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Principles affecting the size of pink and chum salmon populations in British Columbia Neave, Ferris 1951

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L  5  ^ 7  tlft fit PRINCIPLES AFFECTING THE SIZE OF PINK AND CHUM SALMON POPULATIONS IN BRITISH COLUMBIA by Ferris Neave  —oOo—-  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  in ZOOLOGY  —oOo  THE  UNIVERSITY  OF  B R I T I-S H C O L U M B I A  October, 1951  ABSTRACT Changes in population size are governed by the birthrate,the sex ratio and the deathrate. In pink salmon the average egg-production per female is about 1700 and variations from this average are insufficient to account for observed changes in adult populations. The sex ratio is approximately 50-50. Survival during the freshwater phases of the life cycle has been found to vary from approximately 1% to 21$,the average survival being significantly different in different streams. Variation is relatively greater in streams in which average survival is low. Natural survival in the ocean is considered, to average about 5% of the number of young fish reaching the sea. In the central region of the British Columbia coast the annual catch averages about 60% of the adult fish,this percentage being relatively constant for both small and large runs. Pink salmon maturing in "even" and "odd" years represent separate populations. These populations vary in size independently but may maintain a relatively constant ratio for a series of generations. This ratio varies from near equality to extreme disparity. Marked changes in the level of abundance may occur suddenly. Three types of mortality are recognized: (a) mortality which becomes relatively heavier as populations increase in density (compensatory) (b) mortality which becomes relatively heavier as populations decrease in density (depensatory) (c) mortality which is independent of density (extrapensatory).  (a) is especially identified with the period of.  spawning and incubation;(b) is considered to occur mainly during the period of fry migration and to be due to predationj(c) may occur at any stage but is probably most variable during the period between entrance of the adults  into fresh water and emergence of the fry. Population changes initiated by extrapensatory factors,among which stream-flow conditions are considered to be important,are exaggerated by depensatdry factors (notably predation on fry) but tend to be resisted by the compensatory influences which operate during the period of spawning and incubation. Stabilization of a level of abundance depends on a balance between these processes. In general,freshwater mortality is more variable than ocean mortality and plays a greater part in inducing population changes. It is suggested that the average freshwater survival of an unfished population would approximate 2»A% and that this efficiency must be raised to about 6% to permit a sustained catch of 60% of the adult population. A lower freshwater output is likely to result in a reduction in size of the stock. Large runs in both even and odd years are not fundamentally incompatible. The possibility, of promoting a low-level stock to a persisting higher level of abundance is indicated. Chum salmon are subject to the same types of mortality as pink salmon but the results are modified by the higher average egg-production (ca.2700) and the variable length of the life cycle. The species tends to occupy streams in which physical conditions are less stable. Compensatory influences are frequently obscured by these conditions,resulting in irregular fluctuations in abundance. In the application of remedial measures similar principles apply to both species.  The T h e s i s of Hr. F e r r i s Neave entitled " P r i n c i p l e s A f f e c t i n g the S i z e o f P i n k and  Chum Salmon P o p u l a t i o n s i n British is  Dean Angus Clemens Cameron Hoar Hutchinson Robblns Sage Spencer Warren  Columbia"  accepted:  - i CONTENTS Page iv  HEKODUCTION . FINK SALMON  -  1  EGG-PRODUCTION SEX RATIO  2 .  MORTALITY MORTALITY AND SURVIVAL IN FRESH WATER .  4 6 7  1. Total freshwater survival  7  2. Immediate causes of death . . . .  9  3» General conclusions MORTALITY AND SURVIVAL IN THE OCEAN  11 12  CHANGES IN ABUNDANCE OF ADULT PINK SALMON  18  THE MECHANISM CONTROLLING POPULATION LEVELS  21  RELATIVE EFFECTS OF FRESHWATER, NATURAL OCEAN AND FISHING MORTALITY IN DETERMINING POPULATION LEVELS . . . . . CONSERVATION PROBLEMS  28 36  CHUM SALMON  41  EGG PRODUCTION  42  SEX RATIO  42  MORTALITY  42  MORTALITY AND SURVIVAL IN FRESH WATER  42  1. Total freshwater survival  42  2. Immediate causes of death . . . . . .  44  3- General conclusions  45  MORTALITY AND SURVIVAL IN THE OCEAN  47  ' CHANGES IN ABUNDANCE OF ADULT CHUM SALMON  49  THE MECHANISM CONTROLLING POPULATION LEVELS  50  - ii Page RELATIVE EFFECTS OF FRESHWATER, NATURAL OCEAN AND FISHING MORTALITY IN DETERMINING POPULATION LEVELS . . . . . . CONSERVATION PROBLEMS  52 53  ACKNOWLEDGEMENTS  55  REFERENCES  56  TABLES AND FIGURES Table I.  Average egg content of pink salmon-  5  Table II.  Sex ratio of pink salmon escapements  5  Table III.  Freshwater survival of pink salmon  8  Table IV.  Marine survival of pink salmon  14  Table V.  Reported catch and escapement of pink salmon in the central region of British Columbia . . . .  16  Numbers of pink salmon caught annually in traps at Sooke, B. C  19  Table VI. Table VII.  Effect of predation on the fry-migrant production of pink salmon at McClinton creek. . .  25  Table VIII.  Sex ratio of chum salmon escapements  4-3  Table IX.  Freshwater survival of chum salmon  U3  Table X.  Reported catch and escapement of chum salmon in the central region of British Columbia . . . .  48 To face page  Fig. 1. (1) Total mortality required to maintain constant population (2) Egg production required to maintain constant population. Fag. 2.  Decline of population with increased proportion of male fish  6" •  6  Fig. 3. Reported catch and escapement of pink salmon in the central region of British Columbia . . .  18  Fig. U' Runs of pink salmon in the central region of British Columbia  19  - iii To face page Fig. 5« Pink salmon catches reported from Skeena River area  20  Fig. 6. Relation between freshwater survival of pink salmon and size of spawning stock at various levels of fishing intensity.  33  Fig. 7.  •^ig. 8.  Reported catch and escapement of chum salmon in the central region of British Columbia ,  49  Reported catch of chum salmon in Skeena River area  49  *"ig. 9« Reported escapement of chum salmon in Vancouver Island district'  49  Fig. 10. Survival of marked chum fry-migrants in Hooknose Creek, in relation to number of fish migrating and progress f of time  50  Fig. 11. Relation between freshwater survival of c chum salmon and size of spawning stock at various levels of fishing intensity  53  -Jv-  INTRODUCTION The various species of Pacific salmon (Oncorhynchus spp.) exhibit changes in abundance which may be of an annual character or may be associated with longer time periods. Such changes are of general interest because they represent phenomena which in some form and at some time are common to a l l species of animals and indicate the terms on which populations continue to exist.  They are also of specific practical interest be-  cause the fortunes of a large industry depend upon both the short-term and long-term abundance of these fish. The present study represents part of a broader investigation undertaken under the auspices of the Fisheries Research Board of Canada,with the general object of acquiring information necessary for (a) the prediction of changes in abundance of Pacific salmon (b) the maintenance or increase of the stocks<of these fish available for sustained exploitation. The purpose of the account presented herewith is to examine the general factors determining population size in two of the five species of Pacific salmon occurring in British Columbia,with particular reference to the above-mentioned problems of prediction and practical remedial action. These two species (the pink salmon.Oncorhynchus gorbuscha.and the chum salmon .O.ket a) are sufficiently similar in life history and distribution to facilitate a common investigation. At the same time they exhibit certain differences in the details of behaviour,life history and fecundity which help to elucidate general principles applying to both species. In attempting to reach conclusions which may contribute to an understanding of the general processes governing population levels and changes,data have been assembled from various sources.  The investigations at  Nile Creek and Hooknose Creek,to which frequent reference is made,were  initiated by the present writer and have been carried out under his direction since their inception. He has also directed ocean tagging operations on both species and has conducted widespread observations on their numbers, distribution and general ecological relationships in fresh water.  For the  purpose of reaching general conclusions,however,he has not hesitated to use and interpret additional data for which he was in no way personally responsible. The sources of such information are acknowledged in the text.  -1PINK SALMON (0nchorhynchus gorbuscha (Walb.)) The pink Salmon is generally distributed throughout the length of the British Columbia fioastline. In fresh water it is scarce in certain areas of both the east and west coasts of Vancouver Island.  Elsewhere, the numbers of  adults in alternate .years may be small. With these provisos it is the most abundant salmon in British Columbia. The adults enter fresh water mainly in August and September, utilizing both large and small streams,  In moderately large unobstructed rivers such as  the Bella Coola they may travel upstream for as much as 50 to 70 miles, while in the Skeena watershed many thousands annually ascend the Babine River to a distance of nearly 4-00 miles (by water) from the ocean.  In general, however, a  great part of the spawning takes place close to salt water, sometimes beginning within the limits of tidal influence. Deposition of eggs occurs mainly in September and early October.  In observed instances the hatching of the alevins is  completed during February and the emergence of the fry from the gravel, followed by their immediate migration to salt water, reaches a peak at the end of April or early in May. The young fish can be observed feeding in schools at or near the surface of the sea in May and June, when they may already be found at a distance of many miles from their presumed source of origin.  With the advancing  season they appear to disperse somewhat and to descend to a depth which, combined with their increased size and activity, makes capture difficult (Observations of writer and. others in Strait of Georgia).  Their subsequent marine distribution  is not ordinarily witnessed until the following summer, when the maturing fish begin to be taken in July by the fishery, So far as is known, the total life cycle invariably occupies two year,s.  EGG-PRODUCTION Since changes in the size of any population are determined by the birthrate and the ceathrate, the number of eggs produced is of basic importance in the present study.  Total egg-production is obviously a product of the average  egg-production per female and the number of females in a population. Egg-production per female Available data on the average production of eggs by female pink salmon are shown in Table I. From these figures it appears that (1) the general average egg-production per female in pink salmon is about 1700, which is lower than that of any other species of Pacific salmon (2) differences in average number of eggs occur between different localities and in the same locality in different years. The first point carries the obvious implication that i f the species is to maintain itself the mortality occurring during the life cycle (from egg -to spawning adult) must be less than in the other species of Oncorhynchus.  If we  assume an average egg-production of 2700 per female chum salmon (see Page $2), with equality of sexes, then a total mortality of 99-33% applied to the product of a spawning pair would just permit replacement of the parents.  The application  of the same mortality rate to pink salmon eggs (taking an average of 1700) would reduce the spawning stock to less than one quarter of its original size in three generation*. To maintain stability it would be necessary to apply a mortality rate of 99.88$ to the pink salmon. This latter rate would permit a chum salmon spawning stock to triple itself in tfeo generations.  A steady state would of  course be represented in either instance by: M ° 100(E-2) , where M is percent E total mortality and E is the average number of eggs per spawning female. It does not follow that i f the average egg-production of either species were changed, the mortality rate at present characteristic of the species would remain unaltered.  It does follow, however, that i f average egg-production remains  constant, a change of a fraction of 1% in the total mortality rate would quickly  " 3 produce a spectacular increase or decrease in the spawning stock.  Hence the  average egg-production per female of a population which is maintaining a fairly constant level over a prolonged period, must be a very sensitive measure of the mortality which is actually prevailing. While these considerations can be applied to long-term averages, considerable variation in egg-production is apparent from generation to generation and from place to place.  In so far as topographic differences remain constant,  they must reflect local differences in mortality rate or active changes in population size.  Variations in fecundity from generation to generation may, however,  merely represent fluctuations around an average. Pritchard's (1948a) figures, for average egg counts in a series of consecutive generations of a pink salmon population range from 1535 to 1899.  This represents a fluctuation of about 11$  above or below the overall mean, of 1718.  Such a variation is certainly far from  insignificant but it is evident that other major factors must be found to account for the much greater variations which occur in pink salmon populations. The extent to which egg-production is controlled by hereditary factors is unknown. Presumably certain limits are so determined. Well-marked and persistent differences in fecundity between widely separated sockeye populations of the Fraser River system and also between the various stocks of the Skeena watershed are known to exist.  It can readily be supposed that such differences are geneti-  cally established, although the effects of external conditions have not been experimentally excluded. 2  We can concede the possibility that genetically  controlled changes in fecundity may take place at times.  Such occurrences, however  will not .'account for the fluctuating type of variation which is observed in a I At Port John, where pink salmon runs of the genetically well-segregated "even" and'bdd" year cycles both occur, no significant difference in egg-production is yet apparent (data supplied by J.G. Hunter).  series of generations of a given population.  It is assumed that such devia-  tions can be ascribed to external conditions such as food supplies or selective fishing. , SEX RATIO The sex ratio should be considered in attempting to define the factors which may affect population size, since any continued inequality between the sexes would have to be taken into account in determining reproductive efficiency. The percentage of each sex recorded in the spawning escapement to various streams is shown in Table II. These figures do not vary consistently in one direction and for the most part the deviation from equality is small.  !Ehe tabulated values  refer to populations remaining after the operation of a fishery which has been thought to be sometimes selective in respect to sex of fish caught^ The above figures do not indicate any marked selectivity unless the latter has acted in such a way as to balance an initial inequality in the production of the sexes. However, Robertson (1951) in a sample of 232 migrating pink salmon fry. at Hooknose Creek, found 52% males and 4*$ females, — a difference which is insufficient to suggest a general departure from a 50-50 ratio. The presence of females in excess of 50$ might represent either a potential wastage of eggs or an increased efficiency per capita of population, according to the spawning habits of the fish.  On this point Shuman  (1950) reports "a definite tendency towards monogamy". He adds::  "While it  is probable that a male occasionally will attend more than one female, either because of the early death of the first mate, or because he has been driven from her by a stronger and more pugnacious male, the effectiveness of multiplicity of service must be demonstrated conclusively before it can be  - 5Table I. Average egg content of pink salmon  Locality  Year  McClinton Creek ti tt ii  n tt ti ti n  II  «  Morrison Creek, V.I. n  II  it  Namu Fraser River Port John it  n  Table II.  