i o n s  spring-summer  were from a v e r y  few  are t h e r e f o r e more s t r i c t l y comparable, the o n l y "many"  o c c u r r e n c e s were i n t h a t  period.  84  T a b l e 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 ( S t i c h a e i d a e and P h o l i d a e ) 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  - Nov  No.  7o  40  13.8  12  7.8  22  7  2.4  16  10.4  13  Other  244  83.8  126  81.8  160  T o t a l (n)  291  100.0  154  Sculpins Blennioids D >  2 X  a  (Dec-Mar v s A p r - J u l v s  No.  . 7o  Aug  100.0  No.  185  %  11.9 7.0 81.1 100.0  Aug-Nov) = 15.15, df = 4, p < 0 . 0 1  No. B number o f o c c u r r e n c e s ; % = p e r c e n t o f t o t a l (Sculpins plus blennioids plus other).  occurrences  ^Other: l a r g e l y u n i d e n t i f i e d , many p r o b a b l y i n c l u d i n g u n i d e n t i f i a b l e o c c u r r e n c e s o f s c u l p i n s and b l e n n i o i d s i n v a 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 i t s 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 l i f e and do not adopt the adults' demersal existence until an six months.  age of five or  British Columbia blennioids may follow a similar pattern;  Peppar (1965) noted that the f i r s t small young (25-30 mm) comb which he encountered in his study.were in June.  of the high cocks-  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 mosshead 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 i s 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 i n 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 i s 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 t e r r i t o r i a l i t y 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, " i s considered a mechanism by  which shallow water fishes of rocky shore areas avoid being l e f t 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 contingencies, 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 i t s maximum occurrence during the winter months (Figure 7, Table 3). Birds - Hatler e_t a l . (In Press) recorded 247 vicinity of the study areas.  species of birds in the  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 opportunity 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; undoubtedly some of the birds I have recorded as being eaten by mink were f i r s t killed or wounded by hunters. Studies elsewhere have also found mink feeding upon vulnerable individual birds, but not taking any species at w i l l .  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 a l . (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 blackbirds (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 observations (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 differences 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 i s 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 d i f f i c u l t 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  i n t o crab p o p u l a t i o n s , e s p e c i a l l y l o c a l l y . to have caught a minimum o f 276 and  k e l p crabs  F o r i n s t a n c e , one i n l e s s than two  a l l of these came from waters s u r r o u n d i n g  diameter.  Observations  an i s l e t  are unknown.  l a t i o n s before  s h o r t span o f s h o r e l i n e .  Unfortunately,  zone, and  the extent o f r e c r u i t m e n t  their  the  those from o t h e r  Much must be l e a r n e d about the dynamics o f crab popu-  the e f f e c t s o f mink p r e d a t i o n can be  ascertained.  E f f e c t s on S e a b i r d C o l o n i e s Guiguet  do a l l o f  i n deeper waters are a v a i l a b l e to r e p l a c e  removed from the i n t e r t i d a l sources  w i n t e r months,  o f i n d i v i d u a l mink c a t c h i n g f o u r or more crabs  along a r e l a t i v e l y  degree to which crabs  known  s c a r c e l y 75 m i n  d u r i n g a s i n g l e low t i d e were not uncommon, and many animals hunting  mink was  (1971) s p e c u l a t e d , p r o b a b l y  ,  correctly,  t h a t s e a b i r d s along  the  B r i t i s h Columbia c o a s t are l i m i t e d to n e s t i n g on i s l e t s ' which are not i n h a b i t e d by mink.  According  to C a r l e t a l . (1951), an u n a u t h o r i z e d  d u c t i o n o f mink to Lanz I s l a n d o f f the northwest end  intro-  o f Vancouver I s l a n d i n  the l a t e 1930's a p p a r e n t l y r e s u l t e d i n e l i m i n a t i o n o f n e s t i n g s e a b i r d s i n a p e r i o d of l e s s than  12 y e a r s .  there,  Damage to c o a s t a l b i r d c o l o n i e s has  been r e p o r t e d i n Sweden ( c i t a t i o n s i n G e r e l l  1968)  also  and Norway (Wildhagen  1956). There are t h r e e s m a l l v e g e t a t e d areas which support  i s l a n d s i n the v i c i n i t y o f my  numbers o f n e s t i n g s e a b i r d s .  Burrow-nesting s p e c i e s ,  which one would t h i n k would be more v u l n e r a b l e to p r e d a t i o n by mink those which n e s t on c l i f f s two  or out  than  i n the open i n c o l o n i e s , are common on  ( C l e l a n d I s l a n d west o f Vargas I s l a n d and  B a r k l e y Sound) and occur  study  S e a b i r d Rocks j u s t  i n s m a l l numbers on the t h i r d  south  of  (Florencia Island  94  at the south end o f Long Beach). all  three.  I have v i s i t e d  I have searched  Glaucous-winged G u l l s n e s t i n numbers on  each o f these  twice and,  although  c a r e f u l l y , I have never seen a s i g n o f mink at any.  I have examined a number o f v e g e t a t e d Sound, which support mink but not b i r d s would use  i s l a n d s at l e a s t  all,  or any,  islets,  i n c l u d i n g dozens i n B a r k l e y  seabird colonies.  o f these  Conversely  It i s n o t  c e r t a i n that  i f mink were not p r e s e n t , but on  few B a r k l e y Sound i s l a n d s where both n e s t i n g b i r d s and mink do o c c u r , nest  i n l o c a t i o n s l a r g e l y i n a c c e s s i b l e to mink (cormorants on c l i f f s  sea caves,  gulls  those the b i r d s  and  i n t r e e s and on r o c k p i n n a c l e s at some d i s t a n c e from  or at low d e n s i t i e s ( B l a c k O y s t e r c a t c h e r s  in  cover)  on s e v e r a l c o a s t i s l a n d s and  head-  lands) . It  seems e v i d e n t  lacking s o i l i t s den.  t h a t a mink can not  l o c a t e permanently on an  and v e g e t a t i o n o r at l e a s t a h o l l o w  l o g i n which i t can make  There are s e v e r a l such i s l e t s along the west c o a s t of Vancouver  I s l a n d , and many harbour n e s t i n g b i r d s such as g u l l s , cormorants, o y s t e r c a t c h e r s i n summer, but a p p a r e n t l y more secure,  l i e barren  i n winter.  and  These s p e c i e s  are  t h e r e f o r e , than are the burrow n e s t i n g s p e c i e s which  have some h a b i t a t requirements The  island  ( s o i l , vegetation)  s i m i l a r to those o f mink.  t h r e e i s l a n d s mentioned e a r l i e r as s u p p o r t i n g burrowing s p e c i e s  each two  k i l o m e t e r s o r more from the n e a r e s t m i n k - i n h a b i t e d  t h i s d i s t a n c e appears to be  sufficient  to prevent  are  l a n d mass,  n a t u r a l establishejmht  and of  mink p o p u l a t i o n s . Both i n s t a n c e s o f p r e d a t i o n on n e s t i n g s e a b i r d s recounted on White I s l e t o f f Long Beach and near V i c t o r i a  t h a t observed  earlier,  by Campbell on Chain  that  Islets  (see page 64 ), i n v o l v e d temporary v i s i t s by a s i n g l e mink to  95  the unvegetated rocky  type h a b i t a t d e s c r i b e d  above.  The  long term damage  which mink can do under such c i r c u m s t a n c e s remains u n c e r t a i n . case at White I s l e t ,  the midden and  l a t r i n e were l o c a t e d i n the  cover a v a i l a b l e on the e n t i r e i s l e t , was  an obvious r a i n t r a p .  little  damage to the  The  In  a h o l l o w under a b o u l d e r ,  mink stayed o n l y  a few  the  only and  days, h a v i n g  a p p r o x i m a t e l y 90 n e s t i n g p a i r s o f b i r d s (3  this done  species)  there. At Chain I s l e t s the mink found a dry n e s t d e b r i s and was situation,  able  to s t a y f o r s e v e r a l months.  i t d i d not  appear t h a t p r o d u c t i o n  a l t e r e d i n comparison w i t h (Campbell, p e r s o n a l as a l l d r i f t  adjacent  o f young b i r d s was  no  Again, residence  was  storms.  only  occurred  temporary  According  i t at the end  On  b i r d s ; where c o n c e n t r a t i o n s  the  o f December. P o s s i b l y i f a mink c o u l d f o r more than one  season, i t  the whole, i t appears t h a t f r e e - r a n g i n g mink  i n areas where b i r d s are s p a r s e l y d i s t r i b u t e d  do not  r e g u l a r l y p r e y upon  o f n e s t i n g b i r d s e x i s t , r e s i d e n t mink may  those which cannot e s t a b l i s h permanent r e s i d e n c e  s e r i o u s l y impair  to  s i g n o f the mink, or s u i t a b l e denning s i t e s , on  might have g r e a t e r e f f e c t .  do not  significantly  i s l e t s where no mink  on the b i r d s of a s i n g l e i s l e t  d e s t r u c t i v e , but  However, even i n t h a t  d e b r i s washed away i n w i n t e r  i s l e t when he r e v i s i t e d exert pressure  t h a t on  communication).  l o g s and  Campbell, t h e r e was  s i t e under a p i l e o f beach  an i s l e t ' s c a p a c i t y to support  be  quite  probably  nesting birds.  SUMMARY, FOOD HABITS Mink on the west c o a s t o f Vancouver I s l a n d r e g u l a r l y eat at l e a s t s p e c i e s of c r u s t a c e a n s eat dozens o f o t h e r  and  twelve s p e c i e s of f i s h ,  and have been known to  animals i n c l u d i n g i n v e r t e b r a t e s , f i s h ,  at l e a s t o c c a s i o n a l l y .  six  b i r d s and  Most o f the animals eaten r e g u l a r l y , and  mammals  some o f  96  those which have been d e t e c t e d i n food h a b i t s m a t e r i a l o n l y now and then, must be c l a s s e d as g e n e r a l l y abundant i n o r near o f my study a r e a s .  S e v e r a l animals  ( P o l y c h a e t a ) , ghost  shrimp  l i t t o r a l waters  such as sea cucumbers, sandworms  and p i l e - s e a p e r c h are common to abundant near  shore, but were n o t known t o have been eaten by mink. p r e s e n t , i t i s apparent  along a l l  I n terms o f biomass  t h a t food l i m i t a t i o n p e r . s e . should n o t be a f a c t o r  i n the l i v e s o f c o a s t mink.  However, n o t 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 v a r y 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 Crabs waters  habitats.  are taken most o f t e n i n summer when they move i n t o  f o r mating  n e a r - s h o r e waters  and m o u l t i n g .  intertidal  One s p e c i e s (the k e l p crab) appears i n  i n winter, after  storms have removed the k e l p beds which  c o n s t i t u t e i t s summer r e f u g e , and i t i s preyed upon most o f t e n i n t h a t season.  Intertidal  f i s h occur l e a s t  i n the d i e t i n summer, when t h e i r  b o u l d e r cover i s most s t a b l e , and when o t h e r foods, e s p e c i a l l y c r a b s , are more a v a i l a b l e .  Warm-blooded v e r t e b r a t e s a r e r a r e l y taken except when  s p e c i a l c i r c u m s t a n c e s p e r m i t ; i n the case o f b i r d s , the c i r c u m s t a n c e s 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 h u n t i n g s e a s o n , l e d ! t o J h e a v i e s t use i n f a l l e x p l o i t a t i o n o f migrant  and w i n t e r , although some  c o n c e n t r a t i o n s i n s p r i n g i s suspected.  Invertebrates  o t h e r than c r a b s appear to be taken l a r g e l y when o t h e r food i s n o t immediately available,  again l e s s o f t e n i n summer than a t o t h e r times.  97 FOOD GETTING INTRODUCTION AND METHODS Small m u s t e l i d s a r e l a r g e l y n o c t u r n a l i n most areas, and o b s e r v a t i o n s o f them i n the p r o c e s s o f seeking food have been few. I n h e r r e c e n t Ewer (1973) c i t e d  a few p u b l i s h e d r e c o r d s from the f i e l d which  t h a t t r a c k i n g by scent and response sp.) h u n t i n g methods.  She a l s o concluded,  t h a t the most common k i l l i n g upper neck o r head.  to sounds a r e important  technique  indicated  weasel  (Mustela  as d i d Hewson and H e a l i n g (1971),  f o r these aminals  Burns (1964) r e c o r d e d  review,  i s a b i t e to the  s i x o b s e r v a t i o n s o f mink c a t c h i n g  s m a l l f i s h by d i v i n g from the i c y shores o f sloughs  i n the Yukon-Kuskokwim  D e l t a , A l a s k a , and E r r i n g t o n (1967: 20-29) summarized h i s o b s e r v a t i o n s o f mink i n c o n t a c t w i t h s e v e r a l prey o t h e r accounts  s p e c i e s i n Iowa,marshes.  There a r e few  o f h u n t i n g mink, and those I am aware o f i n v o l v e s i n g l e  a n e c d o t a l o b s e r v a t i o n s such as t h a t r e p o r t e d i n Sargeant  e_t _al.  which a mink jumped from a r i v e r b a n k upon a swimming brood  (1973)  in  of wild ducklings  and k i l l e d o r c r i p p l e d a t l e a s t t h r e e o f them. As  i n d i c a t e d e a r l i e r , my own o p p o r t u n i t i e s f o r watching  mink have been unprecedented.  A basic description of observation  has been g i v e n i n the food h a b i t s s e c t i o n . through  During  August 1972, I saw mink h u n t i n g 432 times.  such as i n t e r f e r e n c e caused  active wild  by my; presence  techniques  the p e r i o d May 1968 F o r a v a r i e t y o f reasons,  and poor v i s i b i l i t y  due t o  adverse weather o r the n a t u r e o f t h e h u n t i n g h a b i t a t , 278 (647o) o f these o b s e r v a t i o n s were s h o r t and p r o v i d e d o n l y minimal d a t a ( d a t e , time, ally  tide level, habitat, size, color, the h u n t i n g method used).  location,  and sometimes sex o f mink, and o c c a s i o n -  An a d d i t i o n a l 56 o b s e r v a t i o n s  (137»)  lasted  o n l y one minute o r l e s s , b e i n g c u t s h o r t i n most cases by the observed  mink's  98 c a t c h i n g something, c a r r y i n g i t away and not r e t u r n i n g . of  2-5  minutes d u r a t i o n numbered 46  minutes each o c c u r r e d  17  times  (117o), those o f 6-10  (47o' each),  Observations minutes and  and o b s e r v a t i o n s  o f more than  minutes d u r a t i o n (up to a maximum o f 80 minutes) numbered 18 out e x c e p t i o n ,  the l o n g e r o b s e r v a t i o n s  mink had  items  sight. its  taken As  away from shore  (47o) .  to cache or eat them, and was  out  time spent  engaged i n o t h e r a s s o c i a t e d  hunting  the l o n g o b s e r v a t i o n s p r o v i d e d success  and  foods  taken  hunting,  activities.  In a d d i t i o n to the b a s i c d a t a l i s t e d e a r l i e r f o r o b s e r v a t i o n s o f one or l e s s ,  of  I n a l l cases  I attempted to p a r t i t i o n the t o t a l o b s e r v a t i o n time i n t o time spent e a t i n g , and  the  r e t u r n i n g to the same a r e a to hunt,  a c t i v i t i e s were c o n s i d e r e d p a r t o f a s i n g l e o b s e r v a t i o n .  time spent  30  With-  i n c l u d e d p e r i o d s d u r i n g which  l o n g as r e c o g n i z a b l e mink kept  11-30  i n f o r m a t i o n on methods of  minute  hunting,  i n r e l a t i o n to v a r i o u s h a b i t a t c o n d i t i o n s .  HUNTING HABITATS All line.  h u n t i n g o b s e r v a t i o n s o c c u r r e d on  shores below the h i g h e s t h i g h water  Mink p r o b a b l y hunt t e r r e s t r i a l prey  a l l y , but I d i d not  see them d o i n g  so.  F o r the purposes o f the  s h o r e l i n e h a b i t a t s were c a t e g o r i z e d i n t o paragraphs.  above t h i s l i n e at l e a s t  occasion-  observations,  f i v e types, d e s c r i b e d i n f o l l o w i n g  When a mink hunted jin water, near shore,  wasereeordedias'.;the shore h a b i t a t from which i t was  i t s hunting h a b i t a t  diving.  Small P a r t i c l e Beach Shores i n t h i s c a t e g o r y  are r e l a t i v e l y homogenous expanses of sand,  s h e l l , or g r a v e l , t y p i c a l l y exposed to heavy wave a c t i o n and p r o v i d i n g cover of  f o r near-shore  marine organisms.  Some s m a l l p a r t i c l e beaches c o n s i s t  compacted, f i n e r p a r t i c l e s approaching  silt  size,  and  i n B a r k l e y Sound  a number o f beaches so c a t e g o r i z e d are composed l a r g e l y o f wave-rounded averaging portrayed,  up  to 20  cm  i n diameter.  An  little  rocks  example o f a s m a l l p a r t i c l e beach i s  from a d i s t a n c e , i n P l a t e l a .  99  Boulder  Beach  T h i s type i s composed p r i m a r i l y o f l o o s e r o c k s o f v a r i o u s shapes and all  s i z e s , b u t i n c l u d i n g many w i t h diameters  g r e a t e r than 20 cm.  The  e s s e n t i a l f e a t u r e o f these beaches i s t h a t they p r o v i d e cover f o r a v a r i e t y o f i n t e r t i d a l organisms eaten by mink. c o n s i s t i n g almost  I n some c a s e s , I examined beaches  e n t i r e l y o f s m a l l r o c k s , averaging  l e s s than 20 cm i n  diameter b u t i r r e g u l a r l y shaped and p i l e d upon each o t h e r i n such a way t h a t chambers and c a v i t i e s  s u i t a b l e as r e f u g e f o r s m a l l f i s h and c r a b s o c c u r -  r e d i n "honeycomb" f a s h i o n throughout.  Boulder beaches, examples o f which  are shown i n P l a t e s 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 s h e l t e r e d waters on the l e e s i d e s o f i s l a n d s and r e e f s , and a l o n g i n l a n d waterways. on these beaches may themselves  The b o u l d e r s  l i e on s u b s t r a t e s r a n g i n g from s m a l l p a r t i c l e s  ( i n which case cover h a b i t a t f o r marine organisms i s l i m i t e d ) to bare  rock.  Rockweed Shore Much o f t h e exposed o u t e r c o a s t and many headlands c o n s i s t o f bare,  s o l i d rock.  i n p r o t e c t e d waters  Below t i d e l i n e , c r a c k s and c r e v i c e s i n t h i s  r o c k and v a r i o u s a t t a c h e d marine a l g a e , e s p e c i a l l y rockweeds (Fucus s p . ) , p r o v i d e h i d i n g p l a c e s f o r s e v e r a l o f the prey s p e c i e s sought  by mink (see  Plate 2). Eelgrass  Flats  Exposed t i d a l  flats  ( P l a t e 3) and s m a l l mud-bottom bays b e a r i n g a  growth o f e e l g r a s s ( Z o s t e r a sp.) and o t h e r marine p l a n t s such as U l v a fit  i n t h i s category.  They d i f f e r  from o t h e r s m a l l p a r t i c l e shores i n t h a t  they u s u a l l y occur i n p r o t e c t e d waters, their ability  to support a p l a n t c o v e r .  t h i s f a c t probably accounting f o r  face page 100  P l a t e 2: a)  Pelage; Hunting h a b i t a t s A m o u l t i n g female mink, showing u n u s u a l l y l i g h t p e l a g e b e i n g r e p l a c e d by a new,darker summer coat.  b,c) Mink h u n t i n g by " p o k i n g " on a t y p i c a l shore. d)  rockweed  Mink h u n t i n g i n a b o u l d e r beach h a b i t a t .  face page 101  P l a t e 3:  Hunting h a b i t a t s a)  Mink h u n t i n g  on an e e l g r a s s  b)  Mink on  c)  A t y p i c a l boulder  a mixed e e l g r a s s beach.  flats  f l a t s - b o u l d e r beach.  102 Estuary Shores  along the lower reaches o f c o a s t a l streams,  p o r t i o n s exposed  to marine t i d a l  specifically  a c t i o n , c o n s t i t u t e t h i s beach  those  category.  Such shores are u s u a l l y composed o f s m a l l p a r t i c l e bottoms, u s u a l l y sand o r mud  but o c c a s i o n a l l y g r a v e l and l a r g e r stones, and most p r o v i d e r e l a t i v e l y  little  cover f o r i n t e r t i d a l  RESULTS AND  animals.  DISCUSSION  HUNTING METHODS Mink hunted by t h r e e main t e c h n i q u e s , which I termed " b i r d - d o g g i n g , poking, and d i v i n g " , but they a l s o employed •-. v a r i a t i o n s o f each and comb i n a t i o n s o f one o r more.  As shown i n T a b l e 8, the method used  to depend mostly upon h a b i t a t , and a s i n g l e mink w i l l i n one h u n t i n g s e s s i o n , changing next.  appears  employ s e v e r a l methods  as i t moves from one h a b i t a t type to the  F o l l o w i n g a r e d e s c r i p t i o n s o f the c i r c u m s t a n c e s and mechanics o f  methods used by mink to f i n d f o o d , t o g e t h e r w i t h accounts o f what p r e y animals were u s u a l l y found i n these ways and how been  they were caught once h a v i n g  found. B i r d - d o g g i n g - T h i s technique was  but I a l s o saw lines,  used most o f t e n on e e l g r a s s f l a t s ,  i t on s m a l l 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  and along expanses o f homogeneous, s p a r s e l y - v e g e t a t e d rockweed  (Table 8 ) . implies,  The c h i e f c h a r a c t e r i s t i c o f t h i s t e c h n i q u e s , as my name f o r i t  i s t h a t the mink h o l d s i t s nose to the ground,  moves a l o n g .  shore  I t u s u a l l y m a i n t a i n s a semi-crouched  s u r p r i s i n g l y q u i c k l y i n t h i s way,  l i n e o r may  as i t  p o s t u r e , sometimes moving  but I a l s o saw mink b i r d - d o g g i n g from a  more u p r i g h t p o s i t i o n . W h i l e b i r d - d o g g i n g , a mink may r e l a t i v e l y straight  sniffing,  move forward i n a  change d i r e c t i o n f r e q u e n t l y , moving e i t h e r i n  a z i g - z a g or back-and f o r t h p a t t e r n ; 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.  Bird-dogging No. %  Hunting Methods Poking Diving No. % No. %  More Than One Method No. 7o  3  Boulder Beach (n=106:86)b  3  2.8  72  67.9  31  29. 2  20 23.2 Poking + Diving = 18 Bird-dogging + Diving = 1 Poking + Bird-dogging = 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 = 15 Bird-dogging + Poking = 1 Diving + Poking = 1 A l l three methods = 1  1  12.5  3  37.5  4  50.0  2 33.3 Poking + Bird-dogging = 1 Poking + Diving = 1  Small Particle Beach and Estuary (n=8; 6)  X* (Three hunting methods vs Boulder Beach + Rockweed Shore + a l l 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 f i r s t 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 i s the total from which the percentage under the category is calculated.  104  to  s i d e so t h a t the area covered  i t may  i n t e n s i v e l y i s wider  than i t s body,  and  slow down o r stop f r e q u e n t l y to extend i t s head i n some d i r e c t i o n ,  a p p a r e n t l y to i n v e s t i g a t e ; ^ c o n c e n t r a t i o n ' o f - o d o r . Commonly i t i s moving q u i c k l y enough to o v e r - r u n p o t e n t i a l food items, and on s e v e r a l o c c a s i o n s I  saw b i r d - d o g g i n g mink stop s h o r t , then run back a few  steps to r e l o c a t e  and c a p t u r e d e t e c t e d prey. Crabs, animals  e s p e c i a l l y Cancer sp. from s p r i n g through  found most o f t e n by mink on e e l g r a s s f l a t s  are o c c a s i o n a l l y found neath of  themselves  shows from above.  r o u g h l y the f o l l o w i n g procedure eelgrass of  i n t o the mud Having  On  so t h a t o n l y a p o r t i o n  with  a few o c c a s i o n s I saw mink a s s i s t the d i g g i n g by  can f l i p  the c r a b onto  i t s back.  i t aside.  to get i n t o a p o s i t i o n from which  The mink u s u a l l y d i g s at s e v e r a l  l o c a t i o n s around the p e r i m e t e r o f the c r a b u n t i l  the c r a b commits  by t a k i n g a d e f e n s i v e p o s t u r e , r a i s i n g up on i t s w a l k i n g c h e l a e ; at t h i s time the mink moves behind the c r a b over i s q u i c k , and may  a l l cases I observed,  it.  used  to g i v e an a s s i s t i n g push. vary.  l e g s and waving  i n more than one  c r a b s were grabbed i n the mouth and  o f the head, but on a few o c c a s i o n s i t appeared The  itself  The movement which  be accomplished  flip  grabbed may  Once v e g e t a t i o n  I f the crab has b u r i e d i t s e l f , more  d i g g i n g w i t h the f o r e f e e t i s r e q u i r e d .  