Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

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

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1951_A1 N3 P7.pdf [ 4.22MB ]
Metadata
JSON: 831-1.0302621.json
JSON-LD: 831-1.0302621-ld.json
RDF/XML (Pretty): 831-1.0302621-rdf.xml
RDF/JSON: 831-1.0302621-rdf.json
Turtle: 831-1.0302621-turtle.txt
N-Triples: 831-1.0302621-rdf-ntriples.txt
Original Record: 831-1.0302621-source.json
Full Text
831-1.0302621-fulltext.txt
Citation
831-1.0302621.ris

Full Text

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 T H E U N I V E R S I T Y OF B R I T I-S H C O L U M B I A October, 1951 L 5 ^ 7 tlft fit 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 ob-served 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 sig-nificantly 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 relative-ly constant for both small and large runs. Pink salmon maturing in "even" and "odd" years represent separ-ate populations. These populations vary in size independently but may main-tain a relatively constant ratio for a series of generations. This ratio varies from near equality to extreme disparity. Marked changes in the lev-el of abundance may occur suddenly. Three types of mortality are recognized: (a) mortality which becomes relatively heavier as populations increase in density (compensat-ory) (b) mortality which becomes relatively heavier as populations de-crease in density (depensatory) (c) mortality which is independent of dens-ity (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 exagger-ated by depensatdry factors (notably predation on fry) but tend to be re-sisted 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 mort-ality and plays a greater part in inducing population changes. It is suggested that the average freshwater survival of an un-fished 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 popul-ation. 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 persist-ing 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 irreg-ular fluctuations in abundance. In the application of remedial measures similar principles apply to both species. Dean Angus Clemens Cameron Hoar Hutchinson Robblns Sage Spencer Warren The Thesis of Hr. F e r r i s Neave e n t i t l e d " P r i n c i p l e s A f f e c t i n g the S i z e of Pink and Chum Salmon Populations i n B r i t i s h Columbia" i s accepted: - i -CONTENTS Page HEKODUCTION . iv FINK SALMON - 1 EGG-PRODUCTION 2 SEX RATIO . 4 MORTALITY 6 MORTALITY AND SURVIVAL IN FRESH WATER . 7 1. Total freshwater survival 7 2. Immediate causes of death . . . . 9 3» General conclusions 11 MORTALITY AND SURVIVAL IN THE OCEAN 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 . . . . . 28 CONSERVATION PROBLEMS 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 - i i -Page RELATIVE EFFECTS OF FRESHWATER, NATURAL OCEAN AND FISHING MORTALITY IN DETERMINING POPULATION LEVELS . . . . . . 52 CONSERVATION PROBLEMS 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 Table VI. Numbers of pink salmon caught annually in traps at Sooke, B. C 19 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. 6" Fag. 2. Decline of population with increased proportion of male fish • 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 - i i i -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. Reported catch and escapement of chum salmon in the central region of British Columbia , 49 •^ig. 8. 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 - J v -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 all species of animals and indicate the terms on which popul-ations 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 under-taken 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 sal-mon .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 chan-ges,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 dir-ection 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 respons-ible. The sources of such information are acknowledged in the text. -1-PINK 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 Sept-ember 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 birth-rate 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, consider-able 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 popu-lation 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 selec-tive 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 repro-ductive 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 product-ion 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 popula-tion, 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 multi-plicity of service must be demonstrated conclusively before it can be - 5 -Table I. Average egg content of pink salmon Locality Year No. of fish McClinton Creek 1930 • 97 ti n 1932 73 tt tt 1934 165 ii ti 1936 911 II ti 1938 40 « n 1940 41 Morrison Creek, V.I. 1943 38 II n it 1945 27 Namu 1934 a Fraser River 1934 48 Port John 1947 38 n it 1950 20 Average no. Authority of eftRS 1535 Eritchard (1948a) 1758 »' » 1799 » " 1899 " " 1698 " " 1619 " " 1779 " (unpub.) 1862' Neave » 1841 Foerster & Pritchard (1936) I755 « it 11 11 1520 Hunter (unpub.) 1593 " " Table II. Sex ratio (percent) of pink salmon escapements Locality Year No. of Males Females Authority fish McClinton Creek 1930 66,153 49.8 50.2 Pritchard (1948a) n n 1932 15,600 51.3 48.7 tt n "tt 11 1934 155,196 49.9 50.1 tt it 11 11 1936 52,312 46.3 53.7 n tt 11 n 1938 10,577 52.5 47.5 tt 11 tt 11 1940 35,521 53.7 46.3 ti tt 11 ti 1942 36,893 47.6 52.4 n n Morrison Creek, V.I. 1943 15,755 48.6 51.4 ti (unpub.) it n it 1945 13,411 ' 5,576 52.0 48.0 Neave n Hooknose Creek 1947 47.1 52.9 Hunter it tt tt 1948 1,160 48.5 51.5 ti ti ti n 1949 1,173 55.4 44.6 tt 11 tt it 1950 1,857 44.7 55.3 11 11 - 6 -considered an important factor in salmon propagation under natural condi-tions". 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 popula-tion 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 pre-ponderance, 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 ) Total m o r t a l i t y required to maintain constant population, with egg production of 1700 and varying preponderance of male f i s h . (2) Egg production required to maintain constant population with m o r t a l i t y rate of 99«8824£and varying preponderance pf male f i s h . ' Fig.2. Decline of population with egg production and mortality rate , constant and increased proportion of 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 egg-content 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 periodSB of time, utilizing weirs and pens for trapping the fish. 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. The figures obtained by careful and competent investigators are accepted here as being the best 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 Survival ($) Authority McClinton Creek 1930 10.6 Pritchard (1948a) it it 1932 17.4 tt tt it ii 1934- 9.0 ti ti ti II 1936 6.9 tt ti tt it 1938 23.8 ti tt n it 1940 19.0 tt tt McClinton Creek Average 14.45 Morrison Creek, V.I. 1943 4.7 Pritchard (unpub.) tt n tt 1945 6.7 Neave ti Hboknose Creek 1947 0.87 Hunter (1948) it tt 1948 8.02 tt (1949) ti tt 1949 6.24 tt (unpub.) tt ti 1950 , 14.76 tt ti Hooknose Creek Average 7.47 Sashin Creek, Alaska Average 1.93 U.S.Fish & Wildlife (1940-50, 11 years) Servicex x The writer is indebted to M.G. Hanavan, Acting Chief, Alaska Fishery Investigations, for these figures which ere of great interest for comparative purposes. The average recorded freshwater survival in the three streams for which several years1 data are available is as:follows: McClinton Creek, 14«45$j Hooknose Creek, 7.47$; Sashin Creek, 1.93$. 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 magni-tude 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 No. of migrants produced Ratio of range ($) (a) Maximum (b) Minimum (a) to (b) McClinton 76.2 - 93.1 404 118 3«45:1 Hooknose 85 - 99.1 255 15 17.00:1 Sashin 95«6 - 99.8 109 3*4 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 fertil-ization. - 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, expos-ure 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 si lt , 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 pre-vention 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 earlier-spawning 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 "silting11 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 fresh-water 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 mor-tality 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 mort-ality 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) if (a) all 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 fish-ing 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 al 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 economic-ally significant". Also, "For all 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". In a later paper Pritchard (1948b) supported his conclusion 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 undoubt-edly "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 individ-uals 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 consecutive-ly, 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 Brood- No. of Range {%) Average Authority year experiments  McGlinton Creek 1930-4-0 6 , 0.29-6.75 1.71 Er itchard (1948b) Morrison Creek 194-3 1 - 2.10' Neave (unpub.) Hooknose Creek 194-7-48 2 2.7 -3-7 3.20 Hunter " Sashin Creek 1940-45 6~ 0.6-3-5 1.93 U.S.Fish & Wild-life 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 reason-ably 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 for sockeye. Although the latter species commonly spends a year longer i n salt water than the pink salmon, this factor i n the opinion of the writer, would by no means offset the vulnerability 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 certain areas. For such comparisons to be valid, 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 is no certainty whatever that the fi s h caught are related to the spawning streams of the same area. This is particularly true of the fi s h which enter the straits between Vancouver Island and the mainland which, as tagging experiments show, may be proceeding to any of a wide variety of l o c a l -i t i e s from Queen Charlotte Strait to Puget Sound. There i s , however, a large region of the central part of the British Columbia coastline, corresponding with the Department of Fisheries 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 direction of the writer at the south and north ends of t h i s region in 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 Fig. 3) between catches and escapements reported for this region by . officers of the Department of Fisheries. "Catches" were reported i n hundred-weights and have been converted to approximate equivalent numbers of individ-uals. The escapements quoted have been arrived at by summing the estimates of spawning populations made by inspectors on about 180 individual streams and rivers (I am indebted to Mr. J.G. Hunter for this compilation). Estimates have usually been recorded by observers by means of letters indicating 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 2,250,000 2,046,675 4,296,675 52 1935 1,682,000 1,205,875 2,887,875 58 1936 3,502,000 2,577,500 6,079,500 58 1937 1,841,000 909,050 2,750,050 67 1938 1,619,000 1,512,425 3,131,425 52 1939 2,800,000 970,800 3,770,800 74 1940 663,000 622,875 1,285,875 52 1941 1,073,000 916,250 1,989,250 54 1942 1,175,000 1,045,600 2,220,600 53 1943 5,250,000 1*805,150 7,055,150 74 1944 2,510,000 856,400 3,366,400 74 1945 6,900,000 2,517,275 9,417,275 73 1946 1,685,000 724,775 2,409,775 70 1947 2,076,000 662,275 2,738,275 76 1948 2,834,000 985,100 2,835,100 74 1949 3,233,000 1,720,350 4,296,675 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$ res-pectively from the two operations). Even these figures are undoubtedly too low. 