McClinton Creek 11  11  11  11  n  tt  11  11  ti  n  Morrison Creek, V.I. it  n  Hooknose Creek tt ti tt  tt n it  • 97 73 165 911 40 41  38 27  a 48 38 20  Average no.  Authority  of eftRS  1535 1758 1799 1899 1698 1619 1779 1862' 1841 I755 1520 1593  Eritchard (1948a) »' » » " " " " " " " " (unpub.) Neave » Foerster & Pritchard (1936) « it 11 11 Hunter (unpub.) " "  Sex ratio (percent) of pink salmon escapements  Locality  n "tt  1930 1932 1934 1936 1938 1940 1943 1945 1934 1934 1947 1950  No. of fish  it  Year  No. of fish  Males  Females  Authority  1930 1932 1934 1936 1938 1940 1942 1943 1945 1947 1948 1949 1950  66,153 15,600 155,196 52,312 10,577 35,521 36,893 15,755 13,411 ' 5,576 1,160 1,173 1,857  49.8 51.3 49.9 46.3 52.5 53.7 47.6 48.6 52.0 47.1 48.5 55.4 44.7  50.2 48.7 50.1 53.7 47.5 46.3 52.4 51.4 48.0 52.9 51.5 44.6 55.3  Pritchard (1948a) tt tt n tt ti n ti  Neave Hunter ti tt 11  n it tt 11  tt n  (unpub.) n it ti 11 11  - 6 -  considered an important factor in salmon propagation under natural conditions".  However Cameron (1939) found that there was no permanent pairing  of the sexes and that "males varied their attentions between adjacent females?'.  In view of his definite observations, it may be concluded that  a.deficiency of males, to the small extent noted above, is unlikely to reduce the reproductive efficiency of a population. On the assumption that extra males produce no benefits, the appearance of an excess of this sex in a previously equally divided population would'necessitate a reduced mortality rate or an increased egg-production to maintain a constant population. Fig. 1 shows the adjustments which would be required in either of these directions at various levels of male preponderance, as compared with a standard of 1700 eggs and a 50-50 sex ratio. Fig. 2 shows the percent decline of the population on the assumption that 1700 eggs continued to produce an average survival of two fish, unequally divided as to sex. Since a marked preponderance of males on the spawning grounds appears to be infrequent, this factor is not considered to be generally important in determining population levels.  Although selective fishing may  introduce local inequality at times, it is provisionally assumed that both the young fish and the spawning populations tend to have an equal division of the sexes. The complications introduced by the presence:of "jacks" (precocious males) do not arise in the case of the present species. MORTALITY Since the birthrate can be regarded as relatively constant, major changes in population size must be ascribed to changes in mortality. It is convenient to divide the total mortality which attends the life cycle into three categories, namely:  F i g . l . ( l ) T o t a l m o r t a l i t y r e q u i r e d to m a i n t a i n constant p o p u l a t i o n , w i t h egg p r o d u c t i o n o f 1700 and v a r y i n g preponderance o f male f i s h . (2) Egg p r o d u c t i o n r e q u i r e d to m a i n t a i n constant p o p u l a t i o n w i t h m o r t a l i t y r a t e o f 99«8824£and v a r y i n g preponderance p f male f i s h . '  F i g . 2 . D e c l i n e o f p o p u l a t i o n w i t h egg p r o d u c t i o n and m o r t a l i t y r a t e c o n s t a n t and i n c r e a s e d p r o p o r t i o n o f male f i s h . .  ,  - 7-  Freshwater Mortality Natural Ocean Mortality Fishing Mortality For practical purposes these categories correspond to consecutive portions of the life history, since pink salmon fishing is almost entirely confined to a short period at the close of the ocean phase of existence. MORTALITY AND SURVIVAL IN FRESH WATER 1. Total freshwater survival Counts or quantitative estimates of the number, sex ratio and eggcontent of adult pink salmon migrating up certain streams, and of the number of young migrants subsequently descending to the sea, have been made at several localities over varying for trapping the fish.  periodSB  of time, utilizing weirs and pens  The total number of eggs carried upstream by a given  spawning run has been estimated by sampling an appropriate number of females throughout the period of the upstream migration and has usually been termed the "potential egg depositiontt.  The number of young fish recorded as sea-  going migrants has been expressed as a percentage of the potential egg deposition, thus representing the rtreproductive efficiency*1 or percentage survival between these two points in the life cycle. The operation of counting weirs is attended by difficulties, most of which are connected with sudden large increases in the volume and speed of the water flow.  Enforced suspension of counting for short periods has  been a not infrequent occurrence in the history of some installations of this kind, necessitating a certain amount of interpolation.  The possible  errors resulting from such lacunae have been discussed by Pritchard with reference to the series of experiments conducted by him at McClinton Creek. Somewhat similar considerations apply to certain other cases. obtained by careful and competent investigators are accepted the  best  The figures here  as being  possible estimates and as being very close to the true values.  - 8 -  They are basic to the consideration of factors causing changes in abundance of pink salmon. From published records and through the courtesy of investigators in making their unpublished results available, the figures presented in Table III have been compiled. Table III.  Freshwater survival of pink salmon. Fry entering the sea expressed as percentage of potential egg deposition.  Locality  Brood-year  McClinton Creek it it ti tt n  1930 1932 19341936  it ii  II  it it  1938  1940  McClinton Creek  Average  Survival ($) 10.6  17.4  Authority Pritchard (1948a) tt ti tt ti tt  9.0 6.9  23.8  19.0  tt ti ti tt tt  14.45  Morrison Creek, V.I.  1943 1945  4.7 6.7  Pritchard (unpub.) ti Neave  Hboknose Creek  1947 1948 1949 1950  0.87  8.02 6.24 , 14.76  Hunter  Average  7.47  tt  it ti tt  n  tt  tt tt ti  Hooknose Creek Sashin Creek, Alaska  x  Average 1.93 (1940-50, 11 years)  tt tt tt  (1948) (1949)  (unpub.) ti  U.S.Fish & Wildlife Servicex  The writer is indebted to M.G. Hanavan, Acting Chief, Alaska Fishery Investigations, for these figures which ere of great interest for comparative purposes. The average recorded freshwater survival in the three streams for  which several years1 data are available is as:follows: 14«45$j Hooknose Creek, 7.47$; Sashin Creek, 1.93$.  McClinton Creek,  These differences are  large and can be considered to be statistically significant (P<.05).  Evi-  dently these populations are not being subjected to the same conditions of environmental pressure.  These different average survival rates may have a profound effect not only on the absolute number of sea-going migrants but also on the magnitude of the fluctuations between different generations of such migrants. Evidently i f a population is subject to an average freshwater mortality of 86$, a variation of as much as 7$ in either direction (79$ to 93$) will only increase or reduce the output by 50$.  On the other hand, with an average  mortality of 98$ a shift of only 2$ in the mortality rate would double the output or eliminate it entirely.  The following comparison shows the effects  of the recorded ranges of freshwater mortality as applied to the offspring of a pair of fish producing 1700 eggs. Stream  Observed mortality range ($)  McClinton Hooknose Sashin  76.2 - 93.1 85 - 99.1 95«6 - 99.8  No. of migrants produced (a) Maximum (b) Minimum 404 255 109  118 15 3*4  Ratio of (a) to (b) 3«45:1 17.00:1 32.00:1  It appears that a much greater range of fluctuation (as measured by the reproductive success of individual pairs of fish) can be expected in streams in which the average survival rate is low. 2.  Immediate causes of death Many more or less specific causes of loss can be recognized as  contributing to the total mortality experienced in the freshwater phases. These can be listed by periods, as follows: A.  Period preceding burial of eggs (1) Predation on adult unspawned fish. (2) Death of adult unspawned fish through other causes, notably as a result of barriers or insufficient water. (3) Losses of eggs through retention in the body or failure of fertilization.  - 10 -  B.  Period of incubation and alevinage (1) Erosion or scouring, i . e . , removal of gravel and contained eggs or alevins by flood, with resultant death by mechanical injury, exposure to predation or deposition in unsuitable situations. (2) Asphyxiation. Mortality caused by insufficient exchange of gases with the environment, due to unsuitability of the original location or to subsequent deposition of s i l t , etc., resulting in reduction of the water supply or reduction of the dissolved oxygen content of the water. (3) Unfavourable temperature for development. (4.) Freezing of eggs or alevins by coincidence of prolonged cold weather with exposure of spawning beds to air. (5) Reduction of water level, resulting in death by desiccation or prevention of the emergence of fry from the gravel. (6) "Superimposition" or "overdigging".  Mortality of the same type as  in (1) but caused by the operations of fish which have occupied spawning sites already containing the developing eggs of earlierspawning parents. (7) Killing of eggs or alevins by fungus. (8) Predation.  (Not usually regarded as serious during this period).  (9) Exposure of eggs to salt water in tidal areas. C.  Free-swimming period (1) Predation. (2) Trapping of fry in pools or backwaters. Breakdowns of total freshwater mortality by assigning percentage  losses to such categories as the above have been made in specific instances by Hobbs (1937) for Oncorhynchus tshawytscha and Salmo fario and by Cameron (I94I) for the present species.  Two main difficulties are encountered in  -11 attempting to reach general conclusions as to the relative importance of such individual factors in determining the final output.  The first lies in the  absence of quantitative data indicating the variation which undoubtedly exists in different streams and different years.  The second difficulty is connected  with the fact that many of the factors recognized are interrelated in such a manner as to make the assignment of losses an arbitrary matter.  For example,  the losses among pre-spawning fish may.be assigned largely to a biological factor, predation.  The extent of this predation, however, may depend to a  large extent on a non-biological factor, namely the prevailing water level (the writer has observed rather heavy predation by bears on unspawned pink salmon which had been prevented from surmounting a waterfall by low water conditions: later, an increase in stream flow largely eliminated the losses at this spot).  On the other hand, to isolate and emphasize such factors as  scouring and silting as being the chief causes of mortality during the period between deposition and fry emergence may underemphasize the part played by biological factors in producing the result.  The size of the run may deter-  mine the proportion of the fish which are forced to select less desirable sites.  Again, a comparatively small difference in high water levels might  greatly alter the ratio of losses between "silting 1 1 and "scouring" without producing a corresponding effect on the total mortality. 3. General conclusions While mortality will be discussed further, on a different basis, on later pages, the foregoing brief review of the extent and nature of freshwater losses favours the following preliminary conclusions: Freshwater mortality is always heavy, accounting for a large majority of the eggs or young fish present. Large fluctuations in the percentage output of fry are generally caused not by major variations in the total mortality rate but by small  - 12 variations which are large in relation to the very small percentage of fish which normally survive. While considerable annual variations in percent survival are evident in a given stream, there are also significant average differences in the mortality rates of populations inhabiting different streams. Populations having a low average freshwater survival are likely to show large relative differences in efficiency of reproduction from year to year. The immediate causes of death are interrelated.  Specific factors  are not necessarily additive in effect but act reciprocally to some extent. Total mortality may therefore be expected to vary relatively less than mortality from a particular cause. MORTALITY AND SURVIVAL IN THE OCEAN As indicated previously, the mortalities which take place in the ocean can conveniently be referred to two categories, - Natural mortality and Fishing mortalityx.  Before attempting'the segregation of these it is  convenient to establish a basis for estimating their combined effect on the young fish which have escaped from fresh water. The number of adult fish entering a given stream from the sea will represent numerically the survival of the young migrants leaving the stream in the appropriate previous season(s) i f (a) a l l the survivors return to the parent stream, or (b) the number of survivors which do not return to the same stream is balanced by an equal number of fish which originated elsewhere. With regard to (a), reference may be.made to marking experiments  x  In the largest rivers, i.e. the Fraser and the Skeena, some commercial fishing takes place in fresh water. Since, however, this fishing is a direct continuation of operations which for the most part are prosecuted in the sea, the distinction is not recognized in the present account.  - 13 -  reported by Pritchard (1939).  This investigator found that in the year for  which the most reliable data were available, 90% of the recoveries were from the "home" stream and only 0.2% were recovered in areas and at times which appeared to preclude their return to McClinton Creek. He concluded on the basis of a l l the evidence "that some of the pink salmon in the natural run may wander from the parent stream but the numbers so doing are not economically significant".  Also, "For a l l the marking experiments on pink salmon it  can be stated that in the case of natural runs the majority of fish returned to the parent stream". conclusion  In a later paper Pritchard (1948b) supported his  by reporting the results of an examination of pink salmon car-  cases in streams other than the parent creek.  He found only two marked fish  in 5922 examined, whereas McClinton Creek showed a proportion of marked fish equivalent to 131 in 5922. (b) Even with respect to the small minority of fish which undoubtedly "wander", it may be presumed that such wandering is not in one direction only but that there is a tendency for absentee fish to be replaced by individuals which originated in other streams. It is therefore concluded that the escapement of adults to a stream usually represents very closely the numerical survival from that stream's output of young fish. In the instances for which data are available, the percentages of returning adults in relation to outgoing fry are given in Table IV. These survival figures represent the adult population after it has been subjected to both natural ocean mortality and fishing mortality. For practical purposes these two factors can be considered to operate consecutively, not concurrently. The fishing for this species takes place during such a relatively short period at the end of the ocean phase that natural ocean • mortality can be regarded as having ceased when fishing mortality begins.  - u Fishing mortality therefore is added to natural mortality without reducing the latter to an important extent. Table IV. Marine survival of pink salmon. Adult escapement expressed as percentage of outgoing fry. Locality McGlinton Creek Morrison Creek Hooknose Creek Sashin Creek  BroodNo. of year experiments  Range {%)  Average  1930-4-0 194-3 194-7-48 1940-45  0.29-6.75 2.7 -3-7 0.6-3-5  1.71 2.10' 3.20 1.93  6 , 1 2 6~  Authority Er itchard (1948b) Neave (unpub.) Hunter " U.S.Fish & Wildlife Servicex  •^he writer is indebted to M.G. Hanavan, Acting Chief, Alaska Fishery Investigations, for these figures which are of great interest for comparative purposes. So far, it has not proved feasible to separate natural mortality from fishing mortality in the case of individual streams.  