In  be-  to subdue i t . I f the crab i s covered  has been c l e a r e d away, the mink attempts  flips  These  ( o r o t h e r marine v e g e t a t i o n ) j t h i s i s removed by a d i g g i n g motion  the f o r e f e e t .  its  13).  found a c r a b , a mink f o l l o w s  g r a s p i n g v e g e t a t i o n w i t h the mouth and p u l l i n g  it  (Appendix  are the prey  l y i n g on top o f the e e l g r a s s , but are u s u a l l y  and have often- dug  the carapace  fall,  way.  turned with a  t h a t the f o r e f e e t were  exact manner i n which the crab i s  Sometimes i t appeared  t h a t the mink b i t the carapace  in  105  s t r a i g h t f o r w a r d manner and then t o s s e d i t s head to the s i d e , but at o t h e r times the mink t u r n e d i t s head over b e f o r e b i t i n g  the c r a b , so t h a t i t s  upper  c a n i n e s c o n t a c t e d the c r a b ' s ventrum and i t s lowers were on the c r a b ' s  upper  surface.  I n t h i s way  i t could e f f e c t  by r e t u r n i n g i t s head to normal by t h i s t e c h n i q u e was  position.  the d e s i r e d t w i s t o f i t s p r e y That more power may  be  applied  suggested by an i n c i d e n t i n which a male mink,  t h r e e times i n s u c c e s s i o n to l i f t  failing  a r e d c r a b from a h o l e 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 o n l y wrenched the c r a b from the h o l e but a l s o threw i t n e a r l y h a l f a meter i n the p r o c e s s . On  two o c c a s i o n s I saw mink c a t c h s m a l l f i s h which had been stranded  on e e l g r a s s f l a t s by the f a l l i n g dogging  tide.  a c r o s s a Vargas I s l a n d bay  bounds back to one  The  first  time a male mink b i r d -  suddenly w h i r l e d , made t h r e e s t i f f - l e g g e d  s i d e , s t r u c k q u i c k l y and came up w i t h a mouthful  e e l g r a s s i n which was  a struggling blennioid.  I t was  apparent t h a t  mink had been a t t r a c t e d by sound, p r o b a b l y caused by the f i s h ' s Another  time a mink rushed forward and caught  similar fashion. have responded,  A g a i n , i t was  of the  wriggling.  an u n i d e n t i f i e d f i s h i n  p r o b a b l y a t t r a c t e d by sound, although i t may  by s i g h t , to a movement i n the e e l g r a s s .  e a r l i e r , most o t h e r items appeared  As  indicated  to have been d e t e c t e d by odor.  Whether  mink were u s u a l l y f o l l o w i n g l i n e s o f scent on the ground o r g r a d i e n t s i n the a i r i s unknown, but they are c a p a b l e o f both.  I saw mink f o l l o w i n g the  t r a i l s o f o t h e r mink which had passed s e v e r a l minutes  earlier  (especially  males on the t r a i l s o f females d u r i n g the b r e e d i n g season), and have a l s o seen mink respond to emanations from a one-spot  source from some d i s t a n c e .  Poking - On b o u l d e r beaches and on some rockwee'd shores t h e r e are numbers o f c r a c k s , c r e v i c e s and n a t u r a l c a v i t i e s i n t o which h u n t i n g mink  106  t h r u s t t h e i r heads.  T h i s "poking" was  "the l a n d h u n t i n g technique I  saw  most o f t e n ( T a b l e 8 ) , because rockweed and b o u l d e r beach h a b i t a t s were more common on my  study areas than were t h e o t h e r k i n d s .  I n a sense,  p o k i n g i s a v a r i a t i o n o f b i r d - d o g g i n g i n t h a t the nose i s b e i n g pushed out ahead and appears  to be d o i n g much o f the b u s i n e s s o f p r e y d e t e c t i o n .  However, i t i s u n l i k e l y t h a t t h e r e are many c a v i t i e s  i n the i n t e r t i d a l  e s p e c i a l l y under b o u l d e r s , which do not harbor odors  tempting  and I s u s p e c t t h a t once the animal has i t s head i n c l o s e , s t i m u l a t i o n are more important  zone,  to a mink,  sounds and  i n informing i t i f anything i s  tactile  immediately  available. F i s h and  small, unidentified  animals p r o b a b l y i n c l u d i n g i s o p o d s , are  the p r e y taken most o f t e n by p o k i n g mink, although I have seen c r a b s on s e v e r a l . o c c a s i o n s i n t h i s way i n the same way  (Appendix  13).  as d e s c r i b e d f o r e e l g r a s s f l a t s  Crabs  caught  are u s u a l l y dug  encounters.  Other  out  items  are simply caught by the t e e t h and p u l l e d from t h e i r h i d i n g p l a c e s .  I f the  s u b s t r a t e a l l o w s , a^mink w i l l d i g at the base o f a b o u l d e r to g a i n a c c e s s , and w i l l undertake  c o n s i d e r a b l e c o n t o r t i o n s i n attempting to squeeze  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, w i t h the f r o n t t w o - t h i r d s o f i t s body under a b o u l d e r , i t s t a i l back and f o r t h , of  and i t s h i n d l e g s f l a i l i n g  s m a l l r o c k f l e w from the opening,  "digging".  the a i r w i l d l y .  excitement.  lashing  Occasionally bits  i n d i c a t i n g t h a t i t s f r o n t l e g s were a l s o  I t caught n o t h i n g t h e r e w h i l e I watched, although I c o u l d hear  f i s h w r i g g l i n g and s p l a s h i n g and o t h e r organisms the r o c k .  into  s c u r r y i n g about  beneath  These sounds had p r o b a b l y c o n t r i b u t e d to the mink's obvious'--  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 w i l l 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  i t 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, shorefringing marine vegetation, although they occasionally culminated a series of  face page 109  F i g u r e 12.  D u r a t i o n o f 264 underwater d i v e s by h u n t i n g mink, Vancouver I s l a n d , B r i t i s h Columbia. The shaded a r e a o f each h i s t o g r a m r e p r e s e n t s the p r o p o r t i o n o f d i v e s i n t h a t c a t e g o r y which r e s u l t e d i n prey captures.  c  0) u  50 40  u c  0  o VO  30 20  cr 10  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 i t 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 b i t 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 i t s 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 w i l l 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 w i l l be very short, often only one or two seconds.  Ill  On no o c c a s i o n was I a b l e t o see d i r e c t l y what was happening  underwater.  From f o l l o w i n g t h e p a t h s o f a i r bubbles r e l e a s e d by submerged mink, I have concluded  t h a t they hunt underwater much as they do above.  When b o u l d e r s a r e  p r e s e n t beneath t h e s u r f a c e , t h e bubble p a t h o f t e n f o l l o w s a z i g - z a g p a t t e r n , s u g g e s t i n g t h a t t h e mink i s moving from b o u l d e r t o b o u l d e r and p r o b a b l y p o k i n g between and under them. Bubble paths on s m a l l p a r t i c l e bottoms a r e 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 f i s h e r m a n  D. A r n e t ( p e r s o n a l communication) watched a mink c a t c h a r e d c r a b i n c l e a r water beneath a p i e r on t h e T o f i n o W a t e r f r o n t .  The mink dove, a p p a r e n t l y  swam d i r e c t l y t o t h e c r a b , which was moving s l o w l y a c r o s s t h e mud bottom, and when t h e c r a b threw up i t s c h e l a e i n d e f e n s i v e p o s t u r e , t h e mink q u i c k l y swam around b e h i n d and p i c k e d i t up.  The c r a b was c a r r i e d u p r i g h t .  a l s o been by o b s e r v a t i o n t h a t c r a b s brought  I t has  from t h e water a r e c a r r i e d up-  r i g h t , b u t i t i s u s u a l f o r the mink t o l a y a c r a b down on i t s back a f t e r emerging w i t h i t from t h e w a t e r , and i f t h e p r e y i s t o be t r a n s p o r t e d 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 t h a t p o i n t . difficult  t o fli>p a c r a b underwater,  I t i s p r o b a b l y more  and may even be dangerous. I f i t g a i n e d  a secure g r i p w i t h i t s c h e l a e and wedged i t s body among b o u l d e r s , a l a r g e c r a b c o u l d p r o b a b l y drown a mink. A r n e t ' s o b s e r v a t i o n (above) suggests t h a t mink hunt underwater c h i e f l y by s i g h t , and t h e f a c t t h a t they o c c a s i o n a l l y b r i n g dead c r a b s and moults t o shore i n d i c a t e s t h a t response  i s t o shape r a t h e r than j u s t t o movement.  The  a b i l i t y o f mink t o 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 _ a l . 1967), a l t h o u g h t h e i r a c u i t y underwater i s poor i n comparison  with  many o t h e r a q u a t i c and s e m i - a q u a t i c mammals ( S i n c l a i r e t a l . , 1974).,  It is  112  p o s s i b l e t h a t some underwater tactile  p r e y d e t e c t i o n i s a l s o accomplished w i t h the  sense, although I have no evidence t h a t t h i s i s so, and would a g a i n  suggest t h a t t h i s c o u l d be dangerous where l a r g e c r a b s are p r e s e n t . A v a r i a t i o n o f d i v i n g which I c a l l e d " h e r d i n g " i s the t e c h n i q u e which mink appear  to use most o f t e n f o r c a t c h i n g f i s h  i n use p r i m a r i l y at water's parallel  to the beach,  shore, and chases beach or  themselves  mink.  I saw  i n t h i s way,  edge on e e l g r a s s f l a t s .  e i t h e r underwater  f i s h toward  shore.  and are caught  become c o n f u s e d and  I have seen i t  T y p i c a l l y the mink swims  o r above and about one meter o f f -  I n t h e i r h a s t e to escape,  some f i s h  t h e r e b e f o r e they can r e t u r n to the  water,  swim d i r e c t l y back i n t o the jaws o f the p u r s u i n g  s e v e r a l mink c a t c h u n i d e n t i f i e d b l e n n i o i d s and s t a g h o r n s c u l p i n s and one male which caught  hunting, l e f t  i n water.  little  room f o r doubt  s y s t e m a t i c a l l y and riot a c c i d e n t a l l y . small objects, apparently f i s h , times they a l s o appeared  three f i s h  t h a t he was On  i n about  10 minutes  of  a p p l y i n g the technique  a few o c c a s i o n s I saw mink c a t c h  from s t a n d i n g p o o l s o f water and at these  to use t h i s d i v e - a n d - c o r n e r t e c h n i q u e .  Judging  from the o b s e r v a t i o n s o f E r r i n g t o n (1967:27),  f i s h hunting procedures  much the same i n f r e s h water.  along a stream the minks  He wrote,  "...  are  minnows o r l a r g e r f i s h e s i n r i f f l e p o o l s , e d d i e s , p o o l s below mouths o f p o o l s under d r i f t e d d e b r i s , o r i n watery roots. and  cavities  find tiles,  i n banks o r up under t r e e  I have seen minks making s h o r t d i v e s and coming up w i t h f i s h e s  now  then but they get many o f t h e i r f i s h e s i n l a n d - l o c k e d puddles o r c o r n e r  them somewhere, as i n c r a y f i s h burrows or under d r i f t w o o d " . HANDLING PREY Once prey i s caught 1)  I t may  be e a t e n . 2)  i t may  be handled i n one o f two main ways:  I t may  be cached.  A f t e r p r e y had been c a r r i e d out o f  113  Table 9. Handling of prey by mink, Vancouver Island, British Columbia, 1968-1972. A. A l l observations B. Observations of two minutes or longer duration.  A. Immediate Fate of Crabs Prey No.  Fish No. 7o  Eaten Cached  24 25  a b  29 66  30.5 69.5  49.0 51.0  Crabs and fish No. 7o  53 91  36.8 63.2  Unident No. 7»  All No. %  52 13  105 104  80.0 20.0  50.2 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  Fish  Eaten Cached  10 9  52.6 47.4  7 '9  43.8 56.2  0.3  1  cO.90  Both  Eaten 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  s i g h t , i t was u s u a l l y n o t p o s s i b l e f o r me to determine which o f these two a l t e r n a t i v e s a p a r t i c u l a r mink was f o l l o w i n g .  