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 avail-able 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 pop-ulations 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 assur-ance at between 50$ and 70$ of the adult run: 60$ would seem to be a reason-able working figure. 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 pop-ulation 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 un-certain. 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 ni 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 con-sidering 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 tol 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 Year Number 1938 1472 1939 169,018 1940 328 1941 55,503 1942 116 1943 33,699 1944 605 1945 221,871 1946 110 1947 168,284 1948 214 1949 109,304 Average 474 Average 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 legi-timate 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 neces-sarily reflect the course of events in individual streams, the following in-ferences 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 tend-encies 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 con-ditions 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 In seeking to understand the above-mentioned features of population l e v e l s and population changes i t i s necessary-to conceive a mechanism which w i l l permit the occurrence of seemingly contradictory r e s u l t s . On the one hand we are confronted with the fact cuat the pink salmon has s u c c e s s f u l l y with-stood a heavy f i s h i n g mortality f o r many years. Since t h i s f i s h e r y , i f i t had been merely added to e x i s t i n g m o r t ality, would have wiped out the commercial importance of the species i n a very few generations, i t i s evident that the r e -duction i n the number of adult f i s h has been accompanied by an increase i n the reproductive e f f i c i e n c y of the escapements. On the other hand, the evidence i s equally p l a i n that two stocks can e x i s t for prolonged periods i n the same area at very d i f f e r e n t d e n s i t i e s , with no tendency to reach a common l e v e l . Further-more, when a large stock s u f f e r s a major "crash" i t may remain f o r many genera-t i o n s near the nevj l e v e l with no apparent tendency t o regain i t s former numerical status unless or u n t i l some equally sudden "jump" takes place. In considering t h i s problem i t i s necessary t o d i s t i n g u i s h between three "types 5 1 of mortality, namely: (1) M o r t a l i t y which becomes r e l a t i v e l y heavier as populations increase i n density (and vice versa), thereby tending to s t a b l i l i z e the p r e v a i l i n g population l e v e l . (2) M o r t a l i t y which becomes r e l a t i v e l y heavier as populations decrease i n den-s i t y , thereby tending 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 causes. (3) M o r t a l i t y which i s independent of population density, that i s , taking a given percentage of the population whatever the density of the l a t t e r may be. M o r t a l i t y of the f i r s t type has long been recognized as being the e c o l o g i c a l basis on which the maintenance of intensive long-term f i s h e r i e s de-pends. One of i t s e f f e c t s , that i s , the tendency f o r a reduced population to show an increased reproductive 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; Pri t c h a r d , 1948a). The present w r i t e r does not 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 . It appears t o emphasize only h a l f of the process,namely, the decreased m o r t a l i t y which takes plase at low l e v e l s of population. The increased mortality which attends high population l e v e l s i s of course equally c h a r a c t e r i s t i c . In other words the r e l a t i o n s h i p i s , or can be, continuous over a very wide range of population density. It i s not an innate power to respond to a condition of depression but merely an expression of the fac t that changes i n population density n e c e s s a r i l y e n t a i l changes i n the death-rates ensuing from c e r t a i n causes. 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 entomological studies (see Solomon, 1949). This accurately designates 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 necessary to recognize "inversely density-dependent mo r t a l i t y " , that i s , the type of mortality referred to i n category (2) above. The present writer suggests "compensatory mo r t a l i t y " as a convenient term for the r e l a t i o n s h i p now under discussion. This kind of mortality appears to toe highly c h a r a c t e r i s t i c of the period from spawning to the emergence of the. f r y . It might be supposed that physical factors such as f l o o d , drouth and s i l t i n g would tend to have the same ef f e c t on a population whether the l a t t e r be large or small. This i s indeed the case to an important degree (see p. 2.7) but these f a c t o r s do not act i n t h i s way alone. In practice the spawning s i t e s a vailable i n the l i m i t e d environment of a stream appear to vary considerably i n f a v o u r a b i l i t y . "Jith increasing numbers of spawners a l a r g e r precenta;:;e of the f i s h are compelled to make t h e i r redds i n marginal or unfavourable pos i t i o n s . This condition, coupled with the increased "superimposition" and perhaps the mutual interference between i n d i v i -duals of a dense population, tend to produce a lower average e f f i c i e n c y of reproduction. Over the whole period of freshwater existence P r i t c h a r d (1948) at - 23 -McClinton Greek observed a general inverse r e l a t i o n s h i p between number of adults and percentage output of f r y migrants. Certain anomalies were apparent, however, e s p e c i a l l y at the highest population l e v e l encountered. These are : discussed '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 greater with diminishing density of population (and vice versa) seems not to have been appreciated by f i s h e r y b i o l o g i s t s . Quite evidently, however, there are i n -fluences which frequently prevent the e f f e c t i v e operation of compensatory mor-t a l i t y , since otherwise the large and small populations of an area could not i n d e f i n i t e l y preserve t h e i r respective numerical pos i t i o n s . There x«jould be a constant tendency to converge. Such i n f luencesybannot be merely neutral; they must a c t i v e l y oppose the compensatory process, since the existence of the l a t t e r i s beyond dispute. A c t u a l l y , as we have seen, pink salmon populations 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 considerable periods at a wide var i e t y of l e v e l s . The anti-compensatory influences must.accordingly act over a wide range of den-s i t i e s . The term "depensatory m o r t a l i t y " i s proposed for the type of 'mortality which i s inversely related to population density. The i d e n t i f i c a t i o n of the fa c t o r s which work i n t h i s way, and the period i n the l i f e h i s t o r y when they operate, are obviously matters of v i t a l importance to OVLY study of population changes. A s p e c i f i c example of depensatory mortality appears t o be provided by the observations of Davidson and Hutchinson (1943). These authors reported that i n January 1942 a wholly exceptional flood caused extremely heavy losses among pink salmon eggs and alevins i n the lower reaches of Sashin Creek. Surviving f r y migrants came mainly from the upper reaches, an area which usually receives l i t t l e seeding, being r e l a t i v e l y rocky. Owing to the population pressure pro-duced by an exceptionally large run of adults i n the f a l l of 1941 an unusually large number of adult f i s h had u t i l i z e d t h i s upstream area. . . - 24 -Such occurrences are probably r e l a t i v e l y infrequent and c e r t a i n l y cannot provide the constant counterbalance to the compensatory process which seems to be demanded by the observed maintenance of diverse population l e v e l s . JJ'isuing undoubtedly could .be prosecuted i n a depensatory mannery. The taking of a fixed quantity each year would mean that small runs would be heavily ex-p l o i t e d and that large runs would be r e l a t i v e l y l i g h t l y fished. But as we have seen the f i s h e r y does not seem to operate i n t h i s way at the present time, since closures are applied with a view to permitting the escapement of a r e l a -t i v e l y constant proportion of the run. Ac t u a l l y , the only form of depensatory mortality which i s adequate to account for the observed f a c t s appears to be associated with the period of f r y migration. This period, although short i n duration, i s a peculiar and c r i t i c a l phase i n the l i f e - c y c l e . It represents the p r e c i p i t a t i o n of a large and very v u l -nerable population into the community of stream-dwelling organisms. The tr a n -sience of the s i t u a t i o n precludes the establishment of any long-term balance bet-ween predators and prey. The migration i s merely an exodus i n which heavy losses are/sustained without any s i g n i f i c a n t compensatory growth or development on.the part of the survivors. In his 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 estimated a 60$ o v e r a l l mortality among f r y passing to.the ocean from a spawning area which extended f o r a distance of two miles from s a l t water (or a 5«j$ mortality based on the p o t e n t i a l egg deposition). The m o r t a l i t y , however, varied with the distance which the f i s h had to traverse and also increased during the week by week progress of the run. He pointed out the probable advantages to the f r y of an early, compact migration i n reducing mortality during t h i s period.^ General observations indicate that the predators mainly concerned with f r y mortality are other f i s h , resident i n the same stream or themselves migrating 7 seaward at the time. In the l o c a l i t i e s investigated trout, coho.