The reasons are  mainly connected with (a) the difficulty of obtaining a complete, or reasonably complete, recovery of marked fish from a fishery which is prosecuted to a large extent with purse-seines and which delivers catches at widely separated points (b) uncertainty as to the relative effects of natural and fishing mortality on marked and unmarked fish (c) the heavy differential mortality which is believed to follow the marking of fish which must be operated upon when only 30 - 40 mm. long. It is possible, nevertheless, to reach certain general conclusions as to the order of magnitude of the two types of mortality to which fish are subjected in the ocean. The average survival represented by the final escapements (Table IV) is about 2$.  Evidently the survival from natural ocean mortality must be  sufficiently greater than 2% to account for a fairly intensive fishery.  At  the same time we might expect that the average survival would be considerably  - 15 -  less than the 9*9$ which Foerster (1936) estimated as the ocean survival f o r sockeye.  Although the l a t t e r species commonly spends a year longer i n s a l t  water than the pink salmon, t h i s factor i n the opinion of the writer, would by no means offset the v u l n e r a b i l i t y associated with the very small size at which pink salmon reach the sea. The most obvious source of information on the point at issue i s a comparison between the catches and escapements of c e r t a i n areas.  For such  comparisons to be v a l i d , the f i s h caught must be proceeding to the streams with the spawning populations of which they are being compared.  In many  areas there i s no certainty whatever that the f i s h caught are related to the spawning streams of the same area.  This i s p a r t i c u l a r l y true of the f i s h  which enter the s t r a i t s between Vancouver Island and the mainland which, as tagging experiments  show, may be proceeding to any of a wide v a r i e t y of l o c a l -  i t i e s from Queen Charlotte S t r a i t to Puget Sound.  There i s , however, a large  region of the c e n t r a l part of the B r i t i s h Columbia coastline, corresponding with the Department of F i s h e r i e s S t a t i s t i c a l Areas 6, 7 and 8, which can reasonably be considered as a self-contained unit i n respect to catches and escapements.  laggings carried out under the d i r e c t i o n of the writer at the  south and north ends of t h i s region i n 194-7 and 1948 showed l i t t l e migration to other areas (only about 3$ of the tags recovered came from "outside" d i s t r i c t s (unpublished data)).  A comparison i s accordingly presented (Table  V and F i g . 3) between catches and escapements reported f o r t h i s region by . o f f i c e r s of the Department of F i s h e r i e s .  "Catches" were reported i n hundred-  weights and have been converted to approximate equivalent numbers of i n d i v i d uals.  The escapements quoted have been arrived at by summing the estimates  of spawning populations made by inspectors on about 180 i n d i v i d u a l streams and r i v e r s (I am indebted to Mr. J.G. Hunter f o r t h i s compilation).  Estimates  have usually been recorded by observers by means of l e t t e r s i n d i c a t i n g that  - 16 the population was within certain limits, e.g. "H" = between 5000 and 10,000 fish.  In order to obtain numerical totals the mid-points of the indicated  ranges were assigned to the estimates. Table V. Reported catch and escapement of pink salmon in the central region of British Columbia Year  Catch  Escapement  Total run  % caught  1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949  2,250,000 1,682,000 3,502,000 1,841,000 1,619,000 2,800,000 663,000 1,073,000 1,175,000 5,250,000 2,510,000 6,900,000 1,685,000 2,076,000 2,834,000 3,233,000  2,046,675 1,205,875 2,577,500 909,050 1,512,425 970,800 622,875 916,250 1,045,600 1*805,150 856,400 2,517,275 724,775 662,275 985,100 1,720,350  4,296,675 2,887,875 6,079,500 2,750,050 3,131,425 3,770,800 1,285,875 1,989,250 2,220,600 7,055,150 3,366,400 9,417,275 2,409,775 2,738,275 2,835,100 4,296,675  52 58 58 67 52 74 52 54 53 74 74 73 70 76 74 65  The difficulty of estimating accurately the salmon population of a stream on the basis of, at most, a few visits during the spawning season is well-known and fully admitted.  The totals, however, represent a very large  number of individual estimates by various observers.  In general, the catch  seems to constitute a rather constant percentage of the run, whether the latter be large or small, although the degree of exploitation appears to have been maintained at a higher level since about 1943*  Accepting this evidence  of a rather constant relationship between catch and escapement , we may defer acceptance of actual figures until another clue has been considered.  x  Such a relationship might well be expected, in view of the efficiency of the fishing fleet and the measures adopted by the Department of Fisheries to ensure the escape of a proportion of the run.  - 17 The taggings to which reference has been made yielded total returns from the fishery of about 30$.  In both instances tagging was carried out over  a period of several weeks which, however, did not include the earliest stages of the fishing season.  The recoveries of tags applied during the latter part  of the fishing period will certainly tend to be below the general fishing mortality rate (returns from the first week of tagging were 40$ and 33$ respectively from the two operations). low.  Even these figures are undoubtedly too  Non-return of recaptured tags is believed to be a far from unimportant  source of loss and, in addition, even the earlier fish tagged had been available to the fishery for various lengths of time and represented a population which had undergone some depletion. On the other hand, the fishing mortality deduced from the comparison of catch and escapement is probably somewhat high. Not all salmon streams are inspected in each year.  A fraction of the escapement is thus unrecorded. The  evaluation of runs in streams on which counting weirs are operated leads to the conclusion that stream patrol observations usually underestimate the populations which actually enter. With these limitations in mind, the writer believes that the average fishing mortality in this region in recent years can be placed with some assurance at between 50$ and 70$ of the adult run: able working figure.  60$ would seem to be a reason-  If we accept 2$ as representing an average survival  after the operation of the fishery (p./4), this would imply a survival from the natural hazards of ocean life of 5$, i.e. a natural ocean mortality of 95$X.  x  The acceptance of a 95$ natural ocean mortality and a 60$ fishing intensity would require a freshwater survival of about 6$ for maintenance of the population level (see p.3^) in this region. This is in good agreement with the average recorded survival (7.5± 2.9) of Hooknose Creek, which is situ-r ated in this district. The fish of the Central region appear to be at least maintaining their abundance (Fig. .  - 18 Hbw far these conclusions can be applied to other districts is uncertain.  The size and mobility of the present day salmon fleet is such that  a heavy fishery can quickly be brought to bear on a run of fish appearing in any section of the coastline.  Pressure of fishing gear thus tends to be dis-  tributed according to the numbers of fish available anywhere. Certain runs, however, are undoubtedly vulnerable for a longer period than others, due to their long migrations through confined straits and channels.  Tag returns  of 40$ were obtained from taggings carried out at Sooke and in the Alert Bay area in 1943 and 1945 (Pritchard and DeLacy, 1944.; DeLacy and Neave, 1947). These figures imply a somewhat higher fishing mortality among salmon which travel to "inside" waters via Juan de Fuca and Johnstone Straits than among the fish of the central region. While 5$ is considered to be an approximation to the average natural ocean survival, there is no good evidence as to the range of variation.  In  only one of the recorded cases (Table IV) does the return to a stream exceed this figure (the 6.75$ observed in one year at McClinton Creek), indicating that natural survival must have reached this level even i f fishing mortality was n i l . The lowest recorded return (0.29$) would imply a natural survival of less than 5$ unless the fishery took 94$ of the adult fish.  This seems  unlikely, although it is obvious that the average fishing mortality for a large district cannot be expected to hold good when applied to the run of a particular stream in a particular year.  Further speculation on the limits of  variation of natural ocean mortality would seem to be unwarranted until more data are available. CHANGES IN ABUNDANCE OF ADULT PINK SALMON Fig. 3 , to which reference has already been made, may be cited as an example of changes in abundance when a large area is considered.  Since  the pink salmon has a constant life span of two years the "even" and "odd"  - 19 -  year runs represent different stocks and can be treated separately in considering changes in population levels.  Fig. 4 shows the same data segregated  into the two components. Figures for the number of pink salmon caught annually in the traps situated near Sooke may be presumed to reflect with considerable accuracy the relative magnitude^-of the runs entering Juan de Fuca Strait, since this gear is nearly constant in quantity and position from year to year and takes its t o l l at an early stage in the passage of the fish through the fishery as a whole. Since the difference in size between the "even" and "odd" year catches is so great as to make graphical representation inconvenient, the figures are tabulated (Table VI). Table VI. Numbers of pink salmon caught annually in traps at Sooke, B. C.  Year  Number  1938  1472  1944 1946  605 110  Average  474  1940 1942  1948  328 116  214  Year  Number  1939  169,018  1945 1947  221,871 168,284  1941 1943  1949  Average  55,503 33,699  109,304  126,280  The evidence (discussed previously) that the annual catch made by other types of gear is closely related to the size of the run, makes it legitimate to use catch figures alone as indices of relative abundance in certain areas. Fig. 5 shows annual catches reported for the Skeena River area over a lengthy period. From these examples, which apply to large areas and do not necessarily reflect the course of events in individual streams, the following inferences can be drawn:  - 20 -  (1) Changes in abundance between one generation and the next do not usually exceed a two- or threefold increase or decrease (unless the numbers are very small, as in the "off" years of some areas).  This is in spite of the fact  that the rigid life span of the pink salmon precludes any buffering effect which might be introduced i f fish matured at different ages. (2) Occasionally a larger "jump" or "crash" takes place, as is exemplified by the big difference between the 1930 and 1932 catches of the Skeena area. Actually this occurrence took place over a much larger area than that to which the graph refers, being a conspicuous feature of catch records for northern and central British Columbia.x A similar failure in the recurrence of the big "odd" year run to the central region appears to have occurred in 1919* The sudden upswing of this stock in 1943 and the sharp recession in 194-7 (Fig. 4) are further examples of these discontinuous changes in abundance. An upward jump of similar relative magnitude is shown by the Sooke trap catches in 1945 (Table VI). (3) After one of these major changes the population is apt to remain near the new level for at least a generation and sometimes for a prolonged period. (4) The "even" and "odd" year stocks fluctuate independently, neither small nor large changes in one cycle being necessarily reflected by similar tendencies in the other cycle. The average level of abundance of the two cycles may be quite similar, as in the central region from 1934 to 1949 and in the Skeena area from 1931 to 1949, or the two stocks may exist for long periods on a totally different scale, as demonstrated by the runs at Sooke, where the odd-year fish exceed the even-year fish in a ratio of more than 250 to 1*  x  The possibility that the low catches of 1932 merely reflected economic conditions is considered by Hoar (1951). The continued low level of the catches provided by this stock in the face of the high market demand of the last 15 years, together with the existence of a reasonable alternative explanation, seems to the writer to constitute convincing evidence that such a "crash" actually occurred. Moreover, chum salmon production, which should have been at least equally affected by economic conditions, showed a markedfrlncrease in this area in 1932 (see fig. 8).  - 21 -  THE  MECHANISLi COHfROLLIITG POPULATION LEVELS  I n s e e k i n g t o understand  t h e above-mentioned f e a t u r e s of p o p u l a t i o n  l e v e l s and p o p u l a t i o n changes i t i s n e c e s s a r y - t o w i l l permit hand we  the occurrence  conceive  a mechanism which  of seemingly c o n t r a d i c t o r y r e s u l t s .  are c o n f r o n t e d w i t h t h e  fact  cuat the p i n k salmon has  stood a heavy f i s h i n g m o r t a l i t y f o r many y e a r s .  On the  s u c c e s s f u l l y with-  S i n c e t h i s f i s h e r y , i f i t had  been merely added t o e x i s t i n g m o r t a l i t y , would have wiped out the importance of the s p e c i e s i n a v e r y few  one  generations,  commercial  i t i s e v i d e n t t h a t the r e -  d u c t i o n i n the number of a d u l t f i s h has been accompanied by an i n c r e a s e i n the r e p r o d u c t i v e e f f i c i e n c y of the escapements. i s e q u a l l y p l a i n t h a t two at very d i f f e r e n t  s t o c k s can e x i s t  On the o t h e r hand, the f o r prolonged  evidence  p e r i o d s i n the same area  d e n s i t i e s , w i t h no tendency t o r e a c h a common l e v e l .  more, when a l a r g e s t o c k s u f f e r s a major " c r a s h " i t may  Further-  remain f o r many genera-  t i o n s near t h e nevj l e v e l w i t h no apparent tendency t o r e g a i n i t s former s t a t u s u n l e s s or u n t i l some e q u a l l y sudden "jump" t a k e s  numerical  place.  I n c o n s i d e r i n g t h i s problem i t i s n e c e s s a r y t o d i s t i n g u i s h between three "types  51  of m o r t a l i t y , namely:  (1) M o r t a l i t y which becomes r e l a t i v e l y h e a v i e r as p o p u l a t i o n s i n c r e a s e i n d e n s i t y (and v i c e v e r s a ) , t h e r e b y t e n d i n g t o s t a b l i l i z e the p r e v a i l i n g  population  level. (2) M o r t a l i t y which becomes r e l a t i v e l y h e a v i e r as p o p u l a t i o n s decrease s i t y , thereby t e n d i n g t o exaggerate f l u c t u a t i o n s i n i t i a t e d by  other  i n den-  causes.  (3) M o r t a l i t y which i s independent of p o p u l a t i o n d e n s i t y , t h a t i s , t a k i n g a g i v e n percentage o f t h e p o p u l a t i o n whatever the d e n s i t y of t h e  l a t t e r may  M o r t a l i t y of the f i r s t t y p e has l o n g been r e c o g n i z e d as b e i n g e c o l o g i c a l b a s i s on which t h e maintenance of i n t e n s i v e long-term pends.  One  of i t s e f f e c t s , t h a t i s , the tendency f o r a reduced  be.  the  fisheries population  deto  show an i n c r e a s e d r e p r o d u c t i v e e f f i c i e n c y , has been termed " r e s i l i e n c e " by some  - 22 -  authors (Thompson, 1944,  1950;  P r i t c h a r d , 1948a).  f i n d t h i s term p a r t i c u l a r l y a t t r a c t i v e .  The p r e s e n t w r i t e r does not  I t appears t o emphasize o n l y h a l f of  the process,namely, the decreased m o r t a l i t y which takes p l a s e at low l e v e l s o f population.  The  i n c r e a s e d m o r t a l i t y which a t t e n d s h i g h p o p u l a t i o n l e v e l s i s  of course e q u a l l y c h a r a c t e r i s t i c .  I n o t h e r words the r e l a t i o n s h i p i s , or can  be, continuous over a v e r y wide range o f p o p u l a t i o n d e n s i t y .  I t i s not an i n n a t e  power t o respond t o a c o n d i t i o n o f d e p r e s s i o n but merely an e x p r e s s i o n o f the f a c t t h a t changes i n p o p u l a t i o n d e n s i t y n e c e s s a r i l y e n t a i l changes i n the r a t e s ensuing from c e r t a i n causes.  death-  The term "density-dependent m o r t a l i t y "  has  been used i n other f i e l d s , p a r t i c u l a r l y i n e n t o m o l o g i c a l s t u d i e s (see Solomon, 1949).  T h i s a c c u r a t e l y d e s i g n a t e s the r e l a t i o n s h i p but seems cumbersome p a r t i -  c u l a r l y when i t becomes n e c e s s a r y t o r e c o g n i z e " i n v e r s e l y m o r t a l i t y " , that  density-dependent  i s , the type o f m o r t a l i t y r e f e r r e d t o i n c a t e g o r y (2) above.  