Thus, f o r the purposes o f  t h i s d i s c u s s i o n , " e a t i n g " r e f e r s to those o c c a s i o n s on which the mink ate the p r e y e i t h e r immediately upon c a t c h i n g i t o r r e l a t i v e l y near spot, but a t l e a s t remained  w i t h i n view o f the o b s e r v e r .  the h u n t i n g  "Caching"j.on the  o t h e r hand, r e f e r s t o those i n s t a n c e s i n which the p r e y was e i t h e r orarily to  l a i d a s i d e nearby  and r e t r i e v e d l a t e r , o r was c a r r i e d  some p o i n t above the t i d a l h i g h water mark.  I n at l e a s t  temp-  immediately  some o f these  cases the mink p r o b a b l y a t e a l l o r p a r t o f the p r e y b e f o r e r e t u r n i n g to hunt. I f one c o n s i d e r s a l l p r e y c a p t u r e s which I saw ( T a b l e 9A), of  the frequency  e a t i n g was v e r y n e a r l y the same as t h a t f o r c a c h i n g , but t h i s i s due  l a r g e l y to the r e s u l t s  f o r u n i d e n t i f i e d p r e y items, which were eaten  times as o f t e n as they were cached. items were u n i d e n t i f i e d  four  I n f a c t , the r e a s o n why most o f these  i s t h a t they were s m a l l enough to be taken  c o m p l e t e l y i n t o the mouth, chewed and swallowed  on the spot.  Most were  p r o b a b l y s m a l l f i s h and i n v e r t e b r a t e s such as i s o p o d s , a l t h o u g h a few may have been s m a l l c r a b s such as h e m i g r a p s i d s . or  fish),  c r a b s were cached more than t w i c e as o f t e n as they were eaten,  w h i l e f i s h were eaten and cached The  Among i d e n t i f i e d p r e y ( c r a b s  i n about equal f r e q u e n c i e s .  above i n f o r m a t i o n suggests t h a t i d e n t i t y and s i z e o f the prey are  important f a c t o r s b e a r i n g on how i t w i l l be t r e a t e d immediately caught, but i t i s l i k e l y shown i n T a b l e 9B,  upon b e i n g  t h a t t h e r e are a l s o o t h e r f a c t o r s i n v o l v e d .  c r a b s which were the f i r s t o r second  As  items caught by  a mink i n a g i v e n o b s e r v a t i o n were eaten s u b s t a n t i a l l y more o f t e n than were  115 crabs caught later.  It would appear that relative "hunger" i s 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 f o l -  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 crabs fish  358 57  Range 60-1080 15-210  n_ 25 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 i t s 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 i t s hunting area, and ensures that the item in question w i l l 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 a l 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), respectively.  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: a l . (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 f u l l 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 penraised 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 l l 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 k i l l i n g " .  Evidence given above  indicates that both surplus k i l l i n g and food storing are common among small mustelids and, in fact, I suspect that this is the pattern for secure existence 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 individual 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 i t should be discussed.  Of the 176 items which I saw  mink catch, they discarded just eight (4.5%); thus, this practice is rare. A l l 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 f i r s t 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 f i r s t .  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 considerable quantities. My eight observations involved six males, one female and one unidentified animal, and occurred in the months of May August (4), and September (1).  (1), July (2),  In summary, the l i t t o r a l 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, i t s dependence upon food storage. However, there is l i t t l e evidence that i t 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 a l l 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 i t 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 h u n t i n g  s i t e and d i d not r e t u r n to r e t r i e v e i t s f i s h  hour l a t e r ,  at which time the r i s i n g  storage  site.  may  i n f a c t , a technique  be,  t i d e had  n e a r l y reached  O v e r a l l i t appears t h a t temporary c a c h i n g  o t h e r hand, a l l f o u r o f my  h i t upon o n l y by  f o r more than the  temporary  i s not common and  some i n d i v i d u a l mink.  On  o b s e r v a t i o n s o f t h i s b e h a v i o r were i n J u l y  August and I l e a v e open the p o s s i b i l i t y seasons, when both h u n t i n g h u n t i n g V difficult.  t h a t i t may  an  the  and  be more common i n o t h e r  c o n d i t i o n s and o b s e r v a t i o n are more  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 a l l 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 i t was Gerell's impression that up to 90 per cent of a l l 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 provided insights into the degree to which these activities are dictated by various environmental conditions.  It was not practical to catalogue a l l 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 interrupted 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 eelgrass 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; a l l 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 a l l 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 d i f f i c u l t such as during darkness. Equipment 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, I l l i n o i s (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 a l . (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 F e b r u a r y 1972,  an a d u l t male was monitored manually  determine h i s a c t i v i t y p a t t e r n . few hundred  meters  a nearby wharf.  "inactive". f o r 261 hours to  T h i s mink had a r a t h e r l i m i t e d range o f a  a l o n g the T o f i n o w a t e r f r o n t , and i t r e g u l a r l y hunted  The r a d i o - c o l l a r was  from  a t t a c h e d on 3 February and d u r i n g the  f o l l o w i n g two days dens were l o c a t e d and m o n i t o r i n g p o s i t i o n s were determined. Simultaneous o b s e r v a t i o n s and r a d i o - t r a c k i n g on one o c c a s i o n w h i l e the animal was  h u n t i n g p r o v i d e d e x p e r i e n c e w i t h v a r y i n g s i g n a l sounds under  conditions;  f o r example I found t h a t the s i g n a l became v e r y i n t e n s e and  s t a c c a t o when the animal was animal dove.  different  i n or v e r y near the water,  and stopped when the  I t t h e r e f o r e became p o s s i b l e to determine when the animal  hunted by d i v i n g ,  and even to time the d i v e s , w i t h o u t v e n t u r i n g from the  camper-truck which  s e r v e d as a base o f o p e r a t i o n s .  By the e v e n i n g o f 7 F e b r u a r y , when the c o n t i n u o u s m o n i t o r i n g s e s s i o n began, the mink had  a p p a r e n t l y become accustomed  into i t s d a i l y routine.  to the c o l l a r and had  When observed t h a t morning,  i t had appeared  settled  to a c t  n o r m a l l y i n a l l r e s p e c t s , and i t had not d i r e c t e d a t t e n t i o n to the c o l l a r at any time w h i l e i n view.  To monitor s i g n a l s from t h i s animal I p o s i t i o n e d  the t r u c k at one o f t h r e e l o c a t i o n s , depending on time o f day and which o f two main den systems was  i n use.  S i g n a l s c o u l d u s u a l l y be r e c e i v e d from both  dens and the main h u n t i n g a r e a at a l l t h r e e l o c a t i o n s , but the l o c a t i o n g i v i n g the w i d e s t coverage  (used most n i g h t s ) was  poor d u r i n g the day  o f i n t e r f e r e n c e from outboard motors and automobile engines nearby; two' l o c a t i o n s were n e a r e r the r e s p e c t i v e dens. checks on f o o t , but took c a r e not to approach disturb while  it.  A l l a c t i v i t y was  because  the o t h e r  I r e g u l a r l y made l o c a t i o n the animal c l o s e enough to  determined by e a r ; I l e f t the r e c e i v e r  playing  a t t e n d i n g o t h e r d u t i e s i n the t r u c k and noted and i n t e r p r e t e d a l l  126  s i g n a l changes.  To a l l o w myself p e r i o d s o f s l e e p and o c c a s i o n a l  respite,  I plugged t h e r e c e i v e r output to the i n p u t o f an E l e c t r a r e e l - t o - r e e l r e c o r d e r p l a y i n g a t a speed o f 1 7/8 i . p . s .  tape  Up to f o u r hours o f c o n t i n u o u s  r e c o r d i n g c o u l d be o b t a i n e d i n t h i s way, and t h i s was l a t e r t r a n s c r i b e d i n the f o l l o w i n g way. minutes  S i n c e most s i g n i f i c a n t bouts o f a c t i v i t y l a s t e d t e n  o r more, the tape was scanned i n playback by u s i n g t h e f a s t  c o n t r o l to p r o v i d e output a t the e q u i v a l e n t o f ten-minute o f the number o f p u l s e s p e r t e n seconds  forward  intervals.  A count  and a s u b j e c t i v e a p p r a i s a l o f t h e  r e l a t i v e s t e a d i n e s s o f the s i g n a l were compared w i t h the p r e v i o u s count to determine whether a change had o c c u r r e d .  Whenever a change was i n d i c a t e d ,  the tape was r e v e r s e d and f u r t h e r checks were made between the two p e r t i n e n t counts. until  When a p e r i o d o f a c t i v i t y was i d e n t i f i e d ,  the tape was r e v e r s e d  the b e g i n n i n g o f the a c t i v i t y p e r i o d was found, and t h i s was monitored  continuously u n t i l  the animal had a g a i n become i n a c t i v e .  t e r m i n a t e d at about mid-day on 18 February.  Monitoring  When the animal was r e c a p t u r e d  f o r removal o f the c o l l a r on 8 March, i t was i n good c o n d i t i o n and was found to have g a i n e d weight d u r i n g the p e r i o d i t had been instrumented. RESULTS DIRECT OBSERVATIONS For each o f 747 o b s e r v a t i o n s , t h e a c t i v i t y o f the mink i n v o l v e d was classified was  i n one o f e i g h t c a t e g o r i e s .  engaged i n a c t i v i t i e s  I n s e v e r a l cases the observed  animal  f i t t i n g more than one c a t e g o r y , i n which case t h a t  l i s t e d was u s u a l l y the one which c l e a r l y predominated.  I n a few cases the  c a t e g o r y chosen was the one which I c o n s i d e r e d most noteworthy;  f o r instance,  n e a r l y a l l o b s e r v a t i o n s o f "mating" and " f i g h t i n g " i n v o l v e d at l e a s t 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 1. 2. 3. 4. 5. 6. 7. 8.  Hunting Traveling Sleeping Eating Fighting Mating Other Undetermined Total  Number of Occurrences 432 101 85 20 19 13 23 53 747  Per Cent of Occurrences 57.8 13.5 11.4 2.7 2.6 1.8 3.0 7.2 100.0  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, v i s i b i l i t y 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 a l l 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  f a c e page  F i g u r e 13.  I n c i d e n c e o f h u n t i n g , t r a v e l i n g , and s l e e p i n g by mink i n r e l a t i o n * to time o f day and the time o f the lowest t i d e . "  129  minutes b e f o r e low low tide  ro o ©  CO  o o  T  T  o  o  =: Oi °? i — + Oi O ro Oi 1 0  .  .  -  -  o >  ^ i i x w o  O  O  - cr  •  ©  ©  ©  >  > •  •  0  Q t> t> |_  5" CO  0  •  > •  0  0  >  • o  0  > •  0  0 > •  o  0  >  > -  o  > •  0" •  •  • CO  > •  0  0  •  •  0  ©  •  •  •  0  o > •  0  0  >  •  ©  •  •  © >  ©  •  Ul  o  >  • •  o  ©  o  o >  o  t>  >  t> •  > •  o > •  o > •  > •  •  o  o  o  • ••  • >  ©  o CD  ©  >  o  •  •  • ©  +  T  T  ©  • 0  tide  Po  ©  •  •  •  •  0  o =r.  