-, smolts, and HL It i s possible, although u n v e r i f i e d , that e a r l i e r entrance into the sea might briny corresponding disadvantages i f ohe seasonal increase of marine zooplankton were les s advanced. - F . l l . - 25 -sculpins are important. The small size and nocturnal habits of the fry-migrants appear to prevent major inroads by t e r r e s t r i a l predators. Losses are therefore l a r g e l y determined by the number and a c t i v i t y of the l o c a l aquatic predators. Even i n years when production i s r e l a t i v e l y low, fry-migrants w i l l o r d i n a r i l y constitute a large and r e a d i l y available food supply during the short period of t h e i r downstream migration, lience the predators w i l l tend to take a f i x e d num-ber rather than a percentage and the mortality w i l l be of the depensatory type. The t o t a l number of predators present from year to year w i l l presumably vary l e s s than the number of pink salmon f r y , since several species are involved and various age-groups are represented. The data provided by Pritc h a r d and by Cameron for HeClinton Creek may be re-examined in' the l i g h t of t h i s viewpoint. Cameron's estimated 60% morta-l i t y represents 7,590,000 f r y l o s t i n 1941. Assuming predation to be constant from year to year, t h i s figure may oe added to each year's recorded fry-migrant output to obtain a t h e o r e t i c a l figure f o r the number of f r y escaping from the gravel. A comparison between Fritchard's figures f o r migrant putput and these hypothetical figures f o r f r y emergence i s presented i n Table VII. Table VII. E f f e c t of predation on the fry-migrant production of pink salmon at McClinton Creek. P o t e n t i a l egg Fry *fo Hypothetical jo $ deposition Migrants e f f i c i e n c y f r y emergents e f f i c i e n c y predation 8,500,000 2,020,000 23.8 (9,610,000) ( n o ) (86.2) 13,200,000 2,300,000 17.4 9,890,000 75 57.6 26,600,000 5,060,000 19.0 12,650,000 48 29.0 50,800,000 5,384,000 10.6 12,974,000 25 14.4 53,200,000. 3,675,000 6.9 11,265,000 21 14.1 139,500,000 12,600,000 9.0 20,190,000 14 5.0 - 26 -As might be anticipated, the c a l c u l a t e d values f o r emergence produce an obvious absurdity at the lowest l e v e l of population, when the predators could hardly be expected to obtain the maximum number of f r y which they are capable of devouring. In spite of the crudeness of the procedure, however, the adjust-ment produces a better r e l a t i o n s h i p (inverse) between the size of the spawning population and the percent e f f i c i e n c y of reproduction, every value now appearing i n i t s proper sequence i n the series. In p a r t i c u l a r , the anomaly of a higher percentage output from a deposition of 139,500,000 eggs than from one of 53,200pC0 and the s i m i l a r discrepancy between the e f f i c i e n c y at the 27,000,000 and 13,000,000 l e v e l s are eliminated. The reduction i n the effectiveness of pre-dation with increasing f r y production i s well i l l u s t r a t e d i n the l a s t column of Table VII. It may therefore be i n f e r r e d that compensatory m o r t a l i t y prevailed over the whole size range of the spawning populations observed at McClinton Creek up to the time of emergence of the f r y . Subsequent predation, on the other hand, introduced depensatory e f f e c t s which i n some instances caused a t r a n s p o s i t i o n of the f i n a l r e s u l t s . Recognition of these d i f f e r e n t kinds of mortality permits a reasonable explanation of the problems posed on a previous page. With other conditions constant, a reduction i n the number of spawners (by f i s h i n g or other cause) w i l l be followed by increased s u r v i v a l up to the free-swimming stage. The absolute number of f r y produced may not be very d i f f e r e n t from the previous average (cf. the r e l a t i v e uniformity of "hypothetical f r y emergents" from depositions of d i f f e r e n t magnitude) and f l u c t u a t i o n s of the population w i l l remain within moderate l i m i t s . If,: on the other hand, the number of emergent f r y i s d r a s t i -c a l l y reduced (as could happen through unusually unfavourable p h y s i c a l condi-t i o n s or through the reduction of spawners beloxv the l e v e l which could produce a large f r y emergence even with the decreased mortality) the predators w i l l - 27 -take a larger r e l a t i v e t o l l and the population w i l l be depressed to a new l e v e l . At the other extreme, a p a r t i c u l a r l y abundant fry. emergence may "break through" the c o n t r o l imposed by predators and enable the population to reach a l e v e l at widen predation takes only a small percentage and c o n t r o l i s effected mainly by compensatory mortality. The depensatory part of the mechanism tends to r e s i s t a return to the former l e v e l . This would account f o r the apparent fact that pink salmon populations can become s t a b i l i z e d f o r long periods at very d i f f e r e n t l e v e l s and that new l e v e l s are sometimes reached by sudden "steps" rather than by gradual t r a n s i t i o n s . The term " l e v e l " i s used of course i n a comparative sinse, to denote a period of r e l . t i v e l y minor f l u c t u a t i o n s . These, too, are presumably accentuated by predation but not to an extent which precludes r e c t i -f i c a t i o n by ordinary c l i m a t i c v a r i a t i o n s or other f a c t o r s . I t i s evidently the r o l e of depensatory mortality to exaggerate changes and, by opposing compensatory mortality, to s t a b i l i z e these changes when they become gr^at. Since, however, neither compensatory nor depensatory influences can initiate changes we must recognize the existence of a t h i r d type of mortality which i s independent of population density. In conformity with the other terms suggested herein, t h i s may be c a l l e d "extrapensatory mortality". As already pointed out, 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 environment may act to some extent i n a compensatory manner, that i s , they constitute an immediate cause of death, although t h e i r effectiveness i s determined by the density of the population. There can be no doubt, however, that numerically s i m i l a r spawning' runs can show great v a r i a t i o n i n e f f i c i e n c y of reproduction. Favourable and unfavourable conditions of stream flow and temperature can o p e r a t e at a l l l e v e l s of population density. The general e f f e c t s of stream flow are discussed by Neave and TJickett (1949). In t h i s - 28 -p u b l i c a t i o n VJickett has also 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 July - August p r e c i p i t a t i o n and the number of adult pink salmon returning i n the next generation i n the central region of B r i t i s h Columbia. Fishi n g m o r t a l i t y (as applied at the present time) seems to be mainly extrapensatory i n that i t operates with f a i r l y equal i n t e n s i t y on large and small runs - at l e a s t i n the area for which comparisons have been made (p. 16 ), Since, however, i t appears to take an approximately constant percentage every year, i t merely conforms to population sizes which have been determined pre-viously and 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 intensive f i s h e r y could undoubtedly produce a d e c l i n i n g trend, or a drop to a new l e v e l , by d r a s t i c a l l y reducing the number of spawning f i s h . On the other hand, natural ocean conditions could act i n a manner s i m i l a r to the freshwater conditions mentioned above, - f o r example, tempera-ture and the quantity or a v a i l a b i l i t y of food supplies could produce v a r i a t i o n s which are unrelated to the size of the population. Further reference to t h i s point i s made i n the following section. RELATIVE EFFECTS OF FRESHWATER, NATURAL OCEAN AMD FISHING l.OF.TALITY IN DE-TERMINING- POPULATION LEVELS Although a large majority of the t o t a l losses attending the l i f e -cycle takes place i n fresh water, i t does not follow from t h i s fact along that f l u c t u a t i o n s i n population size are determined i n t h i s medium. V a r i a t i o n i n mortality, rather t h a n - i t s t o t a l magnitude, w i l l be important i n t h i s connection. A c t u a l l y , since the i n d i v i d u a l s (or p o t e n t i a l individuals) entering e i t h e r en-vironment commonly suffe r a mortality exceeding 90$ before concluding the phase, i t i s evident that a small change i n mortality rate i n e i t h e r instance would exert a profound e f f e c t on the end r e s u l t . P r i t c h a r d (1950) found that at LIcGlinton Greek the t o t a l ocean sur-v i v a l showed greater r e l a t i v e v a r i a t i o n (that i s , r a t i o between highest and - 29 -lowest recorded values) than the freshwater s u r v i v a l (Freshwater s u r v i v a l , 23.8$ to 6.9%, r a t i o 3.5:1. Ocean survival,6.75% to 0.29%, r a t i o 23:1). Ehere are reasons, however, f o r b e l i e v i n g that t h i s i s not a representative comparison between the v a r i a b i l i t y of freshwater and natural ocean s u r v i v a l . Corresponding f i g u r e s f o r other l o c a l i t i e s are: % freshwater freshwater % ocean ocean s u r v i v a l r a t i o s u r v i v a l r a t i o Sashin Creek 6.4 - 0.2 32:3l • 3.S7 - 0.74 5.2:1 Hooknose Creek 15.0 - 0.87 17:1. 