The present w r i t e r suggests "compensatory m o r t a l i t y " as a convenient term f o r the r e l a t i o n s h i p now  under d i s c u s s i o n .  T h i s k i n d of m o r t a l i t y appears t o toe h i g h l y c h a r a c t e r i s t i c o f the p e r i o d from spawning t o the emergence o f the. f r y .  I t might be supposed  that  p h y s i c a l f a c t o r s such as f l o o d , drouth and s i l t i n g would t e n d t o have the same effect  on a p o p u l a t i o n whether the l a t t e r be l a r g e o r s m a l l .  the case t o an important degree way  alone.  (see p. 2.7)  T h i s i s indeed  but these f a c t o r s do not a c t i n t h i s  In p r a c t i c e t h e spawning s i t e s a v a i l a b l e i n the l i m i t e d  of a stream appear t o v a r y c o n s i d e r a b l y i n f a v o u r a b i l i t y .  environment  "Jith i n c r e a s i n g  numbers of spawners a l a r g e r precenta :;e o f the f i s h are compelled t o make t h e i r ;  redds i n m a r g i n a l o r u n f a v o u r a b l e p o s i t i o n s .  T h i s c o n d i t i o n , coupled w i t h t h e  i n c r e a s e d " s u p e r i m p o s i t i o n " and perhaps the mutual i n t e r f e r e n c e between duals o f a dense p o p u l a t i o n , t e n d t o produce  a lower average  indivi-  e f f i c i e n c y of  reproduction. Over the whole p e r i o d of f r e s h w a t e r e x i s t e n c e P r i t c h a r d  (1948) at  - 23 -  M c C l i n t o n Greek observed a g e n e r a l i n v e r s e r e l a t i o n s h i p between number of a d u l t s and percentage output o f f r y m i g r a n t s .  C e r t a i n anomalies were apparent,  however, e s p e c i a l l y at t h e h i g h e s t p o p u l a t i o n l e v e l encountered.  These are :  d i s c u s s e d 'below. The  importance of m o r t a l i t i e s which become r e l a t i v e l y g r e a t e r w i t h  d i m i n i s h i n g d e n s i t y o f p o p u l a t i o n (and v i c e v e r s a ) seems not t o have been a p p r e c i a t e d by f i s h e r y b i o l o g i s t s .  Q u i t e e v i d e n t l y , however, t h e r e a r e i n -  f l u e n c e s which f r e q u e n t l y prevent the e f f e c t i v e  o p e r a t i o n o f compensatory  mor-  t a l i t y , s i n c e otherwise the l a r g e and small p o p u l a t i o n s of an area c o u l d not i n d e f i n i t e l y preserve t h e i r r e s p e c t i v e numerical p o s i t i o n s . constant tendency t o converge.  Such  i n f luencesybannot be merely n e u t r a l ;  must a c t i v e l y oppose the compensatory is  beyond  dispute.  A c t u a l l y , as we  There x«jould be a  p r o c e s s , s i n c e the e x i s t e n c e of the  they latter  have seen, p i n k salmon p o p u l a t i o n s can be  r e l a t i v e l y s t a b i l i z e d f o r c o n s i d e r a b l e p e r i o d s at a wide v a r i e t y o f l e v e l s . The anti-compensatory i n f l u e n c e s m u s t . a c c o r d i n g l y a c t over a wide range of densities.  The  term "depensatory m o r t a l i t y " i s proposed  which i s i n v e r s e l y r e l a t e d t o p o p u l a t i o n d e n s i t y . f a c t o r s which work i n t h i s way,  The  f o r t h e type of 'mortality i d e n t i f i c a t i o n o f the  and t h e p e r i o d i n the l i f e  o p e r a t e , are o b v i o u s l y matters o f v i t a l  h i s t o r y when t h e y  importance t o OVLY study o f p o p u l a t i o n  changes. A s p e c i f i c example o f depensatory m o r t a l i t y appears t o be p r o v i d e d by the  observations o f Davidson and Hutchinson (1943).  i n January 1942  These a u t h o r s r e p o r t e d t h a t  a w h o l l y e x c e p t i o n a l f l o o d caused extremely heavy l o s s e s among  p i n k salmon eggs and a l e v i n s i n the lower reaches of S a s h i n Creek.  Surviving  f r y migrants came mainly from the upper r e a c h e s , an a r e a which u s u a l l y little  seeding, being r e l a t i v e l y rocky.  receives  Owing t o t h e p o p u l a t i o n p r e s s u r e p r o -  duced by an e x c e p t i o n a l l y l a r g e run of a d u l t s i n the f a l l  of 1941 an u n u s u a l l y  l a r g e number of a d u l t f i s h had u t i l i z e d t h i s upstream a r e a .  .  .  - 24 -  Such o c c u r r e n c e s are p r o b a b l y r e l a t i v e l y i n f r e q u e n t and cannot  p r o v i d e t h e constant counterbalance  seems t o be  certainly  t o the compensatory p r o c e s s which  demanded by the observed maintenance of d i v e r s e p o p u l a t i o n l e v e l s .  JJ'isuing undoubtedly  c o u l d .be p r o s e c u t e d i n a depensatory  mannery.  The t a k i n g  of a f i x e d q u a n t i t y each y e a r would mean t h a t s m a l l runs would be h e a v i l y exp l o i t e d and t h a t l a r g e runs would be r e l a t i v e l y l i g h t l y f i s h e d . have seen the f i s h e r y does not  seem t o operate  i n t h i s way  But as  we  at the present  time,  s i n c e c l o s u r e s are a p p l i e d w i t h a view t o p e r m i t t i n g the escapement of a r e l a t i v e l y c o n s t a n t p r o p o r t i o n o f the  run.  A c t u a l l y , the o n l y form of depensatory account  f o r t h e observed  migration.  f a c t s appears  m o r t a l i t y which i s adequate t o  t o be a s s o c i a t e d w i t h the p e r i o d of f r y  T h i s p e r i o d , a l t h o u g h short i n d u r a t i o n , i s a p e c u l i a r and  phase i n the l i f e - c y c l e .  critical  I t r e p r e s e n t s the p r e c i p i t a t i o n of a l a r g e and v e r y  n e r a b l e p o p u l a t i o n i n t o the community of s t r e a m - d w e l l i n g  organisms.  The  tran-  s i e n c e of the s i t u a t i o n p r e c l u d e s the e s t a b l i s h m e n t o f any long-term balance ween p r e d a t o r s and prey. are/sustained without p a r t of the  The m i g r a t i o n i s merely  any s i g n i f i c a n t  vul-  bet-  an exodus i n which heavy l o s s e s  compensatory growth or development on.the  survivors.  In h i s study of the h'cClinton Creek f r y run of 1941,  Cameron (1941)  t e n t a t i v e l y e s t i m a t e d a 60$ o v e r a l l m o r t a l i t y among f r y p a s s i n g t o . t h e ocean from a spawning a r e a which extended  f o r a d i s t a n c e of two  m i l e s from s a l t water ( o r  a 5«j$ m o r t a l i t y based on the p o t e n t i a l egg d e p o s i t i o n ) .  The m o r t a l i t y , however,  v a r i e d w i t h the d i s t a n c e which the f i s h had t o t r a v e r s e and a l s o i n c r e a s e d d u r i n g the week by week p r o g r e s s of the run.  He p o i n t e d out the probable  advantages t o  the f r y of an e a r l y , compact m i g r a t i o n i n r e d u c i n g m o r t a l i t y d u r i n g t h i s p e r i o d . ^ General observations i n d i c a t e that the predators mainly  concerned  f r y m o r t a l i t y are o t h e r f i s h , r e s i d e n t i n the same stream o r themselves  with  migrating  7  seaward at the time.  In the l o c a l i t i e s i n v e s t i g a t e d t r o u t , coho.-, smolts,  and  HL I t i s p o s s i b l e , a l t h o u g h u n v e r i f i e d , t h a t e a r l i e r entrance i n t o the sea might b r i n y c o r r e s p o n d i n g disadvantages i f ohe s e a s o n a l i n c r e a s e o f marine zooplankton were l e s s advanced. - F . l l .  - 25 -  s c u l p i n s are important.  The s m a l l s i z e and n o c t u r n a l h a b i t s o f t h e f r y - m i g r a n t s  appear t o prevent major i n r o a d s by t e r r e s t r i a l p r e d a t o r s . l a r g e l y determined  Losses a r e t h e r e f o r e  by the number and a c t i v i t y o f the l o c a l a q u a t i c p r e d a t o r s .  Even i n y e a r s when p r o d u c t i o n i s r e l a t i v e l y low, f r y - m i g r a n t s w i l l  ordinarily  c o n s t i t u t e a l a r g e and r e a d i l y a v a i l a b l e f o o d s u p p l y d u r i n g t h e s h o r t p e r i o d o f t h e i r downstream m i g r a t i o n ,  lience t h e p r e d a t o r s w i l l tend t o take a f i x e d num-  b e r r a t h e r t h a n a percentage  and t h e m o r t a l i t y w i l l be o f t h e depensatory  The  type.  t o t a l number o f p r e d a t o r s p r e s e n t from year t o y e a r w i l l presumably vary  less  than t h e number o f pink salmon f r y , s i n c e s e v e r a l s p e c i e s are i n v o l v e d and v a r i o u s age-groups are r e p r e s e n t e d . The  data p r o v i d e d by P r i t c h a r d and by Cameron f o r H e C l i n t o n Creek may  be re-examined in' the l i g h t lity  of t h i s viewpoint.  r e p r e s e n t s 7,590,000 f r y l o s t  i n 1941.  Cameron's e s t i m a t e d 60% morta-  Assuming p r e d a t i o n t o be constant  from year t o year, t h i s f i g u r e may oe added t o each y e a r ' s r e c o r d e d f r y - m i g r a n t output t o o b t a i n a t h e o r e t i c a l f i g u r e f o r t h e number of f r y e s c a p i n g from the gravel.  A comparison between F r i t c h a r d ' s f i g u r e s f o r migrant  putput  and t h e s e  h y p o t h e t i c a l f i g u r e s f o r f r y emergence i s p r e s e n t e d i n Table V I I .  Table V I I . E f f e c t  P o t e n t i a l egg deposition 8,500,000 13,200,000 26,600,000 50,800,000 53,200,000. 139,500,000  o f p r e d a t i o n on t h e f r y - m i g r a n t p r o d u c t i o n o f p i n k salmon at M c C l i n t o n Creek.  Fry *fo Migrants e f f i c i e n c y 2,020,000 2,300,000 5,060,000 5,384,000 3,675,000 12,600,000  23.8 17.4 19.0 10.6 6.9 9.0  Hypothetical f r y emergents (9,610,000) 9,890,000 12,650,000 12,974,000 11,265,000 20,190,000  jo efficiency (no) 75 48 25 21 14  $ predation (86.2) 57.6 29.0 14.4 14.1 5.0  - 26 -  As might be  a n t i c i p a t e d , the c a l c u l a t e d v a l u e s f o r emergence produce  an obvious a b s u r d i t y at the lowest  l e v e l o f p o p u l a t i o n , when the p r e d a t o r s  h a r d l y be expected t o o b t a i n the maximum number of f r y which t h e y are of devouring.  capable  I n s p i t e o f the crudeness of the procedure, however, the a d j u s t -  ment produces a b e t t e r r e l a t i o n s h i p ( i n v e r s e ) between the s i z e of the p o p u l a t i o n and the percent i n i t s proper  e f f i c i e n c y of reproduction,  sequence i n the  percentage output  series.  spawning  every v a l u e now  appearing  I n p a r t i c u l a r , the anomaly of a  from a d e p o s i t i o n of 139,500,000 eggs t h a n from one  and t h e s i m i l a r d i s c r e p a n c y  between the  13,000,000 l e v e l s a r e e l i m i n a t e d .  The  dation with i n c r e a s i n g f r y production Table  could  higher of 53,200pC0  e f f i c i e n c y at the 27,000,000 and r e d u c t i o n i n the e f f e c t i v e n e s s of p r e -  is  well  i l l u s t r a t e d i n the l a s t  column o f  VII. I t may  t h e r e f o r e be  i n f e r r e d t h a t compensatory m o r t a l i t y p r e v a i l e d  over t h e whole s i z e range o f t h e  spawning p o p u l a t i o n s  Creek up to the t i m e of emergence o f the f r y .  observed at  McClinton  Subsequent p r e d a t i o n ,  on  the  other hand, i n t r o d u c e d d e p e n s a t o r y e f f e c t s which i n some i n s t a n c e s caused a t r a n s p o s i t i o n of the f i n a l Recognition  results.  of these  d i f f e r e n t kinds of m o r t a l i t y permits  e x p l a n a t i o n of the problems posed on a p r e v i o u s page. constant, be  With o t h e r  a  conditions  a r e d u c t i o n i n the number of spawners (by f i s h i n g o r o t h e r  f o l l o w e d by i n c r e a s e d s u r v i v a l up t o the free-swimming stage.  number of f r y produced may  not be  very d i f f e r e n t  from the p r e v i o u s  reasonable  cause) w i l l  The  absolute  average ( c f .  the r e l a t i v e u n i f o r m i t y of " h y p o t h e t i c a l f r y emergents" from d e p o s i t i o n s o f d i f f e r e n t magnitude) and moderate l i m i t s .  f l u c t u a t i o n s of t h e p o p u l a t i o n w i l l remain w i t h i n  If,: on the other hand, the number o f emergent f r y i s d r a s t i -  c a l l y reduced (as could happen through u n u s u a l l y u n f a v o u r a b l e  p h y s i c a l condi-  t i o n s or through the r e d u c t i o n of spawners beloxv t h e l e v e l w h i c h c o u l d produce a l a r g e f r y emergence even w i t h the decreased  mortality) the predators  will  - 27 -  take a l a r g e r r e l a t i v e t o l l and t h e p o p u l a t i o n w i l l be d e p r e s s e d t o a level.  At the o t h e r extreme, a p a r t i c u l a r l y abundant fry. emergence  "break t h r o u g h " the c o n t r o l imposed by p r e d a t o r s t o r e a c h a l e v e l at widen p r e d a t i o n takes  and  enable t h e  new  may  population  o n l y a s m a l l percentage and c o n t r o l  i s e f f e c t e d mainly by compensatory m o r t a l i t y . The the  depensatory p a r t of t h e mechanism t e n d s t o r e s i s t  former l e v e l .  populations and t h a t new  a return to  T h i s would account f o r the apparent f a c t t h a t pink  can become s t a b i l i z e d l e v e l s are  gradual t r a n s i t i o n s .  f o r l o n g p e r i o d s at v e r y d i f f e r e n t  levels  sometimes reached by sudden " s t e p s " r a t h e r than  The  term " l e v e l " i s used of course  by  i n a comparative  s i n s e , to denote a p e r i o d of r e l . t i v e l y minor f l u c t u a t i o n s . presumably accentuated  salmon  These, t o o ,  are  by p r e d a t i o n but not t o an e x t e n t which p r e c l u d e s  recti-  f i c a t i o n by o r d i n a r y c l i m a t i c v a r i a t i o n s or o t h e r f a c t o r s . It changes and,  i s e v i d e n t l y the r o l e of depensatory m o r t a l i t y t o exaggerate by opposing compensatory m o r t a l i t y , t o s t a b i l i z e t h e s e  when t h e y become g r ^ a t .  S i n c e , however, n e i t h e r compensatory nor  i n f l u e n c e s can i n i t i a t e changes we type  changes  depensatory  must r e c o g n i z e the e x i s t e n c e o f a  of m o r t a l i t y w h i c h i s independent of p o p u l a t i o n d e n s i t y .  w i t h the other terms suggested h e r e i n , t h i s may  be c a l l e d  In  third  conformity  "extrapensatory  mortality". As a l r e a d y p o i n t e d out, environment may  a c t to some extent  c e r t a i n p h y s i c a l f a c t o r s i n the  freshwater  i n a compensatory manner, t h a t i s , t h e y  c o n s t i t u t e an immediate cause o f death, although t h e i r e f f e c t i v e n e s s i s determined by the d e n s i t y of the p o p u l a t i o n . t h a t n u m e r i c a l l y s i m i l a r spawning' r u n s can of r e p r o d u c t i o n .  Favourable  temperature can o p e r a t e  There can be no doubt, however,  show great v a r i a t i o n i n e f f i c i e n c y  and unfavourable  c o n d i t i o n s of s t r e a m f l o w  at a l l l e v e l s o f p o p u l a t i o n d e n s i t y .  The  e f f e c t s of stream flow are d i s c u s s e d by Neave and TJickett (1949).  and  general In t h i s  - 28  p u b l i c a t i o n VJickett has  -  a l s o shown a s i g n i f i c a n t  degree, of c o r r e l a t i o n betvmen.  the J u l y - August p r e c i p i t a t i o n and the number of a d u l t p i n k salmon r e t u r n i n g i n the next g e n e r a t i o n  i n the  c e n t r a l r e g i o n of B r i t i s h  F i s h i n g m o r t a l i t y (as a p p l i e d at the present extrapensatory  Columbia.  time) seems t o be  i n t h a t i t o p e r a t e s w i t h f a i r l y equal i n t e n s i t y on l a r g e  s m a l l runs - at l e a s t i n the area  and  f o r which comparisons have been made (p. 16 ) ,  S i n c e , however, i t appears t o take an a p p r o x i m a t e l y constant y e a r , i t merely conforms t o p o p u l a t i o n v i o u s l y and  mainly  percentage every  s i z e s which have been determined  pre-  cannot be regarded as a potent source of year to year f l u c t u a t i o n s .  A s u f f i c i e n t l y i n t e n s i v e f i s h e r y c o u l d undoubtedly produce a d e c l i n i n g t r e n d , or a drop t o a new  l e v e l , by d r a s t i c a l l y r e d u c i n g  the number of spawning  On the other hand, n a t u r a l ocean c o n d i t i o n s s i m i l a r t o the  could act i n a manner  f r e s h w a t e r c o n d i t i o n s mentioned above, - f o r example, tempera-  t u r e and the q u a n t i t y  or a v a i l a b i l i t y of food  which are u n r e l a t e d t o the point  s u p p l i e s c o u l d produce v a r i a t i o n s  s i z e of the p o p u l a t i o n .  Further  reference  to  this  i s made i n the f o l l o w i n g s e c t i o n . DE-  RELATIVE EFFECTS OF FRESHWATER, NATURAL OCEAN AMD FISHING l.OF.TALITY IN TERMINING- POPULATION LEVELS Although a large majority  of t h e t o t a l l o s s e s a t t e n d i n g  c y c l e t a k e s p l a c e i n f r e s h water, i t does not fluctuations i n population  A c t u a l l y , s i n c e the  s i z e are determined i n t h i s medium.  