CO.  0 >  c  o • >  o  •  w  -  • o  0  >  >  •  T  0  >  n  a o  Ul  O "T"  03 ,° minutes a f t e r low low o -1 co  •  •  o > •  o •  o •  > •  •  o •  •  D  ro  0>  -s. 3'  CQ  621  130  f r e q u e n c i e s o f o b s e r v a t i o n s per most o f t e n near the low  nearest  s l a c k t i d e , a l l a c t i v i t i e s were seen  low t i d e , but t h i s was  c a n t l y g r e a t e r e x t e n t than f o r the o t h e r s  true f o r hunting  (Table  to a  signifi-  10).  While the above i n f o r m a t i o n p r o v i d e s a g e n e r a l p i c t u r e o f what c o a s t a l mink do as w e l l as when and how be b i a s e d i n some ways.  o f t e n they do them, i t may  Perhaps most i m p o r t a n t l y ,  nevertheless  the d i s t r i b u t i o n  s i g h t i n g s i s at l e a s t p a r t l y r e f l e c t i v e o f my  own  Once I had  were at low t i d e , I i n c r e a s e d  l e a r n e d how  observable  o b s e r v a t i o n e f f o r t at such times. out  the animals  t i m e t a b l e i n the  of  What t h i s means i s t h a t had I not gone  to l o o k f o r mink on e a r l y morning summer low t i d e s and  low t i d e s , I would have seen f a r fewer animals  at h i g h e r t i d e l e v e l s .  was  out  was  a v a i l a b l e for observations  i.e.,  from about f i r s t  I t does not had  v a r i e d g r e a t l y but I t was  Thus, I  a f t e r the s l a c k low t i d e , but I was  s i g h t i n g per day.  times.  The  a c o n s e r v a t i v e l y estimated  p o s s i b l e to go  days I  at the e m p i r i c a l l y determined optimum  The  747  most daytime t i d e s are h i g h , without  also  field  duration of " f i e l d  days,  days"  average would be about  f o r days, e s p e c i a l l y i n f a l l s e e i n g any  animals  times,  activity-classified  l i s t e d e a r l i e r were made d u r i n g a minimum o f 798  an average o f under one  I spent more  I n f a c t , d u r i n g most summer f i e l d  f o r f o u r o r more hours beyond those  observations  winter  to at l e a s t noon and o f t e n l o n g e r .  about one hour b e f o r e and  present  hours.  light  evening  than I d i d .  n e c e s s a r i l y mean t h a t I would have seen many more animals time out  field.  five  and w i n t e r when  other  than  those  caught i n l i v e t r a p s ; on the o t h e r hand, d a i l y s i g h t i n g s o f f i v e or s i x d i f f e r e n t animals  were not uncommon and on  but one w i t h i n an hour anda~hal.f ;_df the low  18 June 1969 tide.  I saw  eleven, a l l  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 Hunting versus Traveling Sleeping Eating Other Undetermined^  LL  LH  HH  HL  376  (87)  17  (4)  28  (6)  11  (3)  62 37 13 27 26  (64) (44) (65) (56) (52)  16 20  (16) (24)  10 13  (10) (15)  10 9  (21) (18)  9 14 7 ..5 .,8  (10) (17) . (35) (10) (16)  Traveling versus Sleeping Eating Other Undetermined  62  (64)  16  (16)  10  (10)  9  (10)  37 13 27 26  (44) (65) (56) (52)  20  (24)  13  (15)  10 9  (21) (18)  6 7  (13) (14)  14 7 5 8  (17) (35) (10) (16)  Sleeping versus Eating Other Undetermined  37  (44)  20  (24)  13  (15)  14  (17)  13 27 26  (65) (56) (52)  (13) (14)  7 5 8  (-35) (10) (16)  c  10 9  (21) (18)  6 (13) 7 (14)  6 7  e  e  e  ChiSquare  df  36.3 91.1 6.0 37.0 47.1  3 3 1 3 3  <0.001 <0.001 <0.02 <0.001 <0.001  7.3 0.0 0.8 -•2.5  3 1 3 3  < 0.10 <0.90 <0.90 <.0.50  2.1 2.1 1.0  1 3 3  < 0.20 < 0.70 < 0.90  ^ i d e 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. 0ther = fighting, mating, playing, grooming, etc. (see textO 0bservations for which activities were not certainly identified. Figure represents sum of observations for LH, HL, and HH tides.. c a e  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 i s evident that the tendency for mink to be out and observable increases as the tide drops, (X = 20.87, df = 6, P<0.001), i s greater within an hour of the slack low 2  tide than at other times (X = 13.30, df = 2, P<0.001), and decreases with 2  advancing time of day (X = 11.19, df = 4, P< 0.02) . 2  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  F i g u r e 14.  R e s u l t s o f 84 census t r a n s e c t s n o r t h from R a s s i e r P o i n t , Vargas I s l a n d , 1968-1969.  133  133  o o  1/1  c  moi  o  C  <U CD  O  o  1—  o  d)  _Q  o o  o  o  CD  O  o  >• LU  o  -—  CN  CO  II  II  II  II  z o e c  o o oo  o o oo o o oo  E  i.  o  €  o  Q  0DO < O  in  CM  o  in ©  •  134 Telemetry As mentioned e a r l i e r , one i n f o r m a t i o n about the a c t i v i t y to d a y l i g h t hours. a c t i v e at n i g h t .  I was Tracks  waterfront  schedules  o f mink was  t h a t mink were a l s o  a f t e r dark.  I attempted n i g h t counts by f l a s h l i g h t at; low  and  t i d e , but not  f o r e s t s are d i f f i c u l t  impractical  f a c t o r s , among which was  other  animals which became aware o f  to observe anywhere except along  use o f t e l e m e t r y  at  seen.  shorelines.  detected.  overcame some o f the above d i f f i c u l t i e s but the hazard  o f n i g h t t r a v e l by  several  s m a l l boat among the  r e c o r d i n g system.  F i g u r e 15  time o f day,  f o r the f a i l u r e of the attempted  o f a c t i v e and  i n a c t i v e f i x e s on e i g h t  a g r e a t e r p r o p o r t i o n o f a c t i v i t y at lowest i n t h i s case mink were found to be tide levels.  automatic  (a & b) d e p i c t s the p r o p o r t i o n , i n r e l a t i o n  mink i n the T u r t l e I s l a n d a r e a i n summer 1972.  times and  ob-  A f a r g r e a t e r number of n i g h t f i x e s would have been d e s i r a b l e , and  would have been p o s s i b l e were i t not  t i d e and  Further,  Animals which were  i s l a n d s o f B a r k l e y Sound, l i m i t e d the i n f o r m a t i o n which c o u l d be  tained.  me  to n e g o t i a t e even d u r i n g d a y l i g h t , so i t was  a c t i v e elsewhere would not have been The  found t h a t  e a s i l y seen under these c o n d i t i o n s and were  the f l a s h l i g h t beam reached them c o u l d e a s i l y a v o i d b e i n g  west c o a s t  high  them at n i g h t , and mink were o f t e n seen along the T o f i n o  u s u a l l y d e t e c t e d o n l y by tapetum r e f l e c t i o n ;  rocky  applied mainly  been a c t i v e d u r i n g darkness, most animals caught  However, they were not  before  in providing  made on beaches a f t e r the r e c e s s i o n o f n i g h t  animals c o u l d almost always be found h u n t i n g times.  t h a t they  aware from a v a r i e t y o f evidence  t i d e s i n d i c a t e d animals had i n l i v e t r a p s entered  l i m i t a t i o n of d i r e c t observations  Again,  to  radio-tagged  As w i t h o t h e r d a t a  sources,  tide levels i s indicated,  although  i n a c t i v e more o f t e n than a c t i v e at a l l  t h e r e was  a low  incidence of a c t i v i t y  during  f a c e page  F i g u r e 15.  A c t i v i t y o f r a d i o - t a g g e d mink i n r e l a t i o n to l e v e l s o f t i d e and time o f day. In (a) and ( b ) , open bars i n d i c a t e o b s e r v a t i o n s ( r a d i o f i x e s ) i n which animals were i n a c t i v e , and c r o s s - h a t c h e d bars r e p r e s e n t f i x e s on animals which were moving. I n ( c ) , o n l y a c t i v e f i x e s are c o n s i d e r e d ; shaded bars r e p r e s e n t n o c t u r n a l and c r e p u s c u l a r o b s e r v a t i o n s (evening to morning, 20:00-06:00) and open bars indicate.,; o b s e r v a t i o n s d u r i n g the r e s t o f the day. The number o f p e r t i n e n t o b s e r v a t i o n s i s shown at the top o f each histogram.  135  135  100  X = 6 . 6 8 ; df = 2; p <.05 88 2  60 40 20  11  I  •0-1.0  20  17  22  1.1-2.5  2.6 +  tide level (m)  C  u  (a)  67  80  100  X = 9 . 0 6 ; df = 4 ; 2  p < .10 44  5  80  Q_  ^  29 31 19  60  11 13  1  u « D  (b) 33  20  4:01-8:00  cr  /  19  I  /  8:01-12:00 12:01-16:00 16:01-20:00 20:01-24:00  time of day 100  X  2 =  3 . 9 0 ; df =2; p < . 3 0  (e)  80 60 40 20  < 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, however, 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) series of more extreme "spring" tides (tidal range almost 4 m).  to a  The mink  hunted primarily between the low high (LH) and high low (HL) tides for the f i r s t 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 a l l 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 i t s 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  H i g h Low  High  fligh  m  CO  uj 12-13  TIME SUNSET  (17:19 - 1 7:37)  OF  DAY SUNRISE  (07:36-07:18)  per cent activity  138 general trend after 11 February was for the mink to shift the onset of i t s 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 i t s low activity on subsequent nights may have been due to i t s having an adequate food supply in i t s 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 i t 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 i t s condition, i t s level of hunger, and the nature of i t s 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, i t 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 i s 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 recommended- 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, respectively, 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 a l . (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 i n f o r m a t i o n on energy requirements o f w i l d mink i s a d m i t t e d l y  imprecise,, t h a t on energy  i n t a k e i s even more so.  D e s p i t e the f a c t  animals were f r e q u e n t l y seen h u n t i n g and c a t c h i n g prey, i t was to q u a n t i f y t h e i r success i n a meaningful way.  difficult  The s i z e o f organisms  r e g u l a r l y caught v a r i e d c o n s i d e r a b l y so t h a t a mink which caught items might  that  several  end up w i t h l e s s food than another c a t c h i n g o n l y one.  I  o c c a s i o n a l l y had o p p o r t u n i t y to weigh and measure f i s h and c r a b s caught by observed mink, but f o r most o b s e r v a t i o n s I had to e s t i m a t e prey s i z e . d i d t h i s by v i s u a l l y comparing width o f c r a b s ) w i t h the.head weights,  the s i z e o f a p r e y ( l e n g t h o f f i s h ; s i z e o f the mink h a n d l i n g i t .  I  carapace  Rough g r o s s  i n grams, were then o b t a i n e d f o r each prey, as f o l l o w s : weights o f  c r a b s e s t i m a t e d a t over 80 mm  i n w i d t h were i n t e r p o l a t e d  from F i g u r e 11 (b)  assuming t h a t . a l l c a n c r i d s p e c i e s have a carapace width-weight  relationship  s i m i l a r to t h a t shown f o r r e d c r a b s ; f o r specimens l e s s than 80 mm wide, a weight o f one gram p e r m i l l i m e t e r o f carapace width was assumed. cases I c o u l d n o t e s t i m a t e a p r e y c r a b ' s s i z e . the average if  size  A t such times I assumed  (as found i n middens, see F i g u r e 10) f o r t h a t s p e c i e s , o r ,  i d e n t i f i c a t i o n was n o t p o s s i b l e , the average  eaten s p e c i e s , the r e d c r a b . A n e t weight  I n some  s i z e o f the most  commonly  -  o f two grams was r e c o r d e d f o r most s m a l l o r u n i d e n t i f i e d  items eaten on the s p o t . The g r o s s weights o f f i s h were o b t a i n e d by rough interpolation of f i s h  s i z e data l i s t e d  i n Appendix 9.  d i s s e c t i o n s o f r e p r e s e n t a t i v e p r e y i n d i v i d u a l s , weight d i g e s t i b l e matter  On the b a s i s o f l o s s e s due to un-  amounting to 75 p e r cent f o r k e l p c r a b s and about  67 per  cent ( t w o - t h i r d s ) f o r o t h e r crabs.; were used to c a l c u l a t e n e t weights o f c r a b s  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. A l l 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).  the weight o f prey caught 17),  There was a l s o a tendency f o r  t o i n c r e a s e w i t h i n c r e a s e d h u n t i n g time ( F i g u r e  although the c o n s i d e r a b l e s c a t t e r among d a t a p o i n t s ( r s. 0.504)  t h a t f a c t o r s o t h e r than time may i n f l u e n c e s u c c e s s .  suggests  Indeed, some mink o b t a i n e d  a l a r g e amount o f food i n a s h o r t time, u s u a l l y by c a t c h i n g l a r g e prey. young male caught  One  a s c u l p i n e s t i m a t e d to have y i e l d e d 200 g o f meat l e s s  than  one minute a f t e r i t began i t s hunt, spent an a d d i t i o n a l e i g h t minutes i n the brush e a t i n g and/or c a c h i n g the f i s h ,  then r e t u r n e d and hunted an a d d i t i o n a l  f o u r minutes but w i t h no f u r t h e r success,.  Still,  i t s . t o t a l food r e t u r n f o r  the 12 minute h u n t i n g p e r i o d was the l a r g e s t r e c o r d e d d u r i n g my o b s e r v a t i o n s . Most items caught  by mink were s m a l l e r , however, and the average  f o r 160 items which I saw caught, (+ 3.8 a t 957o c o n f i d e n c e l e v e l ) .  net return  i n c l u d i n g the one above, was 17.7 g On the b a s i s o f known prey weights  conversion factors l i s t e d e a r l i e r ,  and n e t  the above amount i s the e q u i v a l e n t o f a  Cancer o r Telmessus c r a b about 52 mm wide, a 45 mm k e l p c r a b , a 100 mm s c u l p i n , o r two 130 mm b l e n n i o d s . F i g u r e 18 d e p i c t s the h u n t i n g success o f mink i n r e l a t i o n t o some o f the circumstances  under which they hunted. The v a r i a b i l i t y o f these d a t a makes  f i r m c o n c l u s i o n s i m p o s s i b l e , but some o f the t r e n d s i n d i c a t e d a r e c o n t r a r y to e x p e c t a t i o n s and r e q u i r e e x p l a n a t i o n . the A p r i l  to J u l y season, w i t h i t s r e l a t i v e l y s t a b l e weather and w i t h s e v e r a l  s p e c i e s o f c r a b s o c c u p y i n g near-shore abundance and a v a i l a b i l i t y ,  combined) i s an apparent  i s the time o f h i g h e s t food superior hunting  success  (August-November and December-March  contradiction.  7.8 g / m i n u t e , r e s p e c t i v e l y , h a v e  sively  waters,  thus the d i s t i n c t l y  i n d i c a t e d f o r the o t h e r two seasons  and  I have e s t a b l i s h e d p r e v i o u s l y t h a t  Two w i n t e r hunts which y i e l d e d  i n f l a t e d the August-March r e s u l t  17.1  exces-  (no o t h e r success r a t e s i h . e x c e s s o f 5 g/minute were r e c o r d e d f o r  face page 147  F i g u r e 17.  H u n t i n g success (net weight o f p r e y taken per minute o f h u n t i n g ) f o r Vancouver I s l a n d mink, 1968-1972.  face page 148  Figure 18.  Mink hunting success i n r e l a t i o n to several environmental variables, Vancouver Island, B r i t i s h Columbia. Numbers i n parentheses indicate sample sizes and horizontal bars depict standard errors of the means shown.  HUNTING  SUCCESS  (g/min)  TOTAL 6 0  J  L  FOOD 20  RETURN 40  _i  l _  (net wt., gm) 60  80  HUNTING 100 0  TIME  10  (min)  20  J  L  SEASONS: August-March  (18)  A p r i l - July  (29)  0400-0800  (22!)  0801-1200  (16)  TIME:  1201 +  I — • — I  V-»-H  0)  TIDE LEVEL (m): < 1.0  (32)  1.1-2.5  (10)  2.6 +  HUNTING  I—•—I  I—•—I CO  (5) METHOD:  Dive  (11)  Poke  (9) (6)  Bird-dog Combination  I •—I I  •  1  (20)  HABITAT:  ALL  BB + RW  (25)  I  EF + SB + ES  (14)  I—•—I  Combination  (7)  OBSERVATIONS  (47)  I  •  •—I  l-»H  I (—•—I  l-»H  l-»H  • 1  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, The total food return  df = 43, p<0.40).  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 i t s prey. If i t did not reappear, i t s 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 d i f f i c u l t 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,  i t 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 a l l of one and part of another, cached three, and then abandoned the largest, voluntarily stopping i t s hunt after accumulating what was calculated as half i t s 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 proportion 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 a l l of the food they required in about 80 minutes of searching and, under the conditions of a l l observations providing data, would almost never require two f u l l 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 a l l 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 a l . (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 i t s 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 a l . (1955) found that the extent of movement of marten in the field was inversely related to the availability of food.  A l l 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 intertidal 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 a l l coast mink enjoy the level of success listed as average, and none can obtain food at a high rate under a l l 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 h u n t i n g success under the c o n d i t i o n s measured i s t h e r e f o r e not  surprising.  I n d i v i d u a l mink were h u n t i n g , as much as p o s s i b l e , at times and p l a c e s which p r o v i d e d them the h i g h e s t net energy from the a c t i v i t y  section  return.  In t h i s context, information  (mink hunted s i g n i f i c a n t l y more o f t e n at low  tide  than a t o t h e r t i d e l e v e l s ) i m p l i e s more about mean success r a t e s than do comparative  h u n t i n g success d a t a .  the  I t i s the b a s i s upon which I a s s e r t e d t h a t  low t i d e i s the most p r o f i t a b l e h u n t i n g t i d e i n the p r e v i o u s paragraph  and  t h i s w i l l remain a b a s i c premise. There were a l s o c o n d i t i o n s under which mink a p p a r e n t l y d i d not o f t e n hunt and,  i n most c a s e s , p r o b a b l y c o u l d not hunt.  C h i e f among these i s  heavy s u r f , t h e r e f o r e l a r g e s e c t i o n s o f the open c o a s t o f f e r l i t t l e as mink h a b i t a t .  A mink whose range i n c l u d e s an i n t e r t i d a l a r e a w i t h some  p r o t e c t i o n from wave a c t i o n , e.g., or  i n the l e e o f a r e e f ,  islet,  or  headland  even behind b o u l d e r s on a b o u l d e r beach, i s c e r t a i n l y b e t t e r o f f than  which does n o t . had  potential  As shown i n F i g u r e 18,  one  animals which hunted e e l g r a s s f l a t s  a h i g h e r food r e t u r n than those h u n t i n g o t h e r h a b i t a t s d u r i n g my  v a t i o n s , but t h e r e were many stormy days on which animals  obser-  c o u l d not hunt  these open f l a t s a t a l l . In  summary, i t seems e v i d e n t t h a t the r i c h  l i t t o r a l waters along  the  shores o f western Vancouver I s l a n d are p a r t i c u l a r l y b e n e f i c e n t to h u n t i n g mink.  However, i t i s e q u a l l y e v i d e n t t h a t 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 p r o v i d e access  f o o d , p a r t i c u l a r l y under v a r y i n g weather and water c o n d i t i o n s . s h o r t , good h u n t i n g p l a c e s and good h u n t i n g times  to  There are, i n  ( e s p e c i a l l y r e l a t i v e to the  t i d e ) 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 t o even temporary food d e p r i v a t i o n , i n a b i l i t y s e r i o u s f o r the  individual.  to hunt at such p l a c e s and/or times c o u l d be  155 WATER REQUIREMENTS AND INTAKE INTRODUCTION AND METHODS Over most of its range the mink centers i t s 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 207 from metabolic o  water.  They point out that this ratio w i l l 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 o f t e n v e r y t h i r s t y and  sought water upon r e l e a s e . sea, even though drank  On no o c c a s i o n was  t h i s was  the c l o s e s t  a mink seen to d r i n k from the  source o f water i n many c a s e s .  from r a i n p o o l s which seemed somewhat b r a c k i s h to my  by t i d e p o o l s were u s u a l l y i g n o r e d .  immediately  One  Some  t a s t e , but near-  animal, a p p a r e n t l y t h i r s t y n e a r l y t o  the p o i n t o f s t r e s s , " t a s t e d " s e v e r a l t i d e p o o l s i n s u c c e s s i o n but drank v e r y little  from them.  rain pool.  I t was  encountered minutes  I d i d not o f t e n see f r e e roaming  I saw drank fresh' water. On  l a t e r d r i n k i n g h e a v i l y from a animals d r i n k i n g , but a l l which  t h r e e o c c a s i o n s a known j u v e n i l e male t r a v e l e d  i n the open . f o r o n e - f o u r t h m i l e o r more to d r i n k from a stream at one Of a Vargas  I s l a n d beach.  used range,  and I was  T h i s movement was  w i t h i n the.animal's  end  regularly  not aware o f much movement by o t h e r mink to t h a t  stream even d u r i n g the dry  season.  I g a t h e r e d no evidence t h a t mink r e g u l a r l y e n t e r the home ranges o f o t h e r mink to o b t a i n water, but I s u s p e c t t h a t some do.  The  animals  do  n o t "congregate" around water s o u r c e s , however; as w i l l be shown, mink are difficult t h i s might  to t r a p d u r i n g the l a t e summer and at one p o i n t I s p e c u l a t e d that have been due  T h i s remains  to the e f f e c t s o f water shortage on mink movements.  a possibility,  although l i v e t r a p p i n g along i s l a n d streams  was  no more s u c c e s s f u l than t h a t along c o a s t shores d u r i n g the dry season,  and  mink s i g n ( t r a c k s , droppings) was  never u n u s u a l l y abundant along these  streams. Water requirement and Wood 1968c),  i n c r e a s e s w i t h i n c r e a s i n g energy  intake ( F a r r e l l  and the e x t e n t to which mink c o u l d be s a t i s f i e d by m o i s t u r e  i n foods r e g u l a r l y eaten on the c o a s t i s unknown.  As shown p r e v i o u s l y ,  157  v a r i o u s s p e c i e s o f decapod c r a b s predominate  i n the d i e t d u r i n g the d r y  time o f year and, s i n c e these p r o b a b l y p r o v i d e l e s s body m o i s t u r e do marine f i s h e s (see S c h m i d t - N i e l s o n 1964),  i t at l e a s t appears  mink are n o t s e l e c t i n g food to maximize water i n t a k e i n t h a t way. at l e a s t  some f r e e water i s n e c e s s a r y .  than t h a t the Probably  On many mornings d u r i n g the summer,  v e g e t a t i o n i s wet from c o n d e n s a t i o n o r d e p o s i t i o n by f o g , and mink may supplement t h e i r water i n t a k e by l i c k i n g up water at t h i s time. o f how they do i t ,  Regardless  some mink which may be seen day a f t e r d r y day on s m a l l  w a t e r l e s s i s l e t s remain  i n good h e a l t h through t h e summer, thus they are  a p p a r e n t l y o b t a i n i n g a l l o f the n u t r i e n t s , i n c l u d i n g water, which are needed.  158 SIZE DIMORPHISM AS A TROPHIC STRATEGY INTRODUCTION AND  METHODS  I showed e a r l i e r twice  t h a t males among mink from most areas  as l a r g e (weight) as females.  small mustelids, slight  i n the  e s p e c i a l l y those  average almost  Such dimorphism seems t r u e o f most  i n the genus M u s t e l a ,  s m a l l e s t s p e c i e s , the l e a s t weasel, and  although  according  (1951) w i t h i n s p e c i e s i t i s u s u a l l y g r e a t e s t f o r the l a r g e s t Consistent with  i t is to H a l l  subspecies.  t h i s p a t t e r n i s the a p p a r e n t l y minimal d i f f e r e n c e i n s i z e  between the sexes i n the s m a l l ermines (M.  erminea anguinae) o f Vancouver  I s l a n d and M.  e_. haidarum from the Queen C h a r l o t t e I s l a n d s  G u i g u e t 1965)  and  i n a s m a l l subspecies  the Kuskokwim a r e a o f A l a s k a about the b i o l o g i c a l  (Cowan  and  o f mink (M. v i s o n a n i a k e n s i s )  (Burns 1964b).  Thus, whatever one  concludes  s i g n i f i c a n c e o f s i z e dimorphism i n m u s t e l i d s  e v e n t u a l l y be r e c o n c i l e d w i t h  from  must  the f a c t t h a t i t i s not u n i v e r s a l , even w i t h -  i n s p e c i e s . There i s l i t t l e p e r t i n e n t i n f o r m a t i o n about any m u s t e l i d s the w i l d , however, and  f o r the p r e s e n t  Rand (1952) s p e c u l a t e d be concerned w i t h (1966) p r o v i d e d  exceptions  w i l l have to be  t h a t some s e x u a l l y dimorphic  s i z e i n an a n o l i n e l i z a r d . eristic and  elongate  f o r food.  o f t h i s f o r s e v e r a l s p e c i e s o f b i r d s and  (1967) demonstrated f e e d i n g d i f f e r e n c e s c o r r e l a t e d w i t h Brown and L a s i e w s k i  field  S e a l a n d e r (1943), who  o f the f a c t o r s which enables  i n f o r m a t i o n on  Schoener  sex d i f f e r e n c e s i n  i n energetic  " p r e d i c t " that reduction i n i n t r a - s p e c i e s competition  only published  Selander  (1972) show t h a t the  body shape o f weasels i s "a s a c r i f i c e  dimorphism i n s i z e i s one  ignored.  c h a r a c t e r s might  the r e d u c t i o n o f i n t e r - s e x c o m p e t i t i o n  evidence  in  through  them  charactefficiency",  sexual  to e x i s t .  The  t h i s p o i n t appears to be t h a t o f  examined a s e r i e s o f w i n t e r - c a u g h t  mink i n  Michigan.  159  I n at l e a s t p a r t i a l  support Of the above h y p o t h e s i s , he found t h a t males  took inuskrats ( i . e . , and perhaps My  fish  l a r g e p r e y ) s i g n i f i c a n t l y more o f t e n and s m a l l mammals  l e s s o f t e n than d i d females.  d a t a r e l a t i n g to p o s s i b l e d i f f e r e n t i a l n i c h e u t i l i z a t i o n by  the  sexes are from a n a l y s e s o f d i g e s t i v e t r a c t s , examination o f f e e d i n g middens, and a n a l y s e s o f o b s e r v a t i o n s , the g e n e r a l methods f o r which have been described previously.  F o r the o b s e r v a t i o n d a t a I o c c a s i o n a l l y r e c o g n i z e d  i n d i v i d u a l s and c o u l d r e c o r d t h e i r sex w i t h c o n f i d e n c e ; more-..often. 1 :Co.uld list  only r e l a t i v e size,  i n which case the sex i s i m p l i e d but not  certain.  I n f o l l o w i n g a n a l y s e s , these two k i n d s o f d a t a are h a n d l e d s e p a r a t e l y . O p p o r t u n i t i e s to check f i e l d - d e t e r m i n e d s i z e s a g a i n s t sex were a f f o r d e d when I l a t e r l i v e t r a p p e d r e c o g n i z a b l e animals and, 1972,  when I c o l l e c t e d  were almost  specimens o f both sexes.  always females  although i n some cases  they were u n u s u a l l y s m a l l j u v e n i l e males.  i n summers o f 1971  Animals  listed  as " s m a l l "  (fewer than t e n per  A l e a n appearance,  and  cent)  especially in  the neck, and a " h y p e r a c t i v e " b e h a v i o r p a t t e r n were^as c h a r a c t e r i s t i c o f animals c o n s i d e r e d s m a l l as was  size.  The  s i z e c l a s s i f i c a t i o n "medium" was  the one most s u b j e c t to e r r o r (about f i f t e e n per c e n t ) . was  the c a t e g o r y used when I was  p o s i t i v e sense most o f the time.  I n some cases i t  u n c e r t a i n , although i t was  (as d u r i n g pregnancy) were l i s t e d  animals, a l l t h i c k - n e c k e d , r e l a t i v e l y confused f o r a n y t h i n g  else.  in a  Most medium animals were j u v e n i l e males  although both l e a n ( o f t e n a i l i n g ) a d u l t males and u n u s u a l l y females  listed  heavy-bodied  i n t h i s c a t e g o r y at times.  "Large"  slow-moving a d u l t males, were r a r e l y  160 RESULTS  T a b l e 11 l i s t s  f r e q u e n c i e s a t which the two  sexes, (and s i z e s ) o f mink  were seen h u n t i n g under d i f f e r e n t c o n d i t i o n s , p r o v i d e s a comparison frequency w i t h which each caught  (or f a i l e d  to c a t c h ) prey and,  o f the  f o r ob-  s e r v a t i o n s o f s u c c e s s f u l hunts, compares the e x t e n t to which they  caught  d i f f e r e n t kinds of prey.  hunted  I had  the i m p r e s s i o n t h a t females o f t e n  l o n g e r a f t e r the s l a c k low t i d e than d i d males,  but the d a t a shown do not  i n d i c a t e t h a t e i t h e r sex r e g u l a r l y reduced c o m p e t i t i o n by e x p l o i t i n g tide level.  a higher  I observed b o t h sexes h u n t i n g i n p r o t e c t e d , r o c k y h a b i t a t s f a r  more o f t e n than ;on open, low p r o f i l e s h o r e s , but t h e r e was f o r l a r g e animals  ( i . e . , mostly males) to hunt  than d i d s m a l l ones.  This i s reflected  a s t r o n g tendency  the open areas more o f t e n  f u r t h e r i n a comparison  methods observed, which shows t h a t females  of hunting  (and s m a l l animals) were h u n t i n g  c h i e f l y by poking d u r i n g more than h a l f o f the o b s e r v a t i o n s w h i l e males dove and bird-dogged  f o r most o f t h e i r p r e y .  Comparing the r e s u l t s  f o r the  sexes  and t h e i r r e s p e c t i v e s i z e s i n T a b l e 11, t h e r e i s j u s t one case i n which the g e n e r a l t r e n d o f f r e q u e n c i e s l i s t e d do not agree.  During observations of  known females, f i s h were caught more o f t e n than c r a b s , a r e s u l t which would be expected i f these animals were indeed h u n t i n g mostly by poking i n r o c k y habitats. more o f t e n .  However, " s m a l l " animals, presumably  m o s t l y females, caught  I have no e x p l a n a t i o n f o r t h i s d i s c r e p a n c y .  Other  crabs  evidence  i n d i c a t e s , however, t h a t t h e r e i s c o n s i d e r a b l e o v e r l a p i n foods eaten by the two  sexes, at l e a s t d u r i n g some times o f the y e a r . Examination o f d i g e s t i v e t r a c t s from animals c o l l e c t e d  r e v e a l e d p r e y o c c u r r e n c e s as  follows:  i n summer  161 Table 11. Evidence of differential niche utilization by the sexes, Vancouver Island mink, from observations of free-ranging animals.  Sex of Mink Male Female # %  Size of Mink Small Other # % # %  lo  b  TIDE Low Low Other  33 4  89 11  44 6  88 12  <0.90  118 18  87 13  167 24  87 13  <0.99  HUNTING HABITATS BB + RW EF + SB + ES  27 7  79 21  30 13  70 30  <0.90  115 9  93 7  125 42  75 25  <0.001  HUNTING METHOD Diving Poking Bird-dogging  5 11 4  25 55 20  19 10 10  . 48 26 26  <0.10  29. 39 7  39 52 9  50 38 27  43 33 24  <0.01  HUNTING SUCCESS Caught 0 Prey Caught Prey  2 5  29 71  4 20  17 83  <:0.50  11 16  41 59  11 42  21 79  <0.10  7 13 5  28 52 20  30 8 11  61 17 22  <0.01  22 13 16  43 25 32  64 16 39  54 13 33  < 0.20  0  e  PREY CAUGHT Crabs Fish Unident.  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 Habitats: BB (Boulder Beach) + RW (Rockweed Shore) = relatively protected; EF (Eelgrass Flats) + SB (Small Particle Beach) + ES (Estuary) = open (relatively exposed). Caught one or more prey during period of observation.  d  e  162  PREY PRESENT  Frequency (per cent) Female (N=43)  Crabs Fish Other  49 42 9  Male (N=39) 56 28 16  X =1.9, df=2, p < 0.50  PREY DOMINANT (N=30) Crabs Fish Other  50 40 10  (N=29) 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 i s 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  Mink Size  Prey Size:  No. (?„) of Observations  0 - 10 g  11 - 25 g  26+ g  Small  28  (61)  13  (28)  5  (11)  Large  18  (29)  30  (48)  15  (23)  2  xMl.5, df=2, p<0.01 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, oftenunderground 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 e x e r t i n g themselves  c o n s i d e r a b l y to  remove, from the water, l a r g e c r a b s which were c l i n g i n g w i t h t h e i r to marine v e g e t a t i o n , and managed.  Two  persisted  a f t e r they broke  chelae  i t seemed u n l i k e l y t h a t females c o u l d have  males which I saw get s e v e r e l y p i n c h e d by l a r g e r e d c r a b s away and subdued the crabs i n v o l v e d , w h i l e  a female under the same c i r c u m s t a n c e s f l e d when the c r a b f i n a l l y r e l e a s e d her.  An a d u l t female mink which I found f r e s h l y dead at the edge o f a  B a r k l e y Sound b o u l d e r beach was  a u t o p s i e d and appeared  s p e c u l a t e d t h a t some i n t e r t i d a l  animal, perhaps  to have drowned.  I  a l a r g e r e d c r a b , had h e l d  her under water; o t h e r c i r c u m s t a n c e s which might have l e d to t h i s drowning were not  apparent.  Duririggthe - o b s e r v a t i o n s , h u n t i n g success  ( c a t c h i n g prey v e r s u s not  c a t c h i n g prey, r e g a r d l e s s o f o b s e r v a t i o n d u r a t i o n ) was  somewhat g r e a t e r f o r  males than f o r females, but o b s e r v a t i o n s o f known females were too f o r r e s u l t s to be meaningful  statistically.  few  Small mink were u n s u c c e s s f u l i n  hunts much more o f t e n than were l a r g e r ones., (both medium and l a r g e ) , and proportion of successful  hunts  the  f o r l a r g e animals alone i s 85 per c e n t ,  s i g n i f i c a n t l y h i g h e r than the 59 per cent success i n d i c a t e d f o r s m a l l mink 2 X  = 4.20,  d f = 1, p-i-0.05).  Comparing l o n g e r h u n t i n g p e r i o d s ( > 5  which y i e l d e d prey, h u n t i n g success (expressed as n e t weight per minute o f o b s e r v a t i o n ) d i f f e r e d  little  as compared to l a r g e ones (2.1 g; SE =  minutes)  o f prey  caught  f o r s m a l l mink (2.2 g; SE =  0.6)  0.4)  DISCUSSION The  d a t a r e l a t i n g to s i z e dimorphism i n the c o a s t mink suggest  t h e r e are indeed some d i f f e r e n c e s i n f e e d i n g n i c h e between the two  that sexes.  165  Most o b s e r v a t i o n s  were made under the most f a v o r a b l e f e e d i n g c o n d i t i o n s ,  which happen a l s o to be the b e s t  for observation,  d i f f e r e n c e s at r e l a t i v e l y u n f a v o r a b l e  and  i t is likely  times such as d u r i n g storms or neap  t i d e s would be o f g r e a t e s t s i g n i f i c a n c e to the animals. t h a t the degree o f o v e r l a p  that  I t i s also  i n feeding niche v a r i e s with population  probable density,  g r e a t e r o v e r l a p o c c u r r i n g at r e l a t i v e l y low p o p u l a t i o n l e v e l s such as p r e v a i l i n g d u r i n g much o f the study p e r i o d , and  l e s s o c c u r r i n g as  those  competition  increases. The rocky it  tendency f o r females to hunt more o f t e n than males by poking  shore h a b i t a t s remains w i t h me  i s supported  as a s t r o n g s u b j e c t i v e i m p r e s s i o n  r a t h e r w e l l by the data g i v e n .  Females, b e i n g  t h a t poking critical year  the big-headed, t h i c k - n e c k e d  produces s m a l l e r prey  than o t h e r h u n t i n g  Further,  and  can  crevices  the  fact  methods i s p r o b a b l y  less  to a female, whose t o t a l d a i l y food requirement d u r i n g most o f  i s l e s s than t h a t o f a male.  beaches and rockweed shores  While the q u e s t i o n s q u a l i t a t i v e i n nature, between the two successfully  Even though males f r e q u e n t l y hunted  ( T a b l e 11),  ( e s p e c i a l l y d i v i n g ) which y i e l d  other  males.  and  smaller,  hunt such h a b i t a t s more e f f i c e n t l y s i n c e they can e x p l o i t c r a c k s which would exclude  among  l a r g e r prey..  c e n t e r i n g on the degree o f o v e r l a p  sexes, the q u e s t i o n o f whether one  (catches more prey or catches  between the sexes and, be c o n s i d e r a b l e o v e r l a p  boulder  they u s u a l l y hunted by methods  p e r t i n e n t to the s u b j e c t o f dimporphism are  i s also of i n t e r e s t .  the  i n feeding habits  sex r e g u l a r l y hunts more  prey more e f f i c i e n t l y ) than  A l l o f the f o r e g o i n g presupposes  the two  sexes, one  the  competition  s i n c e the data i n d i c a t e t h a t at times t h e r e may i n foods taken by  largely  indeed  might expect  166  the l e s s c o m p e t i t i v e  sex.to be f o r c e d , on the average, t o hunt i n l e s s  profitable situations. .did enjoy is  observations  a h i g h e r p r o p o r t i o n o f s u c c e s s f u l hunts t h a t d i d s m a l l mink  suggestive.  