2.7 - 3.7 1.4:1 The combination of McClinton Creek data with these other records gives a t o t a l freshwater r a t i o of 119:1, without extending the McClinton Creek range f o r ocean v a r i a b i l i t y . The small degree of v a r i a t i o n i n the freshwater s u r v i v a l at McClinton Creek i s e v i d e n t l y due t o the absence of the low values which have been found elsewhere. The r e l a t i v e s t a b i l i t y of the stream conditions at McClinton Creek during the period of the investigations i s attested by the good r e l a t i o n s h i p (already discussed) between the s i z e of the spawning population and the e f f i c i e n c y of f r y production. Evidently no d r a s t i c extrapensatory f a c t o r s intervened to upset t h i s r e l a t i o n s h i p . Under such circumstances i t i s quite l i k e l y that changes i n population si z e c a n b e determined i n large measure by natural ocean conditions. However, i t should be noted that the figures given are the net r e s u l t of natural and f i s h i n g m ortality. Although we have found reason to believe that f i s h i n g m o r t a l i t y takes a r e l a t i v e l y constant percentage from year to year when a large area i s considered, such constancy cannot be expected to p r e v a i l f o r the run t o an i n d i v i d u a l small stream. P r i t c h a r d himself considered that the i n t e n s i t y with which the McClinton Creek run was f i s h e d v a r i e d a gooddaal. The low . ocean s u r v i v a l observed i n most years at McClinton Creek',: contrasted with a very high s u r v i v a l i n one year, suggests the po s s i b i l i t y that t h i s run i s u s u a l l y f i s h e d heavily but can on occasion!escape major e x p l o i t a t i o n . - 30 -As regards natural mortality, the present writer thinks that major importance i n determining changes i n population size can us u a l l y be assigned to freshwater f a c t o r s , i n view of the following considerations: (1) On general grounds, phy s i c a l conditions such as water volume, temperature and av a i l a b l e space are more variable i n f r e s h water than i n the ocean. Moreover, the eggs or a l e v i n s , unlike the ocean f i s h , are unable to seek optimum conditions but must experience the f u l l impact of environmental changes. (2) According to the view expressed on previous pages, v a r i a t i o n s i n f r y production imposed by such changes are r e i n f o r c e d by the depensatory e f f e c t of predation on the outgoing f r y . (3) The apparent independence of even and odd year stocks i n respect to f l u c -tuations suggest that these originate i n f r e s h water, which i s the only phase i n which these two populations are completely separated. (4) A s i g n i f i c a n t c o r r e l a t i o n has been found between a meteorological f a c t o r a f f e c t i n g spawning conditions and v a r i a t i o n s i n the sizes of r e s u l t i n g runs. (5) The output of fry-migrants from f r e s h water i s at times so low that a re-duction i n the adult population must n e c e s s a r i l y r e s u l t unless 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 i n f o r -mation (see F i g . 6). (6) At each of the l o c a l i t i e s f o r which several years' data are a v a i l a b l e (McClinton Creek, Sashin.Creek, Hooknose Creek) there appears to be a r e l a t i o n between the e f f i c i e n c y of freshwater reproduction and the d i r e c t i o n gfi size change i n the r e s u l t i n g adult escapement. The most s t r i k i n g instance has been the change from a l a r g e r odd-year run to a l a r g e r even-year run, observed at Hooknose Creek. This rev e r s a l i n the r e l a t i v e size of the two stocks was c l e a r l y i n i t i a t e d i n f r e s h water. - 31 -In only one important respect does ocean l i f e seem to o f f e r opportun-i t y f o r greater v a r i a b i l i t y i n natural mortality than freshwater existence. This i s i n the a v a i l a b i l i t y of food supplies,^.problem which i s of r e l a t i v e l y small importance i n fresh water. The appreciable differences i n average si z e of i n d i v i d u a l pink salmon i n d i f f e r e n t years (see Hoar, 1951) may r e f l e c t feeding conditions. It i s reasonable to suppose that va r i a t i o n s i n m o r t a l i t y can also be due to t h i s cause. I n c i d e n t a l l y , the growth of the females i n the ocean may also a f f e c t the number of ripe eggs produced, thereby introducing a factor i n t o the production of the next generation. This f a c t o r , however, appears to be small i n r e l a t i o n to observed changes i n population size (see p. 3 I t w i l l , moreover, tend to be o f f s e t by the compensatory m o r t a l i t y attending incubation and alevinage. The role of marine conditions i n determining population s i z e cannot be dismissed as i n s i g n i f i c a n t , although the w r i t e r believes that t h e i r e f f e c t s are u s u a l l y much smaller than those of freshwater f a c t o r s . A question of some' interest and importance i s whether or not ocean mortality tends to be compensatory, - that i s , whether i t tends to o f f s e t the numerical variations i n the f r y populations which enter the sea from year to 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 increasing the output of sea-going migrants. I f the reasons already advanced f o r considering freshwater f a c t o r s to be l a r g e l y res-ponsible f o r determining f l u c t u a t i o n s are v a l i d , then compensatory influences i n the sea, i f they e x i s t , must be small, since on average they do not obscure the e f f e c t s attributed to fresh water. Davidson and Vaughan (1941) have, how-ever, hypothetized the existence of i n t r a s p e c i f i c competition i n the sea as a fa c t o r a f f e c t i n g the average size of i n d i v i d u a l pink salmon of the Clarence S t r a i t area of southeastern Alaska. This view was based on an inverse r e l a t i o n -ship between size of populations and size of individuals, of the adult runs. I t can be maintained on general grounds that pink salmon, however numerous, constitute but a small element i n the cqmplex economy of the sea.• It cannot be supposed that there 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 constant food supply exploited only by pink salmon. I f other species are involved (e.g. herring, chum,salmon and the various other vertebrates and invertebrates which feed on the same kinds of zooplankton) the degree .of competition should be determined by the t o t a l population of t h i s association of species, not merely by the f l u c t u a t i n g abundance of pink salmon. The e f f e c t s would also be d i s t r i b u t e d . In other words, the rather moderate changes i n pink salmon abundance reported by Davidson and Vaughan would have to a l t e r the r e l a t i o n between the available,food supply and a l l the animals using i t before s p e c i f i c e f f e c t s on pink salmon became apparent. O r d i n a r i l y , the chances that such an. i n t e r a c t i o n would be detectable i n the face of annual changes i n food a v a i l a -b i l i t y wrought by such f a c t o r s as water temperature, amount of sunlight or p r e v a i l i n g wind, would see/a to be small. Perhaps a degree of such competition may occur i n areas where the output of young pinks i s very high and the f i s h tend to remain vdthin r e s t r i c t e d ocean areas. On the B r i t i s h Columbia coast Hoar (l.c) found no inverse as s o c i a t i o n between body size and population s i z e . He noted, however, that changes i n body size 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 that odd-year f i s h tend to be l a r g e r than eveh-year f i s h , i n spite of the fact that i n some d i s t r i c t s they are f a r more abundant. Since the even- and odd-year stocks are e f f e c t i v e l y segregated as regards interbreeding, the p o s s i b i l i t y of a genetic f a c t o r may be entertained. ' The idea of compensatory size adjustment i s more or l e s s opposed to compensatory mort a l i t y . The occurrence of the l a t t e r would, of course, tend t o eliminate the differences' i n abundance with which Davidson and Taughan associated large and small f i s h . As mentioned, the evidence at present Fig.6. R e l a t i o n between freshwater survival of pink salmon and eize of spawning stock at various l e v e l s of f i s h i n g i n t e n s i t y . An average natural ocean survival of 5% i s assumed. F.M. • f i s h i n g m o r t a l i t y . -33-available indicates that on the B r i t i s h Columbia coast neither the m o r t a l i t y of a year-class nor the i n d i v i d u a l size of i t s members i s determined to an appreciable degree by i t s own abundance. Perhaps the most convincing argu-ment against the general occurrence of compensatory mortality i n the sea i s to be found i n the large and long-continued d i s p a r i t y i n the numbers of even-and odd-year stocks which has already been mentioned as c h a r a c t e r i s t i c of c e r t a i n areas. A mechanism for e s t a b l i s h i n g and maintaining t h i s d i s p a r i t y has been suggested on previous pages. Such a condition could not be maintained, however, i f ocean mortality tended to favour small populations and reduce large ones. By accepting the view that freshwater and ocean mortality vary i n -dependently and that f i s h i n g m o r tality tends to remove a f a i r l y constant per-centage, c e r t a i n e f f e c t s of varying these elements can be noted. 'The c a l c u l a -tions presented represent averages which might be expected to p r e v a i l over a period long enough to eliminate the e f f e c t s of annual f l u c t u a t i o n s . F i g . 6 shows the e f f e c t on the spawning stock of various rates of f i s h i n g i n t e n s i t y and freshwater s u r v i v a l , on the basis of an average egg-production of 1700 eggs per female and a natural ocean s u r v i v a l of 5$, which has been considered (p. 17) to represent an approximate average f o r t h i s phase.' A 50-50 sex r a t i o i s assumed. The percentage freshwater s u r v i v a l required t o keep the population constant at various l e v e l s of f i s h i n g i n t e n s i t y i s indicated by the " l e v e l of maintenance". I t can be seen that under the postulated conditions the average fr e s h -water s u r v i v a l of an unfished population must be quite low (2.35$) i f the population i s not to keep on increasing. The addition of a 40$ f i s h e r y would require an increase of 67$ ( i . e . to about 4$) i n freshwater s u r v i v a l . A freshwater survival of approximately 6$ ( i . e . about 150$ greater than the s u r v i v a l indicated f o r an unfished population) would permit a f i s h i n g - 34 -mortality of 60%. This i s approximately the condition (both of freshwater 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 ) which i s indicated by the data at present available f o r the c e n t r a l region, i f Hooknose Creek figures can be taken as a guide (p. ). F i g . 6 shows the great increase i n reproductive e f f i c i e n c y which i s required to o f f s e t s t i l l higher l e v e l s of e x p l o i t a t i o n . Whereas a r i s e i n f i s h i n g i n t e n s i t y from 40% to 60% demands an increase of 33% i n f r e s h -water s u r v i v a l , a r i s e from 40% to 80% would demand a 200% improvement i n the e f f i c i e n c y of f r y output. Although compensation on t h i s scale seems s t a r t l i n g i t .has possibly been accomplished at McClinton Creek. The high average fr e s h -water, s u r v i v a l (14.45%) i s accompanied by a low return of adults and i t would be unnecessary to revise our working figure of 5% natural ocean s u r v i v a l i f f i s h i n g m o r t a l i t y amounted to. 80% or 85%. Such a f i s h i n g i n t e n s i t y would not be out of l i n e with the known e x p l o i t a t i o n of Fraser River sockeye runs and could perhaps be r e a d i l y achieved where large populations are funnelled into • a r e s t r i c t e d area such as Masset I n l e t . It may w e l l be, of course, that average ocean s u r v i v a l varies i n d i f f e r e n t areas. If the marine habitat of McClinton Creek f i s h i s l e s s favourable than that of c e n t r a l region f i s h , i t i s not necessary to assume such a high f i s h i n g mortality. Obviously, however, the a b i l i t y of a population to sustain a f i s h e r y w i l l depend upon the extent to which compensation i s possible. With increasing e x p l o i t a t i o n a premium w i l l be placed on stocks which reproduce under such favourable external conditions that, i n the absence of a f i s h e r y , t h e i r con-t i n u a l increase i s l a r g e l y prevented by factors associated with t h e i r own density. The prevalence of compensatory mortality over a wide ranje of popu-l a t i o n l e v e l s has been indi c a t e d at McClinton Creek and under such conditions a high rate of e x p l o i t a t i o n i s possible. It may be confidently expected, however, that i n many streams the e f f i c i e n c y of f r y output i s determined to a much greater extent by extra-pensatory or depensatory fa c t o r s . The p h y s i c a l conditions may be so rigorous - 35 -or so variable that differences in population size do not produce correspond-ing 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 empha-sized that the effect of a fishery is to reduce the mortality which is directly- related 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 distri-buted in the progeny of a pair of fish belonging to a stable, unfished popu-lation. Unfished 60% fishery Parents 2 2 Eggs 1700 1700 Emerging fry 100 250 Fry entering sea 4-0 100 Mature fish 2 5 Escapement 2 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 individ-ual 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 stabil-ized level might not be reached in one generation) but under the mechanism envisaged the initial drop is likely to be sudden and severe. Similarly the building up of a population might continue after a sharp rise. 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 establish-ment 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 excep-tional 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 physi-cal 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 conser-vational 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 fresh-water 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 incom-patible. (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 re-production) 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 depensa-tory mortality, in spite of the increased reproductive efficiency of individ-ual fish. The spawning stock should therefore be increased to the greatest possible extent, even if 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 fish-ing 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 percent-age 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 little fished, in view of the danger that the cycle will be pushed to a persisting low level. The opera-tion 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 c o n n e c t i o n w i t h p i n k s a l m o n , t h e y w i l l be p r e s e n t e d h e r e i n t h e 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 s p e c i e s . EGG-PRODUCTION E g g c o u n t s 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 h a v e b e e n r e p o r t e d a s f o l l o w s b y t h e i n v e s t i g a t o r s whose names a r e a s s o c i a t e d w i t h t h e same l o c a l i t i e s i n T a b l e I . N o . o f N o . o f A v e r a g e N o . L o c a l i t y Y e a r s F i s h o f E g g s Namu 1 21 2760 F r a s e r P a v e r 1 51 2943 N i l e C r e e k 3 47 2726 H o o k n o s e C r e e k 4 114 2254 A v e r a g e o f a l l l o c a l i t i e s - 2671 The g e n e r a l 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 a b o u t 63$ h i g h e r n u m e r i c a l p r o d u c t i o n t h a n t h a t o f p i n k s a l m o n . A s a l r e a d y p o i n t e d o u t ( p . 2.) t h i s i m p l i e s a d i f f e r e n t t o t a l m o r t a l i t y r a t e d u r i n g t h e l i f e c y c l e . O t h e r -w i s e , 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 v e r y r a p i d l y . SEX RATIO The numbers o f e a c h s e x r e c o r d e d i n s p a w n i n g e scapement s t o N i l e C r e e k and H o o k n o s e C r e e k a r e g i v e n i n T a b l e V I I I . I t w i l l be s e e n t h a t t h e d e v i a t i o n s f r o m e q u a l i t y a r e s m a l l and i t seems f a i r t o assume t h a t 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 I n t h e c a s e o f p i n k s a l m o n . MORTALITY : MORTALITY AND SURVIVAL I N FRESH WATER 1 . T o t a l f r e s h w a t e r s u r v i v a l Q u a n t i t a t i v e r e c o r d s f o r t h e e f f i c i e n c y o f r e p r o d u c t i o n d u r i n g t h e f r e s h w a t e r p h a s e i n B r i t i s h C o l u m b i a a r e a v a i l a b l e o n l y f r o m N i l e C r e e k s i n c e 1945 and H o o k n o s e C r e e k s i n c e 1 9 4 7 . The e s t i m a t e d p o t e n t i a l egg d e p o s i t i o n and t h e r e c o r d e d o u t p u t o f f r y m i g r a n t s a t t h e s e t w o l o c a l i t i e s a r e g i v e n i n T a b l e I X . 4 3 T a b l e VIII. S e x r a t i o ( 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 N o . o f L o c a l i t y Y e a r s F i s h M a l e s F e m a l e s A u t h o r i t y N i l e C r e e k 1945 3 , 0 6 2 49 51 N e a v e (1947) 11 n 1946 1 , 8 6 1 51 49 W i c k e t t ( u n p u b . ) tl w 1947 986 52 4 8 tt tt tl tt 1948 386 54 46 « n tt tt 1949 933 4 8 52 n tt H o o k n o s e C r e e k 1947 1 0 , 1 0 6 48 52 H u n t e r (1948) n tt 1948 1 , 0 1 4 51 49 " (1949) tt tt 1949 705 51 49 H ( u n p u b . ) tt n 1 9 5 0 2 , 3 6 2 5 0 50 tt tt T a b l e IX. 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 B r o o d - P o t e n t i a l F r y S u r v i v a l L o c a l i t y y e a r D e p o s i t i o n M i g r a n t s '* A u t h o r i t y N i l e C r e e k 1945 3 , 5 4 0 , 0 0 0 1 3 8 , 3 8 8 3 . 9 0 N e a v e (1947) « M 1946 2 , 1 1 5 , 0 0 0 8 , 3 1 9 0 . 4 0 W i c k e t t ( u n p u b . ) tt tt 1947 1 , 2 7 6 , 0 0 0 4 , 8 0 8 0 . 3 8 tt n tt tt 1948 3 8 5 , 0 0 0 2 3 , 1 8 8 6 . 0 3 it n tt n 1949 1 , 0 2 2 , 0 0 0 782 0 . 0 8 it tt H o o k n o s e C r e e k 1947 1 0 , 9 7 7 , 0 0 0 1 0 8 , 7 4 6 0 . 9 9 H u n t e r (1948) it tt 1948 1 , 0 5 5 , 0 0 0 7 7 , 4 9 7 7 . 3 7 (1949) tt n 1949 7 1 4 , 0 0 0 4 4 , 4 6 3 6 . 2 2 "! ( u n p u b . ) « tt 1950 2 , 8 5 9 , 0 0 0 4 3 1 , 3 4 9 1 5 . 0 9 it tt 44 , The e f f e c t o f t h e r e c o r d e d o u t p u t s , i n t e r m s o f t h e a v e r a g e number 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 spawning f i s h w i t h a p o t e n t i a l d e p o s i t i o n o f 2700 e g g s , w o u l d be a s f o l l o w s : O b s e r v e d m o r - N o o f m i g r a n t s p r o d u c e d R a t i o o f S t r e a m t a l i t y r a n g e ( a ) Maximum .„ ( b ) M i n i m u m (a ) t o ( b ) , N i l e C r e e k 9 3 . 9 7 - 9 9 . 9 2 163 2 . 2 7 5 : 1 H o o k n o s e C r e e k 8 4 . 9 0 - 9 9 . 0 1 410 2 7 . 0 1 5 : 1 W h i l e t h e r a t i o be tween h i g h e s t a n d ' l o w e s t r e c o r d e d e f f i c i e n c y a t H o o k n o s e C r e e k i s a p p r o x i m a t e l y t h e same- a s t h a t o f p i n k s a l m o n i n t h e same s t r e a m ( s e e p . 8 )» much g r e a t e r r e l a t i v e v a r i a t i o n i s e v i d e n t a t N i l e C r e e k , A t b o t h l o c a l i t i e s , h o w e v e r , i t i s o b v i o u s t h a t , 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 s i z e can b e i n i t i a t e d i n f r e s h w a t e r . 2 . I m m e d i a t e . c a u s e s o f d e a t h ' A l l t h e s p e c i f i c c a u s e s 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 s a l m o n ( p p . c a n . b e r e g a r d e d a s p o t e n t i a l s o u r c e s o f m o r t a l i t y f o r t h e p r e s e n t s p e c i e s . 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 o f t h e s e f a c t o r s h a v e a l r e a d y b e e n e m p h a s i z e d b u t t h e r e a p p e a r t o be c e r t a i n q u a n t i t a t i v e d i f f e r e n c e s i n t h e i r e f f e c t s 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 c a n n o t be m e a s u r e d a c c u r a t e l y f r o m e x i s t i n g d a t a . A t t h e t i m e when 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 i n c o a s t a l s t r e a m s h a s u s u a l l y i n c r e a s e d c o n s i d e r a b l y f r o m t h e l o w f l o w s w h i c h p r e v a i l i n A u g u s t and S e p t e m b e r . Hence c e r t a i n f o r m s o f m o r t a l i t y a t t h i s s t a g e o f t h e l i f e c y c l e a r e l i k e l y t o b e s m a l l e r t h a n f o r p i n k s a l m o n . The l o s s e s w h i c h may t h u s be 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 more d i r e c t l y due t o t h e i m p a s s a b i l i t y o f f a l l s ; p r e d a t i o n ; e x c l u s i o n f r o m s p a w n i n g a r e a s b y l a c k o f w a t e r ; u n f a v o u r a b l y h i g h t e m p e r a t u r e . On t h e o t h e r h a n d , t h e l e s s compact s p a w n i n g r u n w h i c h f r e q u e n t l y . c h a r a c t e r i z e s 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 s u p e r i m p o s i t i o n b y t h e l a t e r a r r i v a l s , w h i c h a r e f r e q u e n t l y o b s e r v e d t o s e l e c t t h e same s i t e s a s e a r l i e r 45 f i s h . M o r e o v e r , d e s p i t e t h e l a r g e r number o f e g g s , chum s a l m o n , o w i n g t o t h e much l a r g e r s i z e o f t h e f i s h a n d t h e s p a c e r e q u i r e d f o r r e d d s , p r o b a b l y c a n n o t s e e d a l i m i t e d a r e a a s e f f i c i e n t l y a s p i n k s a l m o n . O v e r c r o w d i n g e f f e c t s may t h u s a p p e a r a t a l o w e r p o p u l a t i o n l e v e l , p a r t i c u l a r l y s i n c e chums a r e a p t t o r e m a i n i n t h e l o w e r p o r t i o n s o f s p a w n i n g s t r e a m s i n s t e a d o f d i s t r i b u t i n g t h e m -s e l v e s t h r o u g h o u t t h e a c c e s s i b l e p o r t i o n s o f t h e w a t e r - s y s t e m . 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 s e a s o n a l o r i r r e g u l a r c h a n g e s i n r u n - o f f . L o s s e s due t o e r o s i o n c a n t h e r e f o r e be s e v e r e . The v e r y l o w p e r c e n t a g e of 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 a t N i l e C r e e k a p p e a r s t o b e a s s o c i a t e d w i t h t h e i n c i d e n c e o f f l o o d s i n a s s o c i a t i o n w i t h a n u n s t a b l e s t r e a m b e d . S e r i o u s l o s s e s a r e a l s o c a u s e d a t t i m e s i n V a n c o u v e r I s l a n d s t r e a m s b y t h e d r y i n g up o f s p a w n i n g beds 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 emergence o f f r y , o r b y t h e t r a p p i n g o f t h e l a t t e r i n p o o l s b e f o r e t h e y c a n r e a c h t h e s e a (Neave 1 9 4 9 ) . 