that  Variation in connection.  i n d i v i d u a l s (or p o t e n t i a l i n d i v i d u a l s ) e n t e r i n g e i t h e r  that a small  concluding  the  change i n m o r t a l i t y r a t e i n e i t h e r i n s t a n c e  exert a profound e f f e c t on the end Pritchard  life-  important i n t h i s  vironment commonly s u f f e r a m o r t a l i t y exceeding 90$ b e f o r e i s evident  the  f o l l o w from t h i s f a c t a l o n g  m o r t a l i t y , r a t h e r t h a n - i t s t o t a l magnitude, w i l l be  it  fish.  enphase,  would  result.  (1950) found t h a t at LIcGlinton Greek the t o t a l ocean s u r -  v i v a l showed g r e a t e r  r e l a t i v e v a r i a t i o n ( t h a t i s , r a t i o between highest  and  - 29 -  lowest r e c o r d e d v a l u e s ) t h a n t h e f r e s h w a t e r s u r v i v a l 23.8$ t o 6.9%, Ehere  (Freshwater  survival,  r a t i o 3.5:1. Ocean s u r v i v a l , 6 . 7 5 % t o 0.29%, r a t i o  23:1).  are reasons, however, f o r b e l i e v i n g t h a t t h i s i s not a r e p r e s e n t a t i v e  comparison  between t h e v a r i a b i l i t y o f f r e s h w a t e r and n a t u r a l ocean s u r v i v a l .  Corresponding f i g u r e s f o r o t h e r l o c a l i t i e s  S a s h i n Creek Hooknose Creek The  % freshwater survival 6.4 - 0.2 15.0 - 0.87  combination  are:  freshwater ratio 32:3l • 17:1.  % ocean survival 3.S7 - 0.74 2.7 - 3.7  ocean ratio 5.2:1 1.4:1  o f M c C l i n t o n Creek d a t a w i t h t h e s e o t h e r r e c o r d s  g i v e s a t o t a l f r e s h w a t e r r a t i o of 119:1, without e x t e n d i n g t h e M c C l i n t o n Creek range f o r ocean v a r i a b i l i t y . The s m a l l degree  o f v a r i a t i o n i n t h e f r e s h w a t e r s u r v i v a l at  M c C l i n t o n Creek i s e v i d e n t l y due t o t h e absence o f the low v a l u e s which have been found elsewhere.  The r e l a t i v e s t a b i l i t y of t h e stream c o n d i t i o n s  at M c C l i n t o n Creek d u r i n g t h e p e r i o d of t h e i n v e s t i g a t i o n s i s a t t e s t e d the good r e l a t i o n s h i p  ( a l r e a d y d i s c u s s e d ) between the s i z e of the  p o p u l a t i o n and t h e e f f i c i e n c y o f f r y p r o d u c t i o n .  E v i d e n t l y no  e x t r a p e n s a t o r y f a c t o r s i n t e r v e n e d t o upset t h i s r e l a t i o n s h i p . circumstances i t i s quite l i k e l y that determined  be n o t e d t h a t the f i g u r e s g i v e n are the net r e s u l t mortality.  Although  spawning  drastic Under such  changes i n p o p u l a t i o n s i z e  i n l a r g e measure by n a t u r a l ocean c o n d i t i o n s .  by  canbe  However, i t s h o u l d  of n a t u r a l and  fishing  we have found r e a s o n t o b e l i e v e t h a t f i s h i n g m o r t a l i t y  t a k e s a r e l a t i v e l y c o n s t a n t percentage  from y e a r t o year when a l a r g e  area  i s c o n s i d e r e d , such c o n s t a n c y cannot be expected t o p r e v a i l f o r t h e r u n t o an i n d i v i d u a l s m a l l stream.  Pritchard himself considered that the  w i t h which t h e M c C l i n t o n Creek run was  f i s h e d v a r i e d a gooddaal.  intensity The  low .  ocean s u r v i v a l observed i n most y e a r s at M c C l i n t o n Creek',: c o n t r a s t e d with a v e r y h i g h s u r v i v a l i n one y e a r , suggests the p o s s i b i l i t y t h a t t h i s r u n i s u s u a l l y f i s h e d h e a v i l y but can on occasion!escape major e x p l o i t a t i o n .  - 30 -  As r e g a r d s n a t u r a l m o r t a l i t y , t h e p r e s e n t w r i t e r t h i n k s t h a t  major  importance i n d e t e r m i n i n g changes i n p o p u l a t i o n s i z e can u s u a l l y be a s s i g n e d t o f r e s h w a t e r f a c t o r s , i n view of t h e f o l l o w i n g  considerations:  (1) On g e n e r a l grounds, p h y s i c a l c o n d i t i o n s such as water volume, temperature and a v a i l a b l e space are more v a r i a b l e i n f r e s h water than i n t h e ocean. Moreover, the eggs or a l e v i n s , u n l i k e t h e ocean f i s h , are unable t o seek optimum c o n d i t i o n s but must e x p e r i e n c e the f u l l  impact of e n v i r o n m e n t a l  changes. (2) A c c o r d i n g t o t h e view expressed on p r e v i o u s pages, v a r i a t i o n s i n f r y p r o d u c t i o n imposed by such changes a r e r e i n f o r c e d by the depensatory e f f e c t of p r e d a t i o n on the o u t g o i n g f r y . (3) The apparent independence of even and odd y e a r s t o c k s i n r e s p e c t t o f l u c t u a t i o n s suggest t h a t t h e s e o r i g i n a t e i n f r e s h water, which i s t h e o n l y phase i n which these two p o p u l a t i o n s are c o m p l e t e l y (4) A s i g n i f i c a n t  separated.  c o r r e l a t i o n has been found between a m e t e o r o l o g i c a l f a c t o r  a f f e c t i n g spawning c o n d i t i o n s and v a r i a t i o n s i n t h e s i z e s of r e s u l t i n g  runs.  (5) The output o f f r y - m i g r a n t s from f r e s h water i s at t i m e s so low t h a t a r e d u c t i o n i n t h e a d u l t p o p u l a t i o n must n e c e s s a r i l y r e s u l t u n l e s s ocean s u r v i v a l i s r a i s e d t o a l e v e l which seems unreasonable i n the l i g h t  of e x i s t i n g  infor-  mation (see F i g . 6 ) . (6) At each of t h e l o c a l i t i e s f o r which s e v e r a l y e a r s ' data are a v a i l a b l e ( M c C l i n t o n Creek, S a s h i n . C r e e k , Hooknose Creek) t h e r e appears t o be a r e l a t i o n between the e f f i c i e n c y of f r e s h w a t e r r e p r o d u c t i o n and t h e d i r e c t i o n gfi s i z e change  i n t h e r e s u l t i n g a d u l t escapement.  The most s t r i k i n g i n s t a n c e has been  the change from a l a r g e r odd-year r u n t o a l a r g e r even-year run, observed a t Hooknose Creek.  This r e v e r s a l i n the r e l a t i v e  c l e a r l y i n i t i a t e d i n f r e s h water.  s i z e of t h e two s t o c k s  was  - 31 -  I n o n l y one  important r e s p e c t does ocean l i f e  seem t o o f f e r  opportun-  i t y f o r g r e a t e r v a r i a b i l i t y i n n a t u r a l m o r t a l i t y than f r e s h w a t e r e x i s t e n c e . T h i s i s i n the a v a i l a b i l i t y of food s u p p l i e s , ^ . p r o b l e m which i s o f s m a l l importance  i n f r e s h water.  The a p p r e c i a b l e d i f f e r e n c e s i n average  of i n d i v i d u a l p i n k salmon i n d i f f e r e n t years feeding conditions.  (see Hoar, 1951)  may  size  reflect  I t i s reasonable t o suppose t h a t v a r i a t i o n s i n m o r t a l i t y  can a l s o be due t o t h i s cause. the ocean may  relatively  I n c i d e n t a l l y , the growth of t h e females i n  a l s o a f f e c t the number o f r i p e eggs produced, t h e r e b y i n t r o d u c i n g  a f a c t o r i n t o the p r o d u c t i o n of t h e next g e n e r a t i o n . appears t o be s m a l l i n r e l a t i o n t o observed  T h i s f a c t o r , however,  changes i n p o p u l a t i o n s i z e  (see p. 3  I t w i l l , moreover, tend t o be o f f s e t by the compensatory m o r t a l i t y a t t e n d i n g i n c u b a t i o n and a l e v i n a g e . The r o l e of marine c o n d i t i o n s i n d e t e r m i n i n g p o p u l a t i o n s i z e be d i s m i s s e d as i n s i g n i f i c a n t , a l t h o u g h the w r i t e r b e l i e v e s t h a t t h e i r are u s u a l l y much s m a l l e r than those  cannot effects  of f r e s h w a t e r f a c t o r s .  A q u e s t i o n of some' i n t e r e s t and importance  i s whether or not ocean  m o r t a l i t y tends t o be compensatory, - t h a t i s , whether i t tends t o o f f s e t the n u m e r i c a l v a r i a t i o n s i n the f r y p o p u l a t i o n s which e n t e r t h e sea from y e a r t o year.  Any  such tendency would of course l i m i t the e f f e c t i v e n e s s of remed-  i a l measures based  on i n c r e a s i n g the output o f sea-going m i g r a n t s .  I f the  reasons a l r e a d y advanced f o r c o n s i d e r i n g f r e s h w a t e r f a c t o r s t o be l a r g e l y p o n s i b l e f o r d e t e r m i n i n g f l u c t u a t i o n s are v a l i d , t h e n compensatory  influences  i n t h e sea, i f t h e y e x i s t , must be s m a l l , s i n c e on average t h e y do not the e f f e c t s a t t r i b u t e d t o f r e s h water.  Davidson  res-  obscure  and Vaughan (1941) have, how-  ever, h y p o t h e t i z e d t h e e x i s t e n c e of i n t r a s p e c i f i c c o m p e t i t i o n i n the sea as a f a c t o r a f f e c t i n g the average  s i z e of i n d i v i d u a l p i n k salmon of the C l a r e n c e  S t r a i t a r e a of s o u t h e a s t e r n A l a s k a .  T h i s view was  based  on an i n v e r s e r e l a t i o n -  s h i p between s i z e of p o p u l a t i o n s and s i z e o f i n d i v i d u a l s , o f the a d u l t  runs.  It  can be maintained  on g e n e r a l grounds t h a t p i n k salmon, however  numerous, c o n s t i t u t e but a s m a l l element i n the cqmplex economy of t h e It  cannot  be supposed t h a t t h e r e i s a s p e c i a l l i m i t e d and r e l a t i v e l y  food s u p p l y e x p l o i t e d o n l y by p i n k salmon.  sea.•  constant  I f o t h e r s p e c i e s are i n v o l v e d  (e.g. h e r r i n g , chum,salmon and t h e v a r i o u s o t h e r v e r t e b r a t e s and i n v e r t e b r a t e s which f e e d on the same k i n d s of zooplankton) be determined merely  the degree .of c o m p e t i t i o n should  by the t o t a l p o p u l a t i o n of t h i s a s s o c i a t i o n of s p e c i e s , not  by the f l u c t u a t i n g abundance o f p i n k salmon.  be d i s t r i b u t e d .  The  e f f e c t s would a l s o  I n o t h e r words, the r a t h e r moderate changes i n p i n k salmon  abundance r e p o r t e d by Davidson  and Vaughan would have t o a l t e r the  relation  between the a v a i l a b l e , f o o d s u p p l y and a l l the animals u s i n g i t b e f o r e e f f e c t s on pink salmon became apparent.  O r d i n a r i l y , the chances t h a t such an.  i n t e r a c t i o n would be d e t e c t a b l e i n the f a c e of annual b i l i t y wrought by such f a c t o r s as water temperature, p r e v a i l i n g wind, would see/a t o be may  small.  o c c u r i n a r e a s where the output  t e n d t o remain v d t h i n r e s t r i c t e d  specific  changes i n food  availa-  amount of s u n l i g h t  or  Perhaps a degree of such c o m p e t i t i o n  of young p i n k s i s very h i g h and the  ocean a r e a s .  On  the B r i t i s h  fish  Columbia  c o a s t Hoar ( l . c ) found no i n v e r s e a s s o c i a t i o n between body s i z e and p o p u l a t i o n size.  He noted, however, t h a t changes i n body s i z e are i n the same d i r e c t i o n  over the whole B r i t i s h Columbia coast and t h a t odd-year f i s h tend t o be l a r g e r t h a n eveh-year f i s h , i n s p i t e o f the f a c t t h a t i n some d i s t r i c t s are f a r more abundant.  S i n c e the even- and odd-year stocks are  they  effectively  s e g r e g a t e d as r e g a r d s i n t e r b r e e d i n g , the p o s s i b i l i t y of a g e n e t i c f a c t o r  may  be e n t e r t a i n e d . The  '  i d e a of compensatory s i z e adjustment  compensatory m o r t a l i t y .  The  i s more o r l e s s opposed t o  occurrence of the l a t t e r would, of course, tend  t o e l i m i n a t e the d i f f e r e n c e s ' i n abundance w i t h which Davidson a s s o c i a t e d l a r g e and s m a l l f i s h .  and Taughan  As mentioned, the evidence at present  F i g . 6 . R e l a t i o n between f r e s h w a t e r s u r v i v a l o f pink salmon and e i z e o f spawning stock a t v a r i o u s l e v e l s o f f i s h i n g i n t e n s i t y . An average n a t u r a l ocean s u r v i v a l o f 5% i s assumed. F.M. • f i s h i n g m o r t a l i t y .  -33-  available  i n d i c a t e s t h a t on the B r i t i s h Columbia coast n e i t h e r the m o r t a l i t y  of a y e a r - c l a s s nor the a p p r e c i a b l e degree by  i n d i v i d u a l s i z e of i t s members i s determined t o  i t s own  ment a g a i n s t the g e n e r a l  abundance.  occurrence  Perhaps the most c o n v i n c i n g  and  odd-year s t o c k s which has  c e r t a i n areas.  d i s p a r i t y i n the numbers of even-  A mechanism f o r e s t a b l i s h i n g and m a i n t a i n i n g  this  and  dependently and  reduce  and  ocean m o r t a l i t y v a r y i n -  t h a t f i s h i n g m o r t a l i t y tends t o remove a f a i r l y c o n s t a n t  centage, c e r t a i n e f f e c t s of v a r y i n g these t i o n s presented  represent  elements can be noted.  'The  shows the e f f e c t freshwater  on the  spawning s t o c k of v a r i o u s r a t e s of f i s h i n g  s u r v i v a l , on the b a s i s of an average egg-production  eggs per female and a n a t u r a l ocean s u r v i v a l of 5$, which has to represent  r a t i o i s assumed. p o p u l a t i o n constant  an approximate average f o r t h i s phase.'  The  per-  calcula-  averages which might be expected t o p r e v a i l over a  p e r i o d l o n g enough t o e l i m i n a t e the e f f e c t s of annual f l u c t u a t i o n s .  the  maintained,  ones. By a c c e p t i n g the view t h a t freshwater  (p. 17)  of  disparity  Such a c o n d i t i o n c o u l d not be  however, i f ocean m o r t a l i t y tended t o f a v o u r s m a l l p o p u l a t i o n s  and  sea i s  a l r e a d y been mentioned as c h a r a c t e r i s t i c  has been suggested on p r e v i o u s pages.  large  argu-  of compensatory m o r t a l i t y i n the  t o be found i n t h e l a r g e and l o n g - c o n t i n u e d  an  percentage freshwater  been  Fig. 6 intensity  of  1700  considered  A 50-50 sex  s u r v i v a l r e q u i r e d t o keep the  at v a r i o u s l e v e l s of f i s h i n g i n t e n s i t y i s i n d i c a t e d by  " l e v e l of maintenance". It  can be  seen t h a t under the p o s t u l a t e d c o n d i t i o n s the average f r e s h -  water s u r v i v a l of an u n f i s h e d p o p u l a t i o n must be q u i t e low p o p u l a t i o n i s not t o keep on i n c r e a s i n g . r e q u i r e an i n c r e a s e of 67$ freshwater  The  ( i . e . t o about 4$)  s u r v i v a l of approximately  6$  a d d i t i o n of a 40$ i n freshwater  ( i . e . about 150$  s u r v i v a l i n d i c a t e d f o r an u n f i s h e d p o p u l a t i o n )  (2.35$) i f the f i s h e r y would  survival.  A  g r e a t e r t h a n the  would permit  a  fishing  - 34  m o r t a l i t y of 60%. s u r v i v a l and  T h i s i s a p p r o x i m a t e l y the  (p.  central region, ).  (both of  increase  h i g h e r l e v e l s of e x p l o i t a t i o n .  i n f i s h i n g i n t e n s i t y from 40% t o 60%  e f f i c i e n c y o f f r y output.  demands an i n c r e a s e  water, s u r v i v a l (14.45%) i s accompanied by a low be unnecessary t o r e v i s e our working f i g u r e of f i s h i n g m o r t a l i t y amounted to. 80% of l i n e w i t h the  c o u l d perhaps be  efficiency  Whereas a  of 33%  i n fresh-  improvement i n the  Although compensation on t h i s s c a l e seems s t a r t l i n g  i t .has p o s s i b l y been accomplished at M c C l i n t o n Creek.  out  taken  i n reproductive  water s u r v i v a l , a r i s e from 40% t o 80% would demand a 200%  be  freshwater  i f Hooknose Creek f i g u r e s can be  F i g . 6 shows the g r e a t  which i s r e q u i r e d t o o f f s e t s t i l l rise  condition  f i s h i n g i n t e n s i t y ) which i s i n d i c a t e d by the d a t a at present  a v a i l a b l e f o r the as a guide  -  or 85%.  The  r e t u r n of a d u l t s and  Such a f i s h i n g i n t e n s i t y would not  r e a d i l y a c h i e v e d where l a r g e p o p u l a t i o n s I t may  ocean s u r v i v a l v a r i e s i n d i f f e r e n t a r e a s . Creek f i s h i s l e s s f a v o u r a b l e  i t would  5% n a t u r a l ocean s u r v i v a l i f  known e x p l o i t a t i o n of F r a s e r R i v e r  a r e s t r i c t e d area such as Masset I n l e t .  h i g h average f r e s h -  w e l l be,  sockeye runs  and  are f u n n e l l e d i n t o • of course, t h a t  I f the marine h a b i t a t  than t h a t o f c e n t r a l r e g i o n  fish,  average  of M c C l i n t o n i t i s not  n e c e s s a r y t o assume such a h i g h f i s h i n g m o r t a l i t y . O b v i o u s l y , however, the w i l l depend upon the  a b i l i t y of a p o p u l a t i o n  extent t o which compensation i s p o s s i b l e .  