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

Abstract

Changes in population size are governed by the birthrate, the sex ratio and the death rate. In pink salmon the average egg-production per female is about 1700 and variations from this average are insufficient to account for observed changes in adult populations. The sex ratio is approximately 50-50. Survival during the freshwater phases of the life cycle has been found to vary from approximately 1% to 24%,the average survival being significantly different in different streams. Variation is relatively greater in streams in which average survival is low. Natural survival in the ocean is considered, to average about 5% of the number of young fish reaching the sea. In the central region of the British Columbia coast the annual catch averages about 60% of the adult fish, this percentage being relatively constant for both small and large runs. Pink salmon maturing in "even" and "odd" years represent separate populations. These populations vary in size independently but may maintain a relatively constant ratio for a series of generations. This ratio varies from near equality to extreme disparity. Marked changes in the level of abundance may occur suddenly. Three types of mortality are recognized: (a) mortality which becomes relatively heavier as populations increase in density (compensatory) (b) mortality which becomes relatively heavier as populations decrease in density (depensatory) (c) mortality which is independent of density (extrapensatory). (a) is especially identified with the period of spawning and incubation;(b) is considered to occur mainly during the period of fry migration and to be due to predation;(c) may occur at any stage but is probably most variable during the period between entrance of the adults into fresh water and emergence of the fry. Population changes initiated by extrapensatory factors, among which stream-flow conditions are considered to be important, are exaggerated by depensatdry factors (notably predation on fry) but tend to be resisted by the compensatory influences which operate during the period of spawning and incubation. Stabilization of a level of abundance depends on a balance between these processes. In general, freshwater mortality is more variable than ocean mortality and plays a greater part in inducing population changes. It is suggested that the average freshwater survival of an un-fished population would approximate 2.4% and that this efficiency must be raised to about 6% to permit a sustained catch of 60% of the adult population. A lower freshwater output is likely to result in a reduction in size of the stock. Large runs in both even and odd years are not fundamentally incompatible. The possibility, of promoting a low-level stock to a persisting higher level of abundance is indicated. Chum salmon are subject to the same types of mortality as pink salmon but the results are modified by the higher average egg-production (ca.2700) and the variable length of the life cycle. The species tends to occupy streams in which physical conditions are less stable. Compensatory influences are frequently obscured by these conditions, resulting in irregular fluctuations in abundance. In the application of remedial measures similar principles apply to both species.

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