3 . G e n e r a l c o n c l u s i o n s I n H o o k n o s e G r e e k , w h e r e s i z a b l e r u n s o f b o t h s p e c i e s o c c u r , t h e p e r c e n t a g e 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 h a s b e e n a l m o s t i d e n t i c a l f o r p i n k s a n d chums i n each r e c o r d e d y e a r ( T a b l e s I I I , ' I X ) . I n t h i s s t r e a m t h e t w o 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 s p a w n i n g p e r i o d s do n o t d i f f e r g r e a t l y i n t i m e . E c o l o g i c a l l y t h e y a p p r o a c h t h e s t a t u s o f a s i n g l e p o p u l a t i o n . H o w e v e r , t h e mere f a c t 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 -t a i n t h e m s e l v e s i n abundance i n t h e same g e o g r a p h i c a l a r e a s , w i t h n o p h y s i c a l b a r r i e r s a g a i n s t t h e e f f e c t s o f c o m p e t i t i o n , i n d i c a t e s t h a t i n g e n e r a l p i n k a n d chum s a l m o n o c c u p y somewhat d i f f e r e n t e c o l o g i c a l n i c h e s . I n f a c t , one o r o t h e r u s u a l l y p r e d o m i n a t e s i n a g i v e n s t r e a m o r p o r t i o n o f a r i v e r s y s t e m and t h e s i t u a t i o n a t Hooknose C r e e k may r e p r e s e n t a n 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 c o n -d i t i o n b e t w e e n two s u c h n i c h e s o r h a b i t a t s . 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 s t r e a m s w h i c h a r e q u i t e u n s t a b l e i n r e s p e c t t o w a t e r f l o w a n d permanency o f g r a v e l b e d s , o r i n w h i c h l a r g e s t o n e s 46 I r e p l a c e f i n e r d e p o s i t s . A r a t h e r e x t r e m e e x a m p l e i s p r o v i d e d b y N i l e C r e e k . F a c t o r s w h i c h must g i v e t h i s s p e c i e s a n a d v a n t a g e o v e r p i n k s a l m o n i n o f f s e t -t i n g t h e s e r i g o r o u s p h y s i c a l c o n d i t i o n s a r e : (1) t h e g r e a t e r e g g - p r o d u c t i o 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 , o n a p e r c e n t a g e b a s i s , (2) t h e l a r g e r s i z e o f t h e f i s h , e n a b l i n g i t t o move l a r g e r s t o n e s a n d a l s o t o d e p o s i t i t s egg s a t a d e e p e r l e v e l , (3) t h e l e s s r i g i d l i f e s p a n , which , p r e v e n t s t h e e f f e c t s o f a " b a d y e a ^ s " r e p r o d u c t i o n f r o m b e i n g e n t i r e l y l o c a l i z e d i n a s i n g l e r e c u r r e n t c y c l e . T h e s e a d v a n t a g e s , w h i c h c o u l d a c c o u n t f o r t h e g r e a t e r s u c c e s s o f 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 , do n o t e x p l a i n t h e f a i l u r e o f t h e 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 p h y s i c a l h a b i t a t s o f t h e l a t t e r . I n t h i s c o n n e c t i o n i t may a l s o be s u p p o s e d t h a t t h e b a l a n c e o f m o r t a l i t y due t o s u p e r i m p o s i t i o n w o u l d f a v o u r t h e l a t e r - s p a w n i n g c h u m . The f o l l o w i n g s u g g e s t i o n s , h o w e v e r , c a n be o f f e r e d . (1) A s a l r e a d y p o i n t e d o u t , chum s a l m o n do n o t d i s t r i b u t e t h e m s e l v e s a s a d v a n t a g e o u s l y i n r e l a t i o n t o t h e p o t e n t i a l s p a w n i n g g r o u n d s a n d p r o b a b l y c a n n o t s e e d u t i l i z e d g r o u n d a s t h o r o u g h l y . (2) The l a t e r a n d more p r o l o n g e d spav in ing s e a s o n o f t h e chum p r o d u c e s a l a t e r and l e s s c o n c e n t r a t e d f r y m i g r a t i o n . T h i s p r o b a b l y r e s u l t s i n h e a v i e r l o s s e s t o p r e d a t o r s ( s ee p . SI ) w h i c h c a n be e x p e c t e d t o be e s p e c i a l l y numerous i n t h e r e l a t i v e l y s t a b l e s t r e a m s w h i c h a r e p r e f e r r e d b y p i n k s a l m o n . I n g e n e r a l s u p p o r t o f t h e s e v i e w s o n t h e d i f f e r e n t a d a p t a t i o n s o f t h e two s p e c i e s , c e r t a i n p o p u l a t i o n c h a n g e s w h i c h a p p e a r t o h a v e t a k e n p l a c e i n some V a n c o u v e r I s l a n d s t r e a m s may b e c i t e d . F o l l o w i n g d e f o r e s t a t i o n , w h i c h i s p re sumed t o have 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 , p i n k salmonj. p o p u l a t i o n s 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 de Cosmos C r e e k , O y s t e r R i v e r a n d N i l e C r e e k . A t t h e same t i m e chum s a l m o n w e r e a b l e t o m a i n t a i n t h e i r numbers s u f f i c i e n t l y w e l l t o become t h e d o m i n a n t s p e c i e s i n t h e s e w a t e r s . 47 I t i s c o n c l u d e d t h a t , 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 s a l m o n i s somewhat l e s s , o n a p e r c e n t a g e b a s i s , t h a n t h a t o f p i n k s a lmon and t h a t i t a l s o t e n d s t o b e more v a r i a b l e . MORTALITY AND SURVIVAL I N THE OCEAN B e c a u s e o f t h e l o n g e r l i f e s p a n o f chum s a l m o n , d a t a h a v e n o t y e t been a c c u m u l a t e d w h i c h w o u l d 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 o u t g o i n g f r y f r o m a g i v e n s t r e a m w h i c h e v e n t u a l l y r e t u r n t o f r e s h w a t e r . H o w e v e r , i f we • a c c e p t 5 $ a s 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 o c e a n s u r v i v a l o f p i n k s a l m o n , a s i m p l e c a l c u l a t i o n w i l l s u g g e s t r a t h e r n a r r o w l i m i t s w i t h i n w h i c h 1 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 . On t h e b a s i s o f e g g - p r o d u c t i o n , t o t a l s u r v i v a l f r o m e g g t o s p a w n i n g a d u l t ( i n s t a b l e p o p u l a t i o n s ) s h o u l d be 1 7 / 2 7 t i m e s p i n k s a l m o n s u r v i v a l . I f t h e h i g h e r m o r t a l i t y o f t h e chum t a k e s p l a c e 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 o c e a n s u r v i v a l s h o u l d be t h e same i n b o t h s p e c i e s . I f t h e d i f f e r e n c e l i e s w h o l l y i n t h e s a l t w a t e r p h a s e , t h e s u r v i v a l 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 = 3 . 1 5 $ . The p o s s i b i l i t y t h a t chum s a l m o n s u r v i v a l i s much l o w e r i n one phase a n d somewhat h i g h e r i n t h e o t h e r i s d i f f i c u l t t o e n t e r t a i n . We h a v e a l r e a d y e x p r e s s e d t h e v i e w t h a t a v e r a g e f r e s h -w a t e r s u r v i v a l i s l o w e r t h a n f o r p i n k s a l m o n . M a r i n e s u r v i v a l w o u l d n o t . b e e x p e c t e d t o b e h i g h e r t h a n f o r p i n k s , i n v i e w o f t h e l o n g e r p e r i o d f o r w h i c h ' t h e chum i s e x p o s e d t o o c e a n h a z a r d s . D u r i n g a c o n s i d e r a b l e p a r t o f t h e i r f i r s t summer, when 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 p re sumed t o o c c u r , t h e t w o 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 be pre sumed t o be o f s i m i l a r m a g n i t u d e . I t i s t h e r e f o r e s u g g e s t e d t h a t a v e r a g e n a t u r a l o c e a n 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 4 $ . 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 b y t h e same methods a s p i n k s a l m o n f i s h i n g . 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 T a b l e X . R e p o r t e d c a t c h and e scapement 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 Y e a r ' C a t c h E sca p e m e nt T o t a l r u n fo c a u g h t 1934 i 5 8 9 , 5 6 0 5 5 8 , 3 7 5 1 , 1 4 7 , 9 3 5 51 1935 1 , 0 2 9 , 6 4 0 9 5 7 , 2 0 0 1 , 9 8 6 , 8 4 0 52 1936 6 6 8 , 0 4 0 1 , 5 1 5 , 8 5 0 2 , 1 8 3 , 8 9 0 3 1 193? 8 9 9 , 4 5 0 8 2 8 , 7 2 5 1 , 7 2 8 , 1 7 5 52 1938 9 9 3 , 8 5 0 9 6 2 , 9 5 0 1 , 9 5 6 , 8 0 0 51 1939 5 9 8 , 9 9 0 4 1 4 , 7 7 5 1 , 0 1 3 , 7 6 5 59 1 9 4 0 5 6 8 , 0 9 0 6 4 0 , 8 7 5 1 , 2 0 8 , 9 6 5 47 1941 8 3 6 , 7 0 0 9 6 2 , 9 7 5 1 , 7 9 9 , 6 7 5 47 194E 5 9 1 , 3 7 0 5 4 4 , 8 0 0 1 , 1 3 6 , 1 7 0 52 1943 8 3 3 , 6 0 0 6 5 4 , 2 5 0 1 , 4 8 7 , 8 5 0 56 1944 5 8 7 , 0 1 0 4 6 4 , 6 5 0 1 , 0 5 1 , 6 6 0 56 1945 1 , 0 3 5 , 4 6 0 1 , 0 9 2 , 2 2 5 , 2 , 1 2 7 , 6 8 5 49 1946 1 , 8 0 1 , 4 1 0 8 7 4 , 4 5 0 2 , 6 7 5 , 8 6 0 67 1947 2 , 2 4 3 , 0 1 0 . 1 , 1 7 8 , 2 0 0 3 , 4 2 1 , 2 1 0 66 1948 1 , 8 0 9 , 1 6 0 3 8 5 , 2 5 0 2 , 1 9 4 , 4 1 0 82 1949 1 , 0 9 8 , 5 9 0 7 0 5 , 0 8 0 1 , 8 0 3 , 6 7 0 56 49 A c c o r d i n g t o t h e s e f i g u r e s t h e a v e r a g e c a t c h 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 c a s e w i t h p i n k s a l m o n , — a b o u t 60% f o r t h e l a s t 8 y e a r s s h o w n , a s a g a i n s t 70% f o r t h e l a t t e r s p e c i e s . A somewhat 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 e x p e r i m e n t s i n v o l v i n g t h e t w o s p e c i e s ( p . 17 ) . 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 o n l y 16% a s compared w i t h a b o u t 30% f o r p i n k s . The c o n c l u s i o n seems q u i t e j u s t i f i e d t h a t t h e t o t a l 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 s a l m o n s t o c k s . T h i s c o n c l u s i o n i s i n g e n e r a l agreement w i t h r e s u l t s o b t a i n e d f r o m f i s h t a g g e d i n J o h n s t o n e S t r a i t i n 1 9 4 5 , a l t h o u g h q u i t e 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 s o u t h e r n 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 . D e L a c y & N e a v e 1 9 4 7 ; F o e r s t e r & C h a t w i n 1 9 5 1 ) . A v e r a g e 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 , b y r e a s o n i n g a l o n g l i n e s i n d i c a t e d on page 17 , s h o u l d a p p r o x i m a t e 4 0 - 50%. A l o w e r r a t e 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 s i n c e 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 compact 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 c o v e r e d b y t h e f i s h i n g s e a s o n ( a l t h o u g h t e m p o r a r y c l o s u r e s o f t h e f i s h e r y a r e a p p l i e d i n a c c o r d a n c e w i t h e s t i m a t e d c o n s e r v a t i o n a l r e q u i r e m e n t s ) . CHANGES I N ABUNDANCE OF ADULT CHUM SALMON E x a m p l e s o f a n n u a l c h a n g e s i n chum s a l m o n r u n s , c a t c h e s o r e s c a p e -ment s a r e p r e s e n t e d i n F i g s . 7 t o 9 . The f l u c t u a t i o n s shown b y t h e s e g r a p h s a r e o f c o n s i d e r a b l e a m p l i t u d e and i r r e g u l a r f r e q u e n c y . T h e r e i s no t e n d e n c y f o r c y c l i c a l r e c u r r e n c e o f l a r g e o r s m a l l r u n s ; n o r i s t h e r e e v i d e n c e o f s u d d e n change t o p e r s i s t e n t new l e v e l s o f h i g h e r o r l o w e r a b u n d a n c e . These f e a t u r e s a r e no d o u b t a s s o c i a t e d w i t h t h e v a r y i n g l i f e 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 o v e r t w o o r more y e a r s t h e e f f e c t s o f m o r t a l i t y changes t a k i n g p l a c e i n a s i n g l e s e a s o n . N e v e r t h e l e s s , p r o n o u n c e d p e a k s a n d d e p r e s s i o n s o c c u r . T h e s e do n o t s y n c h r o n i z e w i t h t h e Fig.7* Reported catch and escapement of chum salmon i n the central region - of B r i t i s h Columbia. (1).Total run (2) Catch.' 50 i l e n g t h , o f t h e l i f e 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 ( w h i c h i s u s u a l l y t r u e f o r a l a r g e m a j o r i t y o f t h e i n d i v i d u a l f i s h ) o r w h e t h e r some a l l o w a n c e be made f o r a p e r c e n t a g e o f f i s h o f o t h e r a g e s . E v i d e n t l y abundance i s commonly d e t e r m i n e d b y f a c t o r s o t h e r t h a n t h e s i z e o f t h e e s c a p e m e n t . THE MECHANISM CONTROLLING POPULATION L E V E L S A s i s t h e ca se w i t h p i n k s a l m o n , t h e m o r t a l i t y a t t e n d i n g t h e l i f e c y c l e o f t h e chum may be c o m p e n s a t o r y , d e p e n s a t o r y o r e x t r a p e n s a t o r y i n i t s e f f e c t s . C o m p e n s a t o r y m o r t a l i t y i s a g a i n a t t e s t e d b y t h e f a c t t h a t t h e s p e c i e s i s s t i l l 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 , even i f somewhat l e s s s e v e r e t h a n t h a t w h i c h h a s b e e n a p p l i e d t o p i n k s a l m o n , c a n y e t be d e s c r i b e d a s i n t e n s i v e . I t must a g a i n be s u p p o s e d t h a t t h i s t y p e o f m o r t a l i t y i s c h a r a c t e r -i s t i c o f t h e p e r i o d b e t w e e n t h e e n t r a n c e o f t h e a d u l t s i n t o f r e s h w a t e r and . t h e emergence o f t h e f r y , t h a t i s , t h e p e r i o d i n w h i c h d e n s i t y i s a t a maximum 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 . O w i n g , h o w e v e r , t o t h e v a r i a b l e p h y s i c a l c o n d i t i o n s i n t h 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 good i n v e r s e r e l a t i o n s h i p be tween p o p u l a t i o n s i z e and r e p r o d u c t i v e e f f i c i e n c y c a n n o t be d e m o n s t r a t e d b y a s h o r t s equence o f a n n u a l r e c o r d s . I t w o u l d p r e s u m a b l y become a p p a r e n t a s a l o n g - t e r m a v e r a g e b u t i n any g i v e n y e a r c o m p e n s a t o r y e f f e c t s a r e l i k e l y t o be o u t w e i g h e d b y e x t r a p e n s a t o r y f a c t o r s . I n v i e w o f t h e c h a r a c t e r o f t h e s t r e a m s commonly u t i l i z e d b y chums, t h i s i s l i k e l y t o be g e n e r a l l y t r u e f o r t h e s p e c i e s . The e f f e c t s o f p r e d a t i o n o n m i g r a t i n g f r y h a v e b e e n i n v e s t i g a t e d a t N i l e C r e e k and Hooknose 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 a d i s t a n c e above t h e c o u n t i n g f e n c e a t i n t e r v a l s d u r i n g t h e p e r i o d o f f r y m i g r a t i o n . A t N i l e C r e e k e x p e r i m e n t s w e r e c o n d u c t e d b y t h e w r i t e r i n 1946 and 1947 a n d i n s u b s e q u e n t y e a r s b y M r . W . P . W i c k e t t . 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 i n I T 1 1 — i — 1 1 rr o \ Fig.10. Survival of marked chum fry-migrants i n Hooknose Creek,in r e l a t i o n to number of f i s h : migrating and progress of time. 0———O average d a i l y number of surviving fry-migrants • % su r v i v a l . o f marked f r y ®" —. estimated average d a i l y number, of emerged f r y (before predation) ^•estimated from samples comprising more than h a l f the t o t a l surviving fry-migrants. 51 i 1951 a t Hooknose C r e e k b y M r , J . G . H u n t e r , a t t h e w r i t e r * s r e q u e s t . The g e n e r a l f e a t u r e s o f p r e d a t i o n d u r i n g t h i s p h a s e o f t h e l i f e h i s t o r y o f t h e chum sa lmon a r e i n c o n f o r m i t y w i t h t h e d a t a o b t a i n e d b y Cameron (1941) f o r p i n k s a l m o n a t M c C l i n t o n C r e e k . C o n c l u s i o n s may be s u m m a r i z e d a s f o l l o w s : 1 . P e r c e n t a g e m o r t a l i t y i n c r e a s e s w i t h t h e d i s t a n c e o v e r w h i c h t h e f r y t r a v e l . 2 . P e r c e n t a g e m o r t a l i t y d e c r e a s e s w i t h i n c r e a s i n g number o f m i g r a n t s . 3 . P e r c e n t a g e m o r t a l i t y i n c r e a s e s d u r i n g t h e p r o g r e s s o f t h e r u n . P o i n t s ( S ) and ( 3 ) , w h i c h a r e o f c h i e f i n t e r e s t f o r o u r p r e s e n t s t u d y , a r e i l l u s t r a t e d b y F i g . 1 0 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 H o o k n o s e C r e e k . 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 p e a k , week o f t h e m i g r a t i o n , a g e n e r a l r e l a t i o n s h i p be tween number o f m i g r a n t s a n d p e r c e n t a g e s u r v i v a l i s i n d i c a t e d , as i s a l s o t h e f a c t t h a t p r e d a t i o n becomes , more i n t e n s i v e d u r i n g t h e l a t e r weeks o f t h e r u n . The end r e s u l t t h e r e f o r e a p p e a r s t o depend o n ( a ) t h e s i z e o f t h e f r y p o p u l a t i o n , (b ) t h e t i m i n g o f t h e m i g r a t i o n , and ( c ) t h e c o m p a c t n e s s o f t h e m i g r a t i o n , ( a ) i n v o l v e s a d e p e n s a t o r y r e l a t i o n s h i p . V a r i a t i o n s i n (b ) and ( c ) w i l l be e x t r a p e n s a t o r y i n e f f e c t a n d may t h e r e f o r e e i t h e r r e i n f o r c e o r r e d u c e t h e e f f e c t s due t o ( a ) . F o r e x a m p l e , t h e a d v a n t a g e p o s s e s s e d b y a l a r g e p o p u l a t i o n w o u l d p r e s u m a b l y be r e d u c e d i f i t s 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 t h e numbers m i g r a t i n g a t a n y one t i m e were r e l a t i v e l y s m a l l . 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 a d i s a d v a n -t a g e u n l e s s a s i m i l a r l a g o c c u r r e d i n t h e p r e s e n c e o r a c t i v i t y o f p r e d a t o r s . I n c o m p a r i n g t h e f r y r u n s o f d i f f e r e n t y e a r s , t h e f o l l o w i n g e s t i m a t e s 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 C r e e k . Y e a r R e c o r d e d f r y E s t i m a t e d S u r v i v a l m i g r a t i o n emergence $ 1946 1 3 8 , 3 8 8 2 2 3 , 0 0 0 62 1947 8 , 3 1 9 1 8 , 5 0 0 45 1948 4 , 8 0 8 8 , 6 0 0 56 1949 2 3 , 1 8 8 6 6 , 8 0 0 35 1950 782 2 , 1 7 0 36 52 The s i z e o f t h e r u n s a t N i l e C r e e k d u r i n g t h e s e y e a r s h a s b e e n so s m a l l t h a t l i t t l e r a n g e i s p r e s e n t e d f o r e x h i b i t i n g d e e s m p e n s a t o r y e f f e c t s a t v a r y i n g l e v e l s o f a b u n d a n c e . H o w e v e r , t h e one " l a r g e " r u n w h i c h p r o d u c e d 1 3 8 , 0 0 0 s u r v i v o r s was a c c o m p a n i e d by 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 f o u r 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 2 3 , 0 0 0 i n d i v i d u a l s showed a n a v e r a g e m o r t a l i t y r a t e o f a b o u t 57%. 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 g r e a t e r , s i n c e t h e 1946 t e s t s .were 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 p r e s u m a b l y i n v o l v e d i n c r e a s e d m o r t a l i t y . I t i s c o n c l u d e d t h a t t h e v e r y c o n s i d e r a b l e e f f e c t s o f f r y l o s s e s 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 o t h e r c a u s e s . Wherea s c o m 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 m a i n t a i n a u n i f o r m abundance 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 and p e r p e t u a t e s u c h d e v i a t i o n s a s c a n s t i l l a r i s e , t h e c o n s i d e r a b l e and i r r e g u l a r f l u c t u -a t i o n s w h i c h i n f a c t o c c u r p o i n t t o t h e v a r i a b i l i t y o f m o r t a l i t y w h i c h c a n n o t 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 . I t h a s a l r e a d y b e e n s u g g e s t e d (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 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 t o 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 a n d t h e s e a r e r e g a r d e d a s p r o v i d i n g a m a j o r s o u r c e o f t h e f l u c t u a t i o n s o b s e r v e d . O t h e r e x t r a p e n s a t o r y v a r i a t i o n s a r e no d o u b t imposed i n t h e o c e a n . The 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 and chum s a l m o n . The f e a t u r e s w h i c h d i s t i n g u i s h t h e p a t t e r n o f v a r i a t i o n o f t h e l a t t e r s p e c i e s a r e a s s o c i a t e d w i t h t h e r e l a t i v e l y g r e a t e r p a r t p l a y e d b y e x t r a p e n s a t o r y m o r t a l i t y and t h e g r e a t e r f l e x i b i l i t y o f t h e l i f e s p a n . RELATIVE EFFECTS OF FRESHWATER, NATURAL OCEAN AND FISHING- MORTALITY I N DETERiaNnsTG POPULATION L E V E L S A s a l r e a d y i n d i c a t e d , a v e r a g e m a r i n e m o r t a l i t y i s c o n s i d e r e d t o be 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 o f p i n k s a l m o n a n d t h e r e seems t o . be no r e a s o n f o r s u p p o s i n g t h a t i t i s more v a r i a b l e . The s i t u a t i o n i s 53 o t h e r w i s e , h o w e v e r , w i t h r e g a r d t o f r e s h w a t e r l o s s e s . The c o m b i n e d r e s u l t s o f 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 1 5 $ t o 0 . 0 8 $ , o r a r a t i o o f 1 8 7 : 1 . S u c h e x t r e m e v a r i a t i o n o f c o u r s e c a n n o t b e e x p e c t e d 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 a r e a s c o n t a i n -i n g numerous s p a w n i n g 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 o p i n i o n e x p r e s s e d p r e v i o u s l y t h a t m a r i n e m o r t a l i t y i s l a r g e l y n o n - c o m p e n s a t o r y , i t g i v e s good g r o u n d s 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 c o n d i t i o n s p l a y t h e m a j o r p a r t i n d e t e r m i n i n g changes i n t h e abundance o f a d u l t p o p u l a t i o n s . T h i s c o n -c l u s i o n i s s u p p o r t e d b y t h e c o r r e l a t i o n e s t a b l i s h e d b y W i c k e t t (Neave a n d W i c k e t t 1 9 4 9 ) b e t w e e n s e a s o n a l s t r e a m f l o w and t h e a b u n d a n c e o f a d u l t chums 4 y e a r s l a t e r , i n t h e V a n c o u v e r I s l a n d d i s t r i c t . The e f f e c t o n t h e s p a w n i n g s t o c k o f v a r y i n g r a t e s o f f r e s h w a t e r s u r v i v a l a n d f i s h i n g i n t e n s i t y a r e shown i n F i g . 1 1 . I n t h e s e g r a p h s a f i g u r e o f 4 $ h a s b e e n assumed t o r e p r e s e n t a v e r a g e n a t u r a l o c e a n s u r v i v a l ( p . 4 / ) . A v e r a g e e g g - p r o d u c t i o n p e r p a i r o f f i s h i s t a k e n a s 2 7 0 0 . 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 t h e a v e r a g e f r e s h w a t e r s u r v i v a l i n t h e a b s e n c e o f a n y f i s h e r y s h o u l d be l e s s t h a n 2 $ . To m a i n t a i n a c o n s t a n t 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 h a s b e e n s u g g e s t e d a s a n a p p r o x i m a t e a v e r a g e ( p . 4 f ) , f r e s h w a t e r s u r v i v a l s h o u l d be 3 . 7 $ . I t can be s e e n f r o m T a b l e I X t h a t i n t h e c o m b i n e d N i l e C r e e k a n d Hooknose C r e e k r e c o r d s t h i s l e v e l h a s b e e n d e f i n i t e l y e x c e e d e d i n f o u r i n s t a n c e s , d e f i n i t e l y n o t a t t a i n e d i n f o u r o t h e r i n s t a n c e s a n d 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 i n s t a n c e . 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 o f l a r g e a r e a s a r e d i f f i c u l t t o a s s e s s b e c a u s e ( a ) t h e c o l l e c t i o n o f a c c u r a t e s t a t i s t i c s h a s been impeded b y 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 c o m m e r c i a l l y p r o c e s s e d and s o l d ( H o a r 1 9 5 1 ) (b ) t h e l a r g e 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 sound 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 y e a r s . u AS + 90 +80 + 70 +60 +so +40 ••JO >Z0 \ +10 > 0 .H -to 1 ^ -20 -30 -AO -50 -60 i r T r 8 10 12. IA (6 |8 ZO °/o freshwater s u r v i v a l Fig.11. R e l a t i o n between freshwater survival of chum salmon and size of spawning stock a t various l e v e l s of f i s h i n g i n t e n s i t y . An average natural ocean survival of 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 calcu-lation, produce a smaller returning population even if 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 w i l l be e x p o s e d . The minimum s i z e o f escapement r e q u i r e d t o p r o d u c e a g i v e n o u t p u t o f s e a - g o i n g f r y w o u l d v a r y g r e a t l y f r o m y e a r t o y e a r . Tne e x i s t e n c e o f a n u n d e r l y i n g c o m p e n s a t o r y 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 e x t r a p e n s a t o r y v a r i a t i o n s w h i c h a r e i m p o s e d u p o n i t a r e so l a r g e t h a t " s l u m p s " and "booms" f r e q u e n t l y o c c u r . The d i f f e r e n c e 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 r e s p e c t i s o f c o u r s e i n d e g r e e o n l y and i s c o n s i d e r e d t o b e l a r g e l y a r e f l e c t i o n o f an a v e r a g e d i f f e r e n c e i n f r e s h w a t e r h a b i t a t . U n d e r s i m i l a r c o n d i t i o n s , a s shown by t h e r e s u l t 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 r e a c t i n a s i m i l a r m a n n e r . S p e a k i n g b r o a d l y , p r o n o u n c e d i n s t a b i l i t y o f p h y s i c a l c o n d i t i o n s i n spawning s t r e a m s i s v i e w e d a s 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 , b y r e a s o n o f t h e l o w e g g - p r o d u c t i o n a n d r i g i d l i f e s p a n . W i t h chum : sa lmon i t i n d u c e s g r e a t v a r i a t i o n i n p o p u l a t i o n s i z e . M a i n t e n a n c e o f f a v o u r a b l e c o n d i t i o n s d u r i n g i n c u b a t i o n and a l e v i n a g e w o u l d t e n d b o t h t o i n c r e a s e t h e a v e r a g e o u t p u t and t o r e d u c e 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 s p a w n i n g s t r e a m s ( o r p o r t i o n s o f t h e m ) , i t c o u l d b e e x p e c t e d t h a t c o m p e n s a t o r y e f f e c t s w o u l d become a p p a r e n t , - - t h a t i s , a s t r i c t e r r e l a t i o n s h i p be tween t h e s i z e o f t h e escapement and t h e 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 w o u l d be o b s e r v e d . 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 i n c r e a s e d s t a b i l i t y m i g h t t e n d t o e n a b l e p i n k s a l m o n t o d i s p l a c e chums i s a l s o p r e s e n t e d . The g e n e r a l e f f e c t s 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 r e s p e c t t o t h e f r y - m i g r a n t s o f b o t h s p e c i e s , i n t h a t s m a l l p o p u l a t i o n s w i l l t e n d t o s u f f e r r e l a t i v e l y h e a v i e r l o s s e s t h a n l a r g e p o p u l a t i o n s . E l i m i n a t i o n o f , o r p r o t e c t i o n f r o m , p r e d a t o r s s h o u l d t h e r e f o r e be a m a j o r c o n s i d e r a t i o n i n e s t a b l i s h i n g new r u n s o r i n b u i l d i n g up 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 o f a b u n d a n c e . A C K N C M ^ G E M E N T S The w r i t e r i s 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 t h e D e p a r t m e n t o f 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 s t i m u l a t i o n , 56 encouragement a n d f a c i l i t i e s f o r t h e p r e p a r a t i o n o f t h i s t h e s i s . The F i s h e r i e s R e s e a r c h B o a r d o f C a n a d a , u n d e r whose a u s p i c e s t h e 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 , h a s a l s o g e n e r o u s l y a f f o r d e d h i m t h e t i m e 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 t h e p r e s e n t p u r p o s e . The s y m p a t h e t i c 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 and D r . J . L . H a r t , s u c -c e s s i v e D i r e c t o r s o f t h e P a c i f i c B i o l o g i c a l S t a t i o n , N a n a i m o , h a s b e e n a n e s s e n t i a l f e a t u r e i n t h e p r o g r e s s o f t h i s s t u d y . The w r i t e r i s i n d e b t e d t o h i s c o l l e a g u e s a t t h e P a c i f i c B i o l o g i c a l S t a t i o n , a n d e s p e c i a l l y t o M r . ¥ . P . W i c k e t t and M r . J . G . H u n t e r , f o r t h e c o l l e c t i o n o f d a t a , f o r g e n e r a l 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 i n t h e f i e l d o f i n q u i r y t o w h i c h t h i s t h e s i s i s r e l a t e d . REFERENCES C a m e r o n , 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 o f t h e n a t u r a l s p a w n i n g , i n c u b a t i o n and a l e v i n a g e o f t h e p i n k s a l m o n ( O n c o r h y o c h u s g o r b u s c h a ) . MS r e p o r t . 1 9 3 9 . M o r t a l i t y d u r i n g t h e f r e s h - w a t e r e x i s t e n c e o f t h e p i n k s a l m o n . MS r e p o r t . 1 9 4 1 . D a v i d s o n , F , A . a n d E l i z a b e t h V a u g h a n . R e l a t i o n o f p o p u l a t i o n s i z e t o m a r i n e g r o w t h and t i m e o f s p a w n i n g m i g r a t i o n i n t h e p i n k , s a l m o n ( O n c o r h y n c h u s  g o r b u s c h a ) o f s o u t h e a s t e r n A l a s k a . J . M a r i n e R e s e a r c h 4 ( 3 ) : 2 3 1 - 2 4 6 . 1 9 4 1 . D e L a c y , A l l a n , C a n d N e a v e , F . M i g r a t i o n o f p i n k s a l m o n i n s o u t h e r n 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 . R e s . B d . C a n a d a , B u l l . 7 4 . 1 9 4 7 . D e p a r t m e n t o f M a r i n e and F i s h e r i e s , C a n a d a . M o n t h l y r e c o r d o f m e t e o r o l o g i c a l o b s e r v a t i o n s . T o r o n t o . F o e r s t e r , 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 s a l m o n ( O n c o r h y n c h u s 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 , s e x p r o p o r t i o n s and p r o g r e s s o f m i g r a t i o n . J . B i o l . B d . C a n a d a , 3 ( l ) : 2 6 - 4 2 . 1 9 3 6 . a n d P r i t c h a r d , A . L . The egg c o n t e n t o f P a c i f i c S a l m o n . P r o g . R e p t s . P a c . C o a s t S t a t i o n s , 2 8 : 3 - 5 . 1 9 3 6 . H o b b s , 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 a n d 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 . , F i s h e r i e s B u l l . 6 . 1 9 3 7 . H o a r , W . S . The chum and 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 . R e s . B d . C a n a d a , B u l l , ( i n p r e s s ) . 57 H u n t e r , J . G . N a t u r a l p r o p a g a t i o n o f s a l m o n i n t h e c e n t r a l c o a s t a l a r e a o f B r i t i s h C o l u m b i a . P r o g . R e p t s . P a c . C o a s t S t a t i o n s , 7 7 : 1 0 5 - 6 . 1 9 4 8 . N a t u r a l p r o p a g a t i o n o f s a l m o n i n t h e c e n t r a l c o a s t a l a r e a o f B r i t i s h C o l u m b i a . I I . The 1948 r u n . P r o g . R e p t s . P a c . C o a s t S t a t i o n s , 7 9 : 3 3 - 4 . 1 9 4 9 . N e a v e , F . N a t u r a l p r o p a g a t i o n o f chum s a l m o n i n a c o a s t a l s t r e a m . P r o g . R e p t s . P a c . C o a s t S t a t i o n s , 7 0 : 2 0 - 2 1 . 1 9 4 7 . Game f i s h p o p u l a t i o n s o f t h e C o w i c h a n r i v e r . F i s h . R e s . B d . C a n a d a , B u l l . 8 4 . 1 9 4 9 . and W i e k e t t , W . P . F a c t o r s a f f e c t i n g t h e f r e s h w a t e r d e v e l o p m e n t o f P a c i f i c s a l m o n i n B r i t i s h C o l u m b i a . M S , P a c i f i c S c i e n c e C o n g r e s s . 1 9 4 9 . P r i t c h a r d , A . 1>. Homing t e n d e n c y and age o f m a t u r i t y o f p i n k s a l m o n ( O n c o r h y n c h u s  g o r b u s c h a ) i n B r i t i s h C o l u m b i a . J". F i s h . R e s . B d . C a n a d a , 4 : 2 3 3 - 2 5 1 . 1 9 3 9 . E f f i c i e n c y o f n a t u r a l p r o p a g a t i o n o f t h e p i n k s a l m o n ( O n c o r h y n -c h u s g o r b u s c h a ) i n M c C l i n t o n C r e e k , B . C . J . F i s h . R e s . B d . C a n a d a , 7 : 2 2 4 - 2 3 6 . 1 9 4 8 a . . A d i s c u s s i o n o f t h e m o r t a l i t y i n p i n k s a l m o n ( O n c o r h y n c h u s g o r b u s c h a ) d u r i n g t h e i r p e r i o d o f m a r i n e l i f e . T r a n s . R o y . S o c . C a n a d a , 4 2 , s e c t . . 5 : 1 2 5 - 1 3 3 . 1 9 4 8 b . a n d D e L a c y , A . C . M i g r a t i o n o f p i n k s a l m o n ( O n c o r h y n c h u s g o r b u s c h a ) i n s o u t h e r n 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 3 . F i s h . R e s . B d . C a n a d a , B u l l . .66. 1 9 4 4 . R o b e r t s o n , J . G . D e t e r m i n a t i o n and d i f f e r e n t i a t i o n o f s e x i n P a c i f i c s a l m o n . M . A . t h e s i s , U n i v . B . C , 1 9 5 1 . Shuman, R . F . On t h e e f f e c t i v e n e s s o f s p e r m a t o z o a o f t h e p i n k s a l m o n ( O n c o r h y n - c h u s gorbuscha i ) a t v a r y i n g d i s t a n c e s f r o m p o i n t - o f d i s p e r s a l . U . S . F i s h and W i l d l i f e S e r v i c e , F i s h . B u l l . 5 1 : 3 5 9 - 3 6 3 . 1 9 5 0 . S o l o m o n , M . E . The n a t u r a l c o n t r o l o f a n i m a l p o p u l a t i o n s . J . A n i m a l I c o l . . 1 8 : 1 - 3 5 . 1 9 4 9 . 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0302621/manifest

Comment

Related Items