e x p l o i t a t i o n a premium w i l l be p l a c e d favourable  external  t i n u a l increase density.  The  conditions that,  With  increasing  on s t o c k s which reproduce under such i n the  i s l a r g e l y p r e v e n t e d by  absence of a f i s h e r y , t h e i r con-  factors associated  with t h e i r  p r e v a l e n c e of compensatory m o r t a l i t y over a wide r a n j e  l a t i o n l e v e l s has a high  to sustain a f i s h e r y  been i n d i c a t e d at M c C l i n t o n Creek and  under such  own of popu-  conditions  r a t e of e x p l o i t a t i o n i s p o s s i b l e . I t may  be c o n f i d e n t l y expected, however, t h a t  i n many streams  e f f i c i e n c y of f r y output i s determined t o a much g r e a t e r pensatory or depensatory f a c t o r s .  The  e x t e n t by  p h y s i c a l c o n d i t i o n s may  be  the  extraso  rigorous  - 35 -  or so variable that differences in population size do not produce corresponding changes in reproductive efficiency.  Under some conditions a small popu-  lation may be nearly as vulnerable to flood or drouth as a large population, in addition to suffering,  relatively heavier losses through predation. The  imposition of even a moderate fishery on such a stock will produce a decline, whatever the total initial size of the population may be. At the risk of repeating an obvious proposition, it may be emphasized that the effect of a fishery is to reduce the mortality which is directlyrelated to population density.  As we have seen, much of the mortality which  attends the life cycle is not of this nature and will be unaffected or even actually increased by a fishery.  We have also expressed our belief that the  compensatory potentialities are wholly or mainly located in that part of the life cycle which occurs between spawning and the emergence of the fry from the gravel.  On this view, the average maximum fishery which a stock can sus-  tain (in the absence of artificial control) is determined by the degree of adjustability of the mortality during this relatively short period, — which also happens to be a period when another type of mortality (extrapensatory) may produce important effects. The left-hand side of the following tabulation gives a generalized representation of the manner in which we might expect mortality to be distributed in the progeny of a pair of fish belonging to a stable, unfished population.  Parents Eggs Emerging fry Fry entering sea Mature fish Escapement  Unfished  60% fishery  2 1700 100 4-0 2 2  2 1700 250 100 5 2  To provide a stable population at the 60$ fishing level the distri bution of mortality must be altered in some such manner as is suggested in  - 36 -  the right-hand column. This scheme applies only to the offspring of individual pairs of fish and leaves out of account the total size of the populations If compensatory influences are so strong that after subtraction of 60$ of the adult population the remaining 4-0$ can produce the same total number of fry (that is, the required increase in reproductive efficiency per pair of fish can be realized immediately), then the average size of adult runs should not decline, in spite of the reduced escapement.  If, on the other hand, the re-  duced escapement cannot make good the losses caused by fishing there will be a tendency for the population to fall until a level is reached at which the individual pairs of fish can attain the required reproductive efficiency. Probably some adjustment along these general lines can frequently be made. However, a marked lowering of the total fry population, as we have seen, is attended by the danger that predation will become relatively more intensive, so that increased percentage survival up to the time of emergence is offset by reduced survival of migrating fry.  There seems to be no inherent reason  why a crash should not be prolonged into a further decline (that is, a stabilized level might not be reached in one generation) but under the mechanism envisaged the initial drop is likely to be sudden and severe. building up of a population might continue after a sharp rise.  Similarly the Some evidence  of this can perhaps be seen in the Skeena and Central Region graphs. The further increases could, however, merely represent seasonal deviations from an established "level". CONSERVATION PROBLEMS In spite of the indicated possibility of continued declines, the writer sees no convincing evidence of gradual long-term decreases in the existing populations, when large areas are considered. Reference has already  x  Suppressive effects of this kind might of course be less pronounced i f certain predators (e.g. coho, trout) were themselves declining at the same time.  - 37 -  been made to the occurrence of jumps or crashes, followed by the establishment of a new level.  A "trend-line" covering a period in which such a "step"  has occurred will slope upward, or downward but such a line may give a false impression of gradual change.  As indicated previously, the current "decline"  of the Skeena and Nass areas appears in the main to represent a change in level which appeared in the odd-year stock in 1927 and in the even-year stock in 1932.  There is some indication that a further drop in the odd-year stock  may have taken place in the 1945-47 cycle (Fig. 5)« The general association between fluctuations in abundance and physical conditions in fresh water has been indicated (p. 2.7).  There is evi-  dence that crashes are the result of the same factors operating with exceptional severity, although it is not always possible to be dogmatic about the specific causes of events which happened many years ago.  Very low water-  levels were faced by the parents of the 1932 fish when entering streams in northern and central British Columbia in 1930.  The August rainfall in these  districts was about 76$ below the mean (Dept. Marine & Fish. 1932).  In 194-5  there was an August rainfall deficiency of similar magnitude, with September precipitation also well below normal. The abrupt drop in population size from 194-5 to 194.7 affected not only the Skeena fish, which were already at a relatively low level, but also the Central region fish, which were at a peak of abundance in 1945 (Fig. 4 ) . In 1925, climatic conditions in the Skeena area at spawning time do not appear to have been unfavourable but an extremely heavy precipitation which occurred in the lower Skeena valley in February 1926, accompanied by unseasonably high temperatures, may well have produced a destructive flood.  The failure of the pink salmon run of the Central region  in 1919 followed wholly exceptional floods in that region in the autumn of 1917. Sudden failures can thus be logically attributed primarily to physical conditions experienced in fresh water.  There remains, however, the  - 38 possibility that intensive fishing may keep populations down to a level at which they are more susceptible to natural disasters.  It may also tend to  prevent a low population from reaching the numerical strength necessary to "break through" and achieve a higher general status. While it is beyond the scope of this paper to deal with specific cases and treatments, certain general conclusions or opinions are implicit in the foregoing discussions which might influence the approach to conservational problems and can be expressed briefly as follows: Freshwater and natural ocean survival are largely independent of each other, with freshwater survival showing the greater range of variation. Therefore improved output of migrants from fresh water will on average be reflected in larger runs of adult fish. Under intermediate conditions of fishing intensity a total freshwater survival of 5% or less is likely to produce a reduction in size of the spawning population.  A freshwater survival of 2% or less will on an average  reduce the population, even i f no fishing takes place. Large runs in both even and odd years are not fundamentally incompatible.  (The Koeye River may be cited as an example of a stream which has  heavy runs of adults in both cycles.) The general objective of remedial measures should be to change natural extrapensatory mortality into fishing mortality, while keeping total mortality (on a percentage basis) constant.  In effect, this means the pro-  motion of the most favourable physical conditions for spawning, incubation add alevinage.  In the absence of lethal factors which ate density-independent,  compensation can exert its full effect in making good the losses inflicted by the fishery.  At the same time, remedial measures should aim at producing a  large total emergence of fry (not merely a high efficiency of individual reproduction) since a large fry population will suffer relatively less predation  - 39 than a small population.  A stock should be large enough to produce a maximal  emergence even at the risk of some loss of individual efficiency. From the point of view of conserving pink salmon alone, it may be better to ameliorate physical conditions merely at the places and times at which incubation takes place, rather than to effect general and permanent stream improvements which may have the effect of also raising the stocks of predators. Predation is believed to be a major factor in keeping stocks at a low level.  The elimination of predators (or the protection of fry. from their  attacks) should greatly increase the contribution to the fishery of stocks which are in this condition and should tend to prevent further drops. The effect on stocks which are at a high level of abundance would be much less pronounced.  It may be argued, however, that certain predators (e.g. coho,  trout) are more valuable than pink salmon. The persistence of a low level of abundance (in the absence of a marked change in the condition or accessibility of spawning grounds) means that the total fry emergence is not large enough to escape excessive depensatory mortality, in spite of the increased reproductive efficiency of individual fish.  The spawning stock should therefore be increased to the greatest  possible extent, even i f no current "decline" is evident.  In thus attempting  to establish a higher level, a brief total closure of the fishery would seem to be sounder conservational practice than the institution of a reduced fishing rate over a longer period. While even a total escapement of the adult run might not suffice to make the "jump", it would offer a chance of success, especially i f it happened to coincide with favourable climatic conditions. A successful result would enable fishing to be continued at the same percentage level but on a larger stock.  - 40 -  The maximum sustained fishery which can be imposed on the existing runs of different streams varies widely (not improbably from B0% to zero), hence an escapement which is adequate for a particular area should not be presumed to be equally applicable elsewhere. In general, light runs should be l i t t l e fished, in view of the danger that the cycle will be pushed to a persisting low level. The operation of the present protective measures, which tend to promote equal pressure on large and small runs, is however definitely preferable to a fixed annual quota or to an attempt to pack the same quantity of fish each year.  Such an  attempt would defeat its own ends by reinforcing the depensatory processes which tend to exaggerate large and small runs. In attempting to establish a run in a new locality or in a cycle which is not represented by existing populations, a satisfactory result would be unlikely unless (a) predators are not numerous or (b) the number of eggs or fry introduced is great enough to be equivalent to the product of a large natural escapement.  <  •  41  CHUM SALMON (Oncorhynchus keta (Walb.)) The chum salmon is as widely distributed as the preceding species in the coastal waters of British Columbia and is common in certain streams, notably in the Vancouver Island district, from which the pink salmon is virtually absent.  In some years i t provides a larger catch, by weight, than any other  species of Pacific salmon. In certain glacier-fed streams of central and northern British Columbia in iiihich a good flow of relatively cold water is maintained during the summer, some chums spawn as early as August 1st.  Usually, however, the main  runs enter fresh water somewhat later than the pink salmon, the most intensive spawning taking place between the end of September and mid-November. In some streams even later runs also occur, with fish s t i l l arriving in late December and January.  In small coastal streams spawning usually takes place within a  few days after entering fresh water.  Although an occasional chum salmon reaches  the upper Babine River, the species shows less inclination or ability than pink salmon to surmount obstacles and travel for long distances up streams. Much spawning takes place very close to salt water.  In conformity with the spawning  habits the downstream migration of fry may extend over a somewhat longer period than that of pink salmon and the peak of this run is frequently reached about the middle of May. The habits of the young fish in salt water, so far as these have been observed, are similar to those of pink salmon, the two species being frequently found in company. The length of the life cycle varies.  While a majority of mature fish  are usually four years of age, 3-year old and 5-year old fish are commonly represented.  In commercial catches third-year fish occasionally constitute  nearly or quite 50% of local catches. Since the problems and factors relating to the population sizes and changes of chum salmon are in many respects similar to those which have been  42  d i s c u s s e d i n connection w i t h p i n k salmon, form, o f a c o m p a r i s o n w i t h t h e l a t t e r  they w i l l  be p r e s e n t e d h e r e i n  the  species.  EGG-PRODUCTION Egg counts f o r as f o l l o w s by t h e localities  chum s a l m o n i n B r i t i s h C o l u m b i a h a v e b e e n  investigators  i n Table  w h o s e names a r e  No. of Years  Namu Fraser Paver N i l e Creek Hooknose Creek  The g e n e r a l  N o . of Fish  1 1 3 4  of a l l l o c a l i t i e s  implies a different  of pink salmon.  -  2671  As already  t h e abundance o f t h e two s p e c i e s w o u l d d i v e r g e  The n u m b e r s o f  each  C r e e k and H o o k n o s e C r e e k a r e  pink  recorded  escapements t o  Nile  VIII. from equality  are  s m a l l and  it  a 5 0 - 5 0 r a t i o r e p r e s e n t s an a v e r a g e c o n d i t i o n , a s  In  salmon.  SURVIVAL  Total freshwater  :  I N F R E S H WATER  survival  Quantitative freshwater  Other-  very r a p i d l y .  i n spawning  the deviations  MORTALITY MORTALITY AND  cycle.  ( p . 2.)  RATIO  g i v e n i n Table  be s e e n t h a t  seems f a i r t o a s s u m e t h a t the case of  sex  higher  p o i n t e d out  t o t a l mortality rate during the l i f e  SEX  1.  2760 2943 2726 2254  a v e r a g e o f r o u g h l y 2700 r e p r e s e n t s about 63$  numerical production than that  It will  same  Average N o . of Eggs  21 51 47 114  Average  wise,  associated with the  I.  Locality  this  reported  records  f o r the e f f i c i e n c y of reproduction during  phase i n B r i t i s h Columbia are  1945 and Hooknose C r e e k s i n c e  1947.  the recorded output of f r y migrants  available  The e s t i m a t e d at  only from N i l e Creek p o t e n t i a l egg  t h e s e two l o c a l i t i e s  are  the since  deposition  and  given i n Table  IX.  43  Table  VIII.  Locality Nile  Creek n tl w tl tt tt tt Hooknose Creek n tt tt tt tt n  11  Sex r a t i o  Years  No. of Fish  1945 1946 1947 1948 1949 1947 1948 1949 1950  3,062 1,861 986 386 933 10,106 1,014 705 2,362  T a b l e IX.  Locality Nile «  Creek M  tt tt tt tt tt n Hooknose Creek it tt tt n « tt  ( p e r c e n t ) o f chum s a l m o n e s c a p e m e n t s  Males  Females  49 51 52 54 48 48 51 51 50  51 49 48 46 52 52 49 49 50  Authority Neave (1947) Wickett (unpub.) tt tt « n n tt H u n t e r (1948) " (1949) H (unpub.) tt tt  F r e s h w a t e r s u r v i v a l o f chum s a l m o n  Broodyear  Potential Deposition  Fry Migrants  1945 1946 1947 1948 1949 1947 1948 1949 1950  3,540,000 2,115,000 1,276,000 385,000 1,022,000 10,977,000 1,055,000 714,000 2,859,000  138,388 8,319 4,808 23,188 782 108,746 77,497 44,463 431,349  Survival  '*  3.90 0.40 0.38 6.03 0.08 0.99 7.37 6.22 15.09  Authority Neave (1947) Wi c k e t t ( u n p u b . ) tt n it n it tt H u n t e r (1948) (1949) "! (unpub.) it tt  44  ,  The e f f e c t  of the recorded outputs,  o f f r y m i g r a n t s p r o d u c e d by a p a i r o f of  2700 e g g s , w o u l d be as  N i l e Creek Hooknose Creek  spawning f i s h w i t h a p o t e n t i a l  N o o f migrants produced ( a ) Maximum .„ ( b ) M i n i m u m  93.97 - 99.92 84.90-99.01  163 410  W h i l e t h e r a t i o between h i g h e s t Hooknose Creek i s a p p r o x i m a t e l y the stream  (see  p . 8 )» much g r e a t e r  At both l o c a l i t i e s , however, i t size 2.  can be i n i t i a t e d i n f r e s h Immediate.causes of ' A l l the  2.2 27.0  and'lowest  same- a s t h a t  of p i n k salmon i n the  Creek,  obvious that, g r e a t v a r i a t i o n s i n p o p u l a t i o n  water.  regarded  causes o f f r e s h w a t e r l o s s e s l i s t e d f o r p i n k salmon  species.  The v a r i a b i l i t y and i n t e r r e l a t i o n s h i p of t h e s e f a c t o r s  been emphasized but t h e r e  as p o t e n t i a l sources o f m o r t a l i t y f o r  a p p e a r t o be  certain quantitative  on t h e two s p e c i e s a l t h o u g h t h e s e d i f f e r e n c e s  the  present  have  already  differences cannot be  in measured  data.  A t t h e t i m e w h e n chum s a l m o n e n t e r f r e s h w a t e r t h e r u n - o f f streams has u s u a l l y increased  p r e v a i l i n August and September. stage of the l i f e  same  evident at N i l e  can.be  coastal  at  death  specific  a c c u r a t e l y from e x i s t i n g  ,  75:1 15:1  recorded e f f i c i e n c y  relative variation is is  Ratio of (a) t o (b)  (pp.  their effects  deposition  follows:  Observed mort a l i t y range  Stream  i n t e r m s o f t h e a v e r a g e number  cycle are  in  considerably from the low flows  Hence c e r t a i n forms o f m o r t a l i t y a t  l i k e l y t o be  which this  smaller than for pink salmon.  The  l o s s e s w h i c h may t h u s b e i n v a r y i n g d e g r e e s a v o i d e d m i g h t b e m o r e d i r e c t l y due to the impassability of f a l l s ; lack of water;  unfavourably high  On t h e characterizes  p r e d a t i o n ; e x c l u s i o n f r o m spawning a r e a s by temperature.  other hand, the l e s s  compact  spawning r u n which f r e q u e n t l y  chum s a l m o n i n c r e a s e s t h e d a n g e r o f  a r r i v a l s , which are f r e q u e n t l y observed  to select  s u p e r i m p o s i t i o n by t h e the  same s i t e s a s  .  later  earlier  45 fish.  Moreover, despite  much l a r g e r  size  o f the f i s h and t h e  seed a l i m i t e d a r e a as thus appear at  t h e l a r g e r number o f e g g s ,  space r e q u i r e d f o r r e d d s ,  e f f i c i e n t l y as  pink salmon.  a lower population l e v e l ,  remain i n the lower portions of  chum s a l m o n , o w i n g t o probably  chums a r e  t o be a s s o c i a t e d w i t h t h e stream bed.  Serious  may  apt  to  spawning streams i n s t e a d o f d i s t r i b u t i n g themwater-system.  Many o f t h e s t r e a m s u t i l i z e d b y chum s a l m o n show e x t r e m e  The v e r y l o w p e r c e n t a g e of  cannot  Overcrowding e f f e c t s  p a r t i c u l a r l y since  selves throughout the a c c e s s i b l e p o r t i o n s of the  i r r e g u l a r changes i n r u n - o f f .  the  L o s s e s due  t o e r o s i o n can t h e r e f o r e  seasonal be  severe.  f r y - m i g r a n t s i n c e r t a i n y e a r s at N i l e Creek incidence of floods  l o s s e s are also  caused a t  i n a s s o c i a t i o n w i t h an  appears  unstable  times i n Vancouver I s l a n d  streams  b y t h e d r y i n g up o f s p a w n i n g b e d s i n s p r i n g , t h u s p r e v e n t i n g t h e e m e r g e n c e fry,  o r by the t r a p p i n g o f the l a t t e r  (Neave 3.  i n pools before  or  they can reach the  of sea  1949).  General  conclusions  I n Hooknose G r e e k , where s i z a b l e  runs of both species occur,  percentage e f f i c i e n c y o f f r y - m i g r a n t p r o d u c t i o n has  been almost  p i n k s a n d chums i n e a c h r e c o r d e d y e a r  (Tables  two s p e c i e s u t i l i z e a p p r o x i m a t e l y t h e  same a r e a s a n d t h e  not d i f f e r greatly population.  i n time.  i n abundance  barriers against the effects  i n t h e same g e o g r a p h i c a l  spawning p e r i o d s s t a t u s of a  do single  areas, w i t h no p h y s i c a l  of c o m p e t i t i o n , i n d i c a t e s t h a t ecological niches.  s i t u a t i o n a t H o o k n o s e C r e e k may r e p r e s e n t  i n g e n e r a l p i n k and  In fact,  i n a g i v e n stream o r p o r t i o n of a r i v e r  d i t i o n b e t w e e n two such n i c h e s o r  the  t h a t b o t h s p e c i e s have c o n t i n u e d t o m a i n -  chum s a l m o n o c c u p y somewhat d i f f e r e n t u s u a l l y predominates  identical for  I n t h i s stream  E c o l o g i c a l l y they approach the  However, t h e mere f a c t  t a i n themselves  III,'IX).  the  one o r  s y s t e m and  other  the  an i n t e r m e d i a t e o r o v e r l a p p i n g  con-  habitats.  V e r y f r e q u e n t l y chum s a l m o n u t i l i z e  streams which are  i n r e s p e c t t o w a t e r f l o w and permanency o f g r a v e l  quite  unstable  beds, or i n which large  stones  46  I replace  finer deposits.  Factors  w h i c h must g i v e t h i s  t i n g these rigorous  A r a t h e r extreme  example  its  size  physical conditions are:  eggs at  effects  of the f i s h ,  (1)  of a "bad y e a ^ s "  recurrent  the greater  (3)  egg-production,  f o r the greater success  s u p e r i m p o s i t i o n would favour the later-spawning  spawning grounds  can be o f f e r e d .  (1)  chum.  As already  as advantageously  the balance  pointed out,  i n r e l a t i o n to the  and p r o b a b l y cannot seed u t i l i z e d ground as  l e s s concentrated f r y migration. (see  This probably results  p . SI ) w h i c h c a n be e x p e c t e d  In general t h e two s p e c i e s ,  support  presumed t o have  s t r e a m s may b e  cited.  potential  thoroughly. a later  waters.  sufficiently well  to  i n the  adaptations  Following deforestation, pink  d e c l i n e d t o a v e r y l o w l e v e l i n s u c h s t r e a m s a s Amor d e  m a i n t a i n t h e i r numbers  and  salmon.  different  A t t h e same t i m e  (2)  i n heavier losses  c a u s e d more e x t r e m e v a r i a t i o n i n s t r e a m f l o w ,  Oyster R i v e r and N i l e Creek.  the  chum s a l m o n  c e r t a i n p o p u l a t i o n changes which appear t o have t a k e n  i n some V a n c o u v e r I s l a n d  populations  o f t h e s e v i e w s on t h e  of  of m o r t a l i t y  t o be e s p e c i a l l y numerous  r e l a t i v e l y s t a b l e streams which are p r e f e r r e d by p i n k  the  The f o l l o w i n g  T h e l a t e r a n d more p r o l o n g e d s p a v i n i n g s e a s o n o f t h e chum p r o d u c e s  these  single  of  of  physical habitats  I n t h i s c o n n e c t i o n i t may a l s o b e s u p p o s e d t h a t  do n o t d i s t r i b u t e t h e m s e l v e s  Creek,  the  cycle.  s u g g e s t i o n s , however,  is  deposit  reproduction from being e n t i r e l y l o c a l i z e d i n a  s p e c i e s t o o u s t p i n k s a l m o n f r o m t h e more f a v o u r a b l e  predators  the  which, p r e v e n t s  chums u n d e r a d v e r s e p h y s i c a l c o n d i t i o n s , d o n o t e x p l a i n t h e f a i l u r e  due t o  (2)  s t o n e s and a l s o t o  the l e s s r i g i d l i f e span,  These advantages, which c o u l d account  latter.  offset-  on a percentage b a s i s ,  e n a b l i n g i t t o move l a r g e r  a deeper l e v e l ,  Creek.  s p e c i e s an advantage o v e r p i n k salmon i n  e n a b l i n g t h e chum t o w i t h s t a n d g r e a t e r l o s s e s , larger  i s p r o v i d e d by N i l e  of  place which salmonj.  Cosmos  chum s a l m o n w e r e a b l e  t o become t h e d o m i n a n t s p e c i e s  in  to  47  It salmon i s that  is  concluded t h a t ,  somewhat l e s s ,  i t a l s o tends  o n a v e r a g e , p r o d u c t i o n o f f r y - m i g r a n t s by chum  on a percentage b a s i s ,  t o be more  t h a n t h a t o f p i n k salmon and  variable.  MORTALITY AND S U R V I V A L I N THE OCEAN Because o f t h e l o n g e r l i f e span of  chum s a l m o n ,  d a t a have not  been accumulated w h i c h would f u r n i s h a g u i d e t o t h e p e r c e n t a g e o f from a g i v e n stream w h i c h e v e n t u a l l y r e t u r n to f r e s h water. accept  outgoing  spawning a d u l t  times p i n k salmon s u r v i v a l .  If  On t h e b a s i s o f e g g - p r o d u c t i o n ,  (in stable populations)  the higher m o r t a l i t y of  e n t i r e l y i n f r e s h w a t e r , t h e n a t u r a l ocean species.  If the difference  s a l t water phase,  = 3.15$.  to entertain.  water s u r v i v a l i s  We h a v e a l r e a d y  The p o s s i b i l i t y t h a t  chum other  expressed the v i e w t h a t average  lower than for p i n k salmon.  first  exposed  to ocean hazards.  place  the s u r v i v a l  During a considerable  summer, w h e n t h e g r e a t e s t m a r i n e m o r t a l i t y i s  similar It  part  presumed t o  of  their  o c c u r , the two to  magnitude. is therefore  suggested that  average n a t u r a l ocean s u r v i v a l f o r  chum s a l m o n i n B r i t i s h C o l u m b i a i s n o t l e s s t h a n Chum s a l m o n f i s h i n g i s p r o s e c u t e d fishing.  fresh-  which'  s p e c i e s h a v e b e e n o b s e r v e d f e e d i n g i n company a n d l o s s e s may b e p r e s u m e d be o f  is  Marine s u r v i v a l would not.be  expected t o be h i g h e r t h a n f o r p i n k s , i n v i e w of t h e l o n g e r p e r i o d f o r chum i s  total  same i n b o t h  s a l m o n s u r v i v a l i s much l o w e r i n o n e p h a s e a n d somewhat h i g h e r i n t h e difficult  1  17/27  t h e chum t a k e s  s u r v i v a l s h o u l d be t h e  l i e s wholly i n the  d u r i n g t h i s p e r i o d s h o u l d be 5 X 1 7 / 2 7  s h o u l d be  •  pink  suggest r a t h e r narrow l i m i t s w i t h i n which  t h e a v e r a g e chum s u r v i v a l s h o u l d l i e . s u r v i v a l from egg t o  fry  H o w e v e r , i f we  5$ as b e i n g a r e a s o n a b l e f i g u r e f o r t h e n a t u r a l ocean s u r v i v a l o f  salmon, a simple c a l c u l a t i o n w i l l  the  yet  4$.  b y t h e same m e t h o d s a s p i n k s a l m o n  The d i v i s i o n b e t w e e n c a t c h e s and e s c a p e m e n t s i n t h e C e n t r a l r e g i o n ,  b a s e d on i n f o r m a t i o n o f t h e k i n d d i s c u s s e d  o n p a g e if , i s  shown i n T a b l e X .  48 Table X .  R e p o r t e d c a t c h a n d e s c a p e m e n t o f chum s a l m o n i n t h e c e n t r a l r e g i o n of B r i t i s h C o l u m b i a  Year  'Catch  1934 1935 1936 193? 1938 1939 1940 1941 194E 1943 1944 1945 1946 1947 1948 1949  i 589,560 1,029,640 668,040 899,450 993,850 598,990 568,090 836,700 591,370 833,600 587,010 1,035,460 1,801,410 2,243,010 . 1,809,160 1,098,590  Escapement  Total run  fo c a u g h t  558,375 957,200 1,515,850 828,725 962,950 414,775 640,875 962,975 544,800 654,250 464,650 1,092,225, 874,450 1,178,200 385,250 705,080  1,147,935 1,986,840 2,183,890 1,728,175 1,956,800 1,013,765 1,208,965 1,799,675 1,136,170 1,487,850 1,051,660 2,127,685 2,675,860 3,421,210 2,194,410 1,803,670  51 52 31 52 51 59 47 47 52 56 56 49 67 66 82 56  49 According to these figures  the average catch c o n s t i t u t e s  a  somewhat  s m a l l e r p r o p o r t i o n o f t h e r u n t h a n i s t h e case w i t h p i n k salmon, — about for  the l a s t  8 years  shown, as  against  70% f o r  s i m i l a r d i f f e r e n c e was o b t a i n e d i n t h e t a g g i n g species 16% a s  (p.  17 ) .  A somewhat  experiments i n v o l v i n g t h e two  T h e c o n c l u s i o n seems q u i t e  only  justified  chum p o p u l a t i o n s a r e g e n e r a l l y somewhat l e s s h e a v i l y f i s h e d  t h a n t h e p i n k salmon s t o c k s . results  species.  T o t a l chum t a g r e t u r n s f r o m t h e c o m m e r c i a l f i s h e r y w e r e  c o m p a r e d w i t h a b o u t 30% f o r p i n k s .  that the t o t a l  the l a t t e r  60%  This conclusion i s  obtained from f i s h tagged i n Johnstone  i n g e n e r a l agreement  Strait  i n 1945,  with  although  p r o b a b l y b o t h s p e c i e s a r e somewhat more h e a v i l y e x p l o i t e d i n t h e s e  quite  southern  a r e a s t h a n i n t h e C e n t r a l r e g i o n ( c f . DeLacy & Neave 1947; F o e r s t e r & C h a t w i n 1951).  Average e x p l o i t a t i o n i n t h e C e n t r a l r e g i o n , by r e a s o n i n g a l o n g  i n d i c a t e d o n p a g e 17  , should approximate 40 -  A lower rate  lines  50%.  o f e x p l o i t a t i o n o f chums w o u l d . b e e x p e c t e d  since f i s h i n g  i s u s u a l l y c l o s e d b e f o r e t h e l a t e r chum r u n s h a v e p a s s e d t h r o u g h t h e f i s h i n g a r e a s , w h e r e a s t h e more c o m p a c t p i n k s a l m o n m i g r a t i o n i s w e l l fishing  season (although temporary closures  accordance w i t h estimated  conservational  covered by the  of the f i s h e r y are a p p l i e d i n  requirements).  CHANGES I N ABUNDANCE OF ADULT CHUM SALMON Examples o f a n n u a l changes ments a r e p r e s e n t e d  in Figs.  The f l u c t u a t i o n s and i r r e g u l a r f r e q u e n c y . or small runs;  the effects  catches  or  are of considerable  There i s no t e n d e n c y f o r c y c l i c a l  These f e a t u r e s  pronounced peaks and depressions  occur.  These  o v e r t w o o r more  i n a single  season.  large  new l e v e l s  a r e no doubt a s s o c i a t e d  s p a n , w h i c h t o some e x t e n t d i s t r i b u t e s taking place  amplitude  recurrence of  n o r i s t h e r e e v i d e n c e o f sudden change t o p e r s i s t e n t  o f m o r t a l i t y changes  escape-  9.  shown b y t h e s e g r a p h s  of h i g h e r o r lower abundance. varying l i f e  7 to  i n chum s a l m o n r u n s ,  with  the  years  Nevertheless,  do n o t s y n c h r o n i z e w i t h  the  Fig.7* Reported c a t c h and escapement o f chum salmon i n t h e c e n t r a l - o f B r i t i s h C o l u m b i a . (1).Total r u n (2) Catch.'  region  50 i length, of the l i f e true  c y c l e , w h e t h e r t h i s be t a k e n a s f o u r y e a r s  f o r a l a r g e m a j o r i t y of t h e i n d i v i d u a l f i s h )  be made f o r  a percentage of f i s h of other a g e s .  determined by f a c t o r s  other than the size  of the  (which i s  usually  o r w h e t h e r some a l l o w a n c e E v i d e n t l y abundance  is  commonly  escapement.  THE MECHANISM CONTROLLING POPULATION L E V E L S As i s t h e c y c l e of  case w i t h p i n k salmon,  t h e chum may b e c o m p e n s a t o r y ,  the m o r t a l i t y attending the  depensatory  or extrapensatory  life  in  its  effects. Compensatory is  still  mortality is  again a t t e s t e d by t h e f a c t  c o m m e r c i a l l y p l e n t i f u l a f t e r many y e a r s o f f i s h i n g w h i c h ,  somewhat l e s s be d e s c r i b e d  as  t h e emergence o f t h e f r y , t h a t  is,  due t o e n v i r o n m e n t a l l i m i t a t i o n s . conditions i n the  t h i s type of mortality i s  if  can  yet  of  the  adults  i n t o f r e s h w a t e r and .  Owing, however, t o t h e v a r i a b l e  chum s a l m o n s t r e a m s h i t h e r t o e x a m i n e d , a g o o d  by a s h o r t  sequence o f  character-  the period i n which density i s at  r e l a t i o n s h i p between p o p u l a t i o n s i z e  apparent as  species  intensive.  i s t i c o f t h e p e r i o d between t h e entrance  demonstrated  even  severe t h a n t h a t w h i c h has been a p p l i e d t o p i n k salmon,  I t must a g a i n be s u p p o s e d t h a t  and r e p r o d u c t i v e e f f i c i e n c y annual r e c o r d s .  It  a l o n g - t e r m average but i n any g i v e n y e a r  a r e l i k e l y t o be o u t w e i g h e d by e x t r a p e n s a t o r y o f t h e s t r e a m s commonly u t i l i z e d b y c h u m s , f o r the  that the  factors.  this  is  a maximum  physical  inverse cannot  be  would presumably  compensatory  effects  I n view of the  l i k e l y to  become  character  be g e n e r a l l y  true  species. The e f f e c t s o f  p r e d a t i o n on m i g r a t i n g f r y have been i n v e s t i g a t e d  N i l e C r e e k and H o o k n o s e C r e e k b y t h e l i b e r a t i o n o f m a r k e d f r y a t above t h e c o u n t i n g f e n c e N i l e Creek experiments  at  a  distance  intervals during the period of f r y migration.  were c o n d u c t e d by t h e w r i t e r i n 1946  subsequent y e a r s by M r . W. P . W i c k e t t .  at  At  and 1947 and i n  A s i m i l a r e x p e r i m e n t was p e r f o r m e d  in  I  T  1  1  — i —1  1  rr  o \  Fig.10. S u r v i v a l o f marked chum f r y - m i g r a n t s i n Hooknose C r e e k , i n r e l a t i o n to number o f f i s h : m i g r a t i n g and p r o g r e s s o f time. 0———O average d a i l y number o f s u r v i v i n g f r y - m i g r a n t s • % s u r v i v a l . o f marked f r y ®" . e s t i m a t e d average d a i l y number, o f emerged f r y ( b e f o r e p r e d a t i o n ) ^ • e s t i m a t e d from samples c o m p r i s i n g more t h a n h a l f t h e t o t a l s u r v i v i n g fry-migrants. —  51 i 1951 a t Hooknose C r e e k b y M r , J . The g e n e r a l  f e a t u r e s of  o f t h e chum s a l m o n a r e  G. Hunter, at  the w r i t e r * s  request.  p r e d a t i o n d u r i n g t h i s phase o f t h e l i f e  i n conformity with the  f o r p i n k salmon at M c C l i n t o n C r e e k .  d a t a o b t a i n e d b y Cameron  C o n c l u s i o n s may b e s u m m a r i z e d a s  1.  Percentage  m o r t a l i t y increases with the distance  2.  Percentage  m o r t a l i t y decreases with  3.  Percentage  m o r t a l i t y increases during the progress of the  Points study,  (S)  and ( 3 ) ,  (1941) follows:  over which the f r y t r a v e l .  i n c r e a s i n g number o f  which are  history  of chief interest  migrants. run.  for  our present  are i l l u s t r a t e d by F i g . 10 w h i c h p r e s e n t s H u n t e r ' s d a t a f o r  Hooknose  Creek.  W h i l e u n f o r t u n a t e l y a p r e d a t i o n t e s t was n o t f e a s i b l e d u r i n g t h e  peak,  week o f  the m i g r a t i o n , a g e n e r a l  and  percentage s u r v i v a l i s  indicated,  more i n t e n s i v e d u r i n g t h e appears to  depend on (a)  as  is also the fact  l a t e r weeks o f the  size  m i g r a t i o n , and  (c)  relationship.  V a r i a t i o n s i n (b)  may t h e r e f o r e  r e l a t i o n s h i p between number o f m i g r a n t s  the r u n .  t h a t p r e d a t i o n becomes  The end r e s u l t  of t h e f r y p o p u l a t i o n ,  the compactness of t h e m i g r a t i o n , and  (c)  e i t h e r r e i n f o r c e o r reduce  involves a  w i l l be e x t r a p e n s a t o r y the e f f e c t s  due t o  e x o d u s w e r e s p r e a d o v e r a l o n g p e r i o d so t h a t  one t i m e w e r e r e l a t i v e l y s m a l l .  t h e numbers m i g r a t i n g a t  tage unless a s i m i l a r l a g occurred i n the presence o r a c t i v i t y of  h a v e b e e n made f o r chum s a l m o n a t N i l e  different years,  Recorded f r y migration  Estimated emergence  1946 1947 1948 1949 1950  138,388 8,319 4,808 23,188 782  223,000 18,500 8,600 66,800 2,170  if any  disadvan-  predators.  the following estimates  Creek.  Year  a  and  example,  be r e d u c e d  A l a t e m i g r a t i o n w o u l d a l s o be a t  In comparing t h e f r y runs o f  For  the  depensatory  i n effect  (a).  the advantage possessed by a l a r g e p o p u l a t i o n would presumably its  therefore  (b) t h e t i m i n g o f  (a)  ,  Survival $ 62 45 56 35 36  52  The s i z e of t h e r u n s a t N i l e Creek d u r i n g t h e s e y e a r s has been s m a l l t h a t l i t t l e range  i s presented f o r e x h i b i t i n g deesmpensatory  v a r y i n g l e v e l s o f abundance. 138,000  H o w e v e r , t h e one " l a r g e "  so  effects  run which produced  s u r v i v o r s w a s a c c o m p a n i e d b y a n e s t i m a t e d l o s s , o f 38% w h e r e a s t h e  r u n s w h i c h y i e l d e d s u r v i v a l s o f f r o m 800 t o 23,000 mortality rate  o f a b o u t 57%.  at  four  i n d i v i d u a l s showed a n a v e r a g e  A c t u a l l y t h e d i s c r e p a n c y was p r o b a b l y  greater,  s i n c e t h e 1 9 4 6 t e s t s . w e r e made o v e r a l o n g e r d i s t a n c e o f t r a v e l w h i c h  presumably  involved increased m o r t a l i t y . It  is  concluded that the very considerable  effects  of fry  losses  w o r k g e n e r a l l y t o t h e d e t r i m e n t o f p o p u l a t i o n s w h i c h h a v e become s m a l l t h r o u g h other  causes. Whereas compensatory m o r t a l i t y t e n d s  to maintain a uniform  f r o m y e a r t o y e a r and d e p e n s a t o r y m o r t a l i t y t e n d s t o i n c r e a s e such d e v i a t i o n s as  can s t i l l  ations which i n fact  arise,  the considerable  and  abundance  perpetuate  and i r r e g u l a r  fluctu-  occur point to the v a r i a b i l i t y of m o r t a l i t y which  be r e f e r r e d t o e i t h e r o f t h e s e c a t e g o r i e s . (p.^-S") t h a t t h e f r e s h w a t e r h a b i t a t s  I t has a l r e a d y been  cannot  suggested  o f chum s a l m o n a r e p a r t i c u l a r l y s u s c e p -  t i b l e to v a r i a t i o n i n p h y s i c a l c o n d i t i o n s and t h e s e are regarded as p r o v i d i n g a major source o f the f l u c t u a t i o n s observed. a r e no d o u b t imposed i n t h e  Other extrapensatory  variations  ocean.  T h e same g e n e r a l p r o c e s s e s d e t e r m i n e p o p u l a t i o n l e v e l s i n b o t h p i n k a n d chum s a l m o n .  The f e a t u r e s  which d i s t i n g u i s h the pattern of v a r i a t i o n of  the l a t t e r s p e c i e s are a s s o c i a t e d extrapensatory  with the r e l a t i v e l y greater  m o r t a l i t y and t h e g r e a t e r  flexibility  p a r t played by  of the l i f e  span.  R E L A T I V E E F F E C T S OF FRESHWATER, NATURAL OCEAN AND FISHING- MORTALITY I N DETERiaNnsTG POPULATION L E V E L S As already  i n d i c a t e d , average marine m o r t a l i t y i s  a t most o n l y s l i g h t l y d i f f e r e n t f r o m t h a t be no r e a s o n f o r s u p p o s i n g  that  it  considered to  be  o f p i n k s a l m o n a n d t h e r e seems t o .  i s more v a r i a b l e .  The s i t u a t i o n  is  53  otherwise, however, w i t h regard t o freshwater l o s s e s .  The combined r e s u l t s  of  N i l e C r e e k a n d H o o k n o s e C r e e k show a n e f f i c i e n c y o f f r y - m i g r a n t p r o d u c t i o n r a n g i n g f r o m 15$ t o  0.08$,  or a ratio of 187:1.  Such extreme v a r i a t i o n of  course cannot be expected t o a p p l y t o t h e p o p u l a t i o n s o f l a r g e  areas contain-  i n g numerous spawning s t r e a m s b u t , t a k e n i n c o n j u n c t i o n w i t h t h e expressed gives part  p r e v i o u s l y t h a t marine m o r t a l i t y i s l a r g e l y non-compensatory,  good grounds f o r s u p p o s i n g t h a t f r e s h w a t e r i n d e t e r m i n i n g changes  clusion is  supported by the c o r r e l a t i o n e s t a b l i s h e d  later,  i n F i g . 11.  i n the Vancouver I s l a n d d i s t r i c t .  I n these graphs  The e f f e c t  on the  and chums  spawning shown  represent  Average egg-production p e r p a i r  of  2700. t h e average freshwater  i n t h e absence o f any f i s h e r y s h o u l d be l e s s t h a n 2 $ . l e v e l w i t h a 50$ f i s h i n g m o r t a l i t y , w h i c h has ( p . 4 f ), f r e s h w a t e r  Table IX that  (Neave  a f i g u r e o f 4$ h a s been assumed t o  On t h e b a s i s o f t h e s e a s s u m p t i o n s  average  This con-  s u r v i v a l and f i s h i n g i n t e n s i t y are  average n a t u r a l ocean s u r v i v a l ( p . 4 / ) . i s t a k e n as  by W i c k e t t  it  major  s t r e a m f l o w and t h e abundance o f a d u l t  stock of v a r y i n g r a t e s of freshwater  fish  conditions p l a y the  i n t h e abundance o f a d u l t p o p u l a t i o n s .  W i c k e t t 1949) between s e a s o n a l 4 years  opinion  To m a i n t a i n a  been suggested as  s u r v i v a l s h o u l d be 3 . 7 $ .  survival  It  an  constant  approximate  can be s e e n f r o m  i n t h e combined N i l e C r e e k and Hooknose Creek r e c o r d s t h i s  has been d e f i n i t e l y exceeded four other instances  i n four instances,  d e f i n i t e l y not attained  and a p p r o x i m a t e l y e q u a l l e d i n t h e r e m a i n i n g  level in  instance.  CONSERVATION PROBLEMS T r e n d s i n t h e chum s a l m o n s t o c k s assess because (a)  the c o l l e c t i o n of accurate  t h e v a r i e d manner i n w h i c h t h i s s p e c i e s i s (Hoar 1951)  of large  (b) t h e l a r g e  areas are  statistics  difficult  to  has been impeded by  commercially processed  and  sold  and i r r e g u l a r f l u c t u a t i o n s p r e v e n t t h e d r a w i n g o f  s o u n d c o n c l u s i o n s f r o m d a t a w h i c h do n o t e x t e n d o v e r a l o n g p e r i o d o f  years.  i  T  r  r  + 90 +80 + 70 +60  +so +40  u AS  ••JO >Z0  \  +10  >  0  .H  -to 1  ^  -20 -30  -AO  -50 -60  8  10  12.  °/o freshwater  IA  (6  |8  ZO  survival  Fig.11. R e l a t i o n between f r e s h w a t e r s u r v i v a l o f chum salmon and s i z e o f spawning stock a t v a r i o u s l e v e l s o f f i s h i n g i n t e n s i t y . An average n a t u r a l ocean s u r v i v a l o f k% i s assumed. F.M. = f i s h i n g m o r t a l i t y .  54  The data available for the Central region (Fig. 7) do not show a • pronounced upward or downward trend. However a decline, at least in the size of escapements, is certainly suggested by the graph for the Vancouver Island region (Fig. 9).  A decline in escapement, caused by increasing fishing  intensity, might of course be made good by increased reproductive efficiency. The downward trend in this instance, however, seems to have continued over a long period and there must come a point at which compensation cannot close the gap.  This is an area of relatively intensive fishing but i f Nile Creek  results are indicative, some of the streams are in fact yielding a very low percentage output in many years, instead of responding to the reduced density of the spawning populations.  As far as this stream is concerned, the fry  output in three out of five observed seasons would, by any reasonable calculation, produce a smaller returning population even i f no fishery were operating. While Nile Creek probably represents an extreme case, concern may well be felt regarding the productivity of other streams in this region at the present time. A chronic condition of low reproductive efficiency caused by the physical condition of the spawning streams cannot be cured by regulation of the fishery. The fact that relatively small spawning escapements may produce large returning populations, and vice-versa, may suggest that the size of adult runs, within existing limits, is independent of the size of escapements and that a more intensive fishery could be prosecuted without harming the stocks.  It may be strongly urged, however, that the observed fluctuations  are in large measure caused by mortality which is non-compensatory. Observed instances of large spawning populations producing, small returns do not permit the establishment of a generalization regarding the inefficiency of large escapements. The probability must be envisaged that a smaller population would have produced an even smaller return in the particular cycles concerned. Runs could, of course, be utilized more economically if it were possible to foresee the particular climatic conditions to which their eggs and alevins  55  v will  be e x p o s e d .  output of  The minimum s i z e  of sea-going f r y would v a r y g r e a t l y  an u n d e r l y i n g compensatory  v a r i a t i o n s which are frequently occur. i s of  of escapement r e q u i r e d t o produce a  imposed upon i t  are  so l a r g e t h a t  existence  extrapensatory  "slumps"  and  "booms"  b e t w e e n p i n k a n d chum s a l m o n i n t h i s  c o u r s e i n degree o n l y and i s  shown by t h e r e s u l t s  Tne  p r o c e s s c a n n o t be d o u b t e d b u t t h e  The d i f f e r e n c e  an average d i f f e r e n c e  from year to y e a r .  given  respect  c o n s i d e r e d t o be l a r g e l y a r e f l e c t i o n of  i n freshwater  habitat.  Under s i m i l a r c o n d i t i o n s ,  a t Hooknose C r e e k , p o p u l a t i o n s  o f t h e two s p e c i e s  as react  i n a s i m i l a r manner. Speaking  b r o a d l y , pronounced i n s t a b i l i t y of p h y s i c a l c o n d i t i o n s  spawning streams i s v i e w e d as t e n d i n g t o e l i m i n a t e p i n k s a l m o n , the low egg-production and r i g i d l i f e span. variation  i n population size.  by r e a s o n  W i t h chum : s a l m o n i t  Maintenance of favourable  induces  conditions during  would t e n d both t o  t o reduce f l u c t u a t i o n s .  I f t h i s were a c h i e v e d by c o n t r o l l i n g t h e f l o w o f  (or p o r t i o n s  of them),  it  the average output  c o u l d be e x p e c t e d  that  and  compensatory  e f f e c t s w o u l d become a p p a r e n t ,  -- that  size  e f f i c i e n c y o f i n d i v i d u a l r e p r o d u c t i o n would be  o f t h e escapement and t h e  observed.  The i n t r i g u i n g p o s s i b i l i t y t h a t  enable p i n k salmon t o The g e n e r a l  displace effects  respect to the fry-migrants tend to of,  suffer  a s t r i c t e r r e l a t i o n s h i p between  increased  chums i s a l s o  s t a b i l i t y might tend  the  to  presented.  o f p r e d a t i o n h a v e ^ h o w n t o be s i m i l a r w i t h  of both species,  i n that  small populations  r e l a t i v e l y heavier losses than large populations.  or protection from, predators  establishing of  is,  of great  i n c u b a t i o n and a l e v i n a g e  spawning streams  increase  in  should therefore  be a m a j o r  will  Elimination  consideration  in  new r u n s o r i n b u i l d i n g u p s t o c k s w h i c h h a v e r e a c h e d a l o w l e v e l  abundance.  ACKNCM^GEMENTS The w r i t e r i s the Department  u n d e r g r e a t o b l i g a t i o n t o D r . W. A . C l e m e n s , Head o f  of Z o o l o g y , U n i v e r s i t y o f B r i t i s h C o l u m b i a , f o r  stimulation,  56  encouragement  and f a c i l i t i e s  The F i s h e r i e s  f o r the p r e p a r a t i o n of  Research Board of Canada,  w o r k p e r f o r m e d by t h e w r i t e r h a s b e e n c a r r i e d o u t ,  this  thesis.  under whose a u s p i c e s has  also generously  the afforded  h i m t h e time and o p p o r t u n i t y t o u t i l i z e t h i s work f o r  the p r e s e n t purpose.  sympathetic  and D r . J .  c o n s i d e r a t i o n shown b y D r . R . E . F o e r s t e r  L . Hart,  c e s s i v e D i r e c t o r s o f the P a c i f i c B i o l o g i c a l S t a t i o n , Nanaimo, has essential  feature i n the progress of t h i s The w r i t e r i s  S t a t i o n , and e s p e c i a l l y collection of data, the f i e l d  of  indebted to h i s  suc-  an  study.  colleagues at the P a c i f i c B i o l o g i c a l  t o M r . ¥ . P . W i c k e t t and M r . J .  f o r general  been  The  G. H u n t e r , f o r the  c o - o p e r a t i o n and f o r h e l p f u l d i s c u s s i o n  inquiry to which t h i s t h e s i s i s  in  related.  REFERENCES Cameron, W. F . A p r e l i m i n a r y i n v e s t i g a t i o n of the n a t u r a l spawning, incubation and a l e v i n a g e o f t h e p i n k salmon (Oncorhyochus g o r b u s c h a ) . MS r e p o r t . 1939. Mortality during the fresh-water MS r e p o r t . 1 9 4 1 .  existence  of the pink  salmon.  D a v i d s o n , F , A . and E l i z a b e t h Vaughan. R e l a t i o n of p o p u l a t i o n s i z e t o marine g r o w t h and t i m e o f spawning m i g r a t i o n i n t h e p i n k , s a l m o n (Oncorhynchus gorbuscha) of southeastern A l a s k a . J . Marine Research 4(3):231-246. 1941. DeLacy, A l l a n , C and Neave, F . M i g r a t i o n o f p i n k salmon i n southern B r i t i s h C o l u m b i a and W a s h i n g t o n i n 1 9 4 5 . F i s h . Res. B d . Canada, B u l l . 74. 1947. Department of M a r i n e and F i s h e r i e s , observations. Toronto. Foerster,  Canada.  M o n t h l y r e c o r d of  meteorological  R. E . The r e t u r n f r o m t h e s e a o f s o c k e y e salmon (Oncorhynchus n e r k a ) w i t h s p e c i a l r e f e r e n c e t o p e r c e n t a g e s u r v i v a l , sex p r o p o r t i o n s and progress of m i g r a t i o n . J . B i o l . B d . Canada, 3 ( l ) : 2 6 - 4 2 . 1936. and P r i t c h a r d , A . L . Pac. Coast S t a t i o n s ,  The e g g c o n t e n t o f P a c i f i c S a l m o n . 28:3-5. 1936.  Prog.  Repts.  Hobbs, D e r i s l e y , F . N a t u r a l r e p r o d u c t i o n o f q u i n n a t s a l m o n , brown and r a i n b o w t r o u t i n c e r t a i n New Z e a l a n d w a t e r s . New Z e a l a n d M a r i n e D e p t . , Fisheries B u l l . 6. 1937. H o a r , W. S . T h e chum a n d p i n k s a l m o n f i s h e r i e s o f B r i t i s h C o l u m b i a , 1 9 1 7 - 4 7 . F i s h . 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