probably  The f a c t t h a t l a r g e mink among my  I n t e n s i v e o b s e r v a t i o n s o f a few known animals  would  be more i n s t r u c t i v e on t h i s p o i n t than my s e r i e s o f e x t e n s i v e  r e c o r d s has been.  167  I I I . POPULATION ECOLOGY DESCRIPTION AND M a t e r i a l presented  i n f o r e g o i n g pages was  a t i o n s o f f r e e - r a n g i n g animals o f d a t a on i n d i v i d u a l s  EVALUATION OF TECHNIQUES  and  based  their "sign".  l a r g e l y upon observ-  To o b t a i n c e r t a i n  (movements, range, r e p r o d u c t i v e c o n d i t i o n , pathology)  and p o p u l a t i o n s ( c o m p o s i t i o n , numbers, t u r n - o v e r ) i t was examine animals  kinds  i n hand.  A v a r i e t y o f techniques was  also necessary  to  employed i n these  s t u d i e s , and most w i l l be d e s c r i b e d where a p p r o p r i a t e i n f o l l o w i n g s e c t i o n s . However, c a p t u r e and h a n d l i n g o f l i v e animals p r o v i d e d the b a s i s f o r most o f the r e s u l t s p r e s e n t e d  later,  the c a p t u r e t e c h n i q u e s used  and  i t is essential  to o u t l i n e ,  i n terms o f the extent o f e f f o r t and  the d i s p o s i t i o n o f specimens a f t e r c a p t u r e , and a s s o c i a t e d w i t h these c a p t u r e a c t i v i t i e s .  the b i a s e s and  success,  limitations  A l s o b a s i c to many o f the  c l u s i o n s drawn i n subsequent pages i s the r e l i a b i l i t y o f my determining  in detail,  con-  methods f o r  age o f specimens, t h e r e f o r e s p e c i a l a t t e n t i o n to t h i s s u b j e c t  i s a l s o warranted LIVETRAPPING AND  i n t h i s opening  s e c t i o n on p o p u l a t i o n p r o c e s s e s .  HANDLING  METHODS Two  models o f c o l l a p s i b l e wire-mesh l i v e t r a p s from the N a t i o n a l L i v e -  t r a p Company, Tomahawk, W i s c o n s i n , were used. trap  (15 x 15 x 48 cm)  (15 x 15 x 60 cm)  was  more convenient  model because i t was  The  s m a l l e r , s i n g l e door  to use than the  double-door  l i g h t e r and more compact, and c o u l d  be s e t i n l o c a t i o n s at which the l a r g e r t r a p would not f i t . was  somewhat l e s s e f f i c i e n t  i n t h a t animals,  were o c c a s i o n a l l y a b l e to remove b a i t w i t h o u t  The  small trap  e s p e c i a l l y experienced a c t i v a t i n g the  treadle.  ones  168  The  double-door f e a t u r e o f the  s i n c e such s e t s were not l e f t closed.  l a r g e t r a p i s to f a c i l i t a t e runway s e t s ,  e f f e c t i v e i n my  S i x t y - t h r e e per cent  areas,  the r e a r door was  but  usually  o f a l l c a p t u r e s were made w i t h the  larger  trap. Most s e t s i n v o l v e d to i t and  because i t was  which e n t e r e d  the  f e c t i v e b a i t was use;  the use o f b a i t , both because mink were a t t r a c t e d  traps fish,  advisable  to p r o v i d e  i n poor c o n d i t i o n .  food  The  to h e l p  s u s t a i n those  most convenient and  and more than 90 per cent o f a l l s e t s i n v o l v e d i t s  b i r d remains, when they were a v a i l a b l e , a l s o worked w e l l , and  c a p t u r e s r e s u l t e d from use  o f m o l l u s k s and  crustaceans.  l e f t up  to s e v e r a l days i n w i n t e r .  per cent or more o f the  t r a p s out  In some areas i t was  i n any  given night  or b a i t l e s s as a r e s u l t o f i n t e r f e r e n c e from o t h e r deer mice and  land slugs  (Limacidae, A r i o n i d a e ) .  for  most t r a p p i n g  i t was  few  to be  they  could  common f o r  50  incapacitated  animals, e s p e c i a l l y Where raccoons  i n numbers, s u c c e s s f u l l i v e t r a p p i n g f o r mink c o u l d be because of t h e i r i n t e r f e r e n c e , and  a  I n summer i t was  n e c e s s a r y to change b a i t s almost d a i l y because o f s p o i l a g e , but be  ef-  occurred  almost  impossible  n e c e s s a r y to a v o i d  such areas  operations.  Most t r a p s were s e t w e l l above the c u r r e n t h i g h water mark, o c c a s i n a l l y among d r i f t d e b r i s , but most o f t e n along cavities line.  (hollow  E a r l y e x p e r i e n c e i n d i c a t e d t h a t mink r a r e l y t r a v e l e d more than about  wasted e f f o r t . f l o o d i n g by low  i n natural  l o g s , s m a l l caves, exposed r o o t systems) above the beach-  50 m from the beach, and  on  runways and  that ..trapping beyond t h a t d i s t a n c e was  Traps set too near c u r r e n t h i g h water were s u b j e c t  storm-heightened t i d a l  t i d e beaches i f I c o u l d be  d a t i o n , but  largely  surges.  I o c c a s i o n a l l y put  traps  sure t h a t I would r e t u r n b e f o r e  success r a t e s f o r such e f f o r t was  no  greater  to  than t h a t  out  inunfor  169  t r a p s s e t as d e s c r i b e d  above. I n a l l cases,  l o c a l materials, e s p e c i a l l y driftwood,  t r a p s were w e l l covered  with  t o reduce exposure to r a i n and sun.  Traps were u s u a l l y checked i n the morning, s i n c e most c a p t u r e s  occurred  at n i g h t . The and  objective of l i v e t r a p p i n g operations  examination o f as many animals u s i n g  possible.  was to enable  a given  classification  s e c t i o n o f s h o r e l i n e as  Trap d i s t r i b u t i o n , t h e r e f o r e , was based on the d i s t r i b u t i o n and  abundance o f " s i g n " and the known l o c a t i o n s o f animals r a t h e r than on a r e g u l a r g r i d o r t r a n s e c t system. to be p r e s e n t  but was d i f f i c u l t  For instances  i n which a mink was known  to c a t c h , I might c o n c e n t r a t e  t e n o r more  t r a p s w i t h i n 100m o f s h o r e l i n e ; f o r long s e c t i o n s o f shore w i t h no s i g n and  l i t t l e p o t e n t i a l as mink h u n t i n g h a b i t a t , adjacent  separated  by up to 500 m.  t r a p s might be  However, an attempt was made to keep s e v e r a l  t r a p s w i t h i n the range o f any g i v e n mink, to enable documentation o f i t s movements.  On Vargas I s l a n d , t r a p s were d i s t r i b u t e d i n an average d e n s i t y  o f about 10 p e r k i l o m e t e r along  the more d e n s e l y  meter.  of shoreline, while  populated  shores o f B a r k l e y  Up to 55 t r a p s were tended d u r i n g  though the average was about 35. Vargas I s l a n d and B a r k l e y  the c o r r e s p o n d i n g  figure  Sound was 17 per k i l o -  a single trapping  session, a l -  I n i n t e n s i v e l y s t u d i e d areas, e s p e c i a l l y  Sound, t r a p p i n g was conducted once each month  i f possible. Traps were u s u a l l y l e f t first but  year o f study " t r a p p i n g  s e t i n one p l a c e f o r f o u r days.  D u r i n g the  s e s s i o n s " o f up to 10 days were attempted,  the number o f new animals caught a f t e r f o u r to f i v e days was n o t  worth the e x t r a s t r e s s imposed upon those animals which might be caught repeatedly.  Occasionally trapping  s e s s i o n s had to be terminated  a t two o r  170 t h r e e days because o f l o g i s t i c a l problems (equipment f a i l u r e ; procure  b a i t ) and/or v e r y severe weather.  i n a b i l i t y to  When a i n d i v i d u a l a n i m a l was  caught i n the same t r a p two days i n s u c c e s s i o n , t h a t t r a p was n o t r e s e t d u r i n g the remainder o f t h e s e s s i o n , and i n some cases where such a r e c a p t u r e o f an i n d i v i d u a l was p r e d i c t a b l e on t h e b a s i s o f p a s t the t r a p was l e f t c l o s e d a f t e r t h e f i r s t c a p t u r e . 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  experience,  This procedure helped  i n f o r m a t i o n on movements.  On the f i r s t c a p t u r e d u r i n g a g i v e n s e s s i o n , a mink was f o r c e d from the l i v e t r a p i n t o a w i r e h a n d l i n g cone ( s e e P l a t e 4) where i t c o u l d be s a f e l y examined.  D u r i n g the f i r s t y e a r o f study animals were a n e s t h e t i z e d  w i t h e t h e r as d e s c r i b e d by L o c k i e and Day (1964), but t h i s t e c h n i q u e inconvenient animals.  and i t o b v i o u s l y caused a t l e a s t temporary i l l n e s s  was  i n some  Once i n the h a n d l i n g cone, t h e animal was examined f o r g r o s s  a b n o r m a l i t i e s , g e n e r a l p h y s i o l o g i c a l and r e p r o d u c t i v e c o n d i t i o n , and age, then was marked, weighed and r e l e a s e d a t t h e c a p t u r e  site.  The s p e c i f i c  c r i t e r i a f o r d e t e r m i n a t i o n o f r e p r o d u c t i v e s t a t u s and c o n d i t i o n a r e d e s c r i b e d i n f o l l o w i n g s e c t i o n s (pp. 195 and 2 2 8 , r e s p e c t i v e l y ) } t e c h n i q u e s for  d e t e r m i n a t i o n o f age a r e d e t a i l e d a t the end o f t h i s s e c t i o n .  numbered f i n g e r l i n g  A  t a g ( S i z e Number 1) from t h e N a t i o n a l Band and Tag  Company, Newport, K e n t u c k y , was p l a c e d on each e a r , and these tags cons t i t u t e d the p r i m a r y 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 t a g  r e t e n t i o n was poor e a r l y i n t h e study, each e a r was a l s o t a t t o o e d w i t h a s i n g l e a l p h a b e t i c o r numeric c h a r a c t e r ( T a t t o o K i t No. 101C from Ketchum Manufacturing  S a l e s L t d . , Ottawa, O n t a r i o ) .  However, I l e a r n e d t h a t tags  f a c e page 171  P l a t e 4: Steps i n the h a n d l i n g o f a l i v e t r a p p e d mink a & b) C h a s i n g the animal from the l i v e t r a p i n t o a w i r e h a n d l i n g cone. c) Examining mink so c o n f i n e d . d) A t t a c h i n g an ear t a g . 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 f i r s t 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 a l l 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 f i r s t 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 f i r s t year (1968-69) resulted largely from my experimentation 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 f i r s t 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 ' 68-Jun '69 Jul '69-Jun '70 Jul ' 70-Jun '71 Jul ' 71-Jun '72 Jul '72-Jun '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 f i r s t 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 l i s t s 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 i n i t i a l 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 f i r s t caught along the Tofino Village Waterfront, where chances for mortality from recreational trapping and domestic dogs is high.  That is  f a c e 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  "2  6  CL < O CO  o LO  o eo  o CM  ° W  CL -C o co £ Z  177  probably  many were not  recaptured  d a t a o f Appendix 15 r e p r e s e n t  because they were no  longer  varying periods of contact with  and under w i d e l y v a r y i n g c i r c u m s t a n c e s ,  and  probably  do not  alive.  individuals,  accurately  p o r t r a y d i f f e r e n c e s i n v u l n e r a b i l i t y to t r a p p i n g among,the sex and c l a s s e s which c o u l d be  identified.  I t i s n e c e s s a r y to c o n s i d e r  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 , bias trapping r e s u l t s ,  Comparisons d e v i s e d of trapping  age  the  since i t could  i . e . , some c l a s s e s might be caught out o f  p o r t i o n to t h e i r o c c u r r e n c e i n the  The  pro-  population.  to d e t e c t b i a s i n t r a p response u t i l i z e  s e s s i o n s which, as d e s c r i b e d  earlier,  are  time ( u s u a l l y 4.days) d u r i n g which the s t a t u s of any should have remained r e l a t i v e l y u n i f o r m .  The  results  short peri