Estimations of Ocean Mortality Rates for Pacific Salmon (Oncorhynchus)

Abstract

A conceptual model representing natural marine mortality rates of Pacific salmon is developed. Ocean mortality rate (q) is taken as the base to which coastal mortality rates of juveniles (c) and of adults (k) are additive factors. The effect of marking is taken as a multiplicative factor (m) of the instantaneous rate (i) where i = q + c +k. Together with time these values are incorporated into the balanced equation [image] where N0 denotes the population entering the sea and R1, R2, R3 denote the returns at succeeding times of maturity. The locus of c + k = f(q) is used to graphically depict all possible combinations of c + k and q within the limits [q, c + k = 0]. Intersections of loci are taken as estimates of values of q and c + k which satisfy two sets of data. Available data for sockeye salmon (O. nerka) from Cultus Lake, Chiko Lake and Hooknose Creek, British Columbia, Karluk River and Bare Lake, Alaska, and Dalnee River, Kamchatka, pink salmon (O. gorbuscha) and chum salmon (O. keta) from Hooknose Creek, Chinook salmon (O. tshawytscha) from the coast of Southeast Alaska and coho (O. kisutch) from the Eel River, California, are utilized. It is concluded that ocean mortality is relatively constant, of the order of magnitude q = 0.32 or S = 73% annually. A juvenile coastal mortality factor (c) apparently exists and is characteristic of the species and race through the media of size of migrants, time spent in the costal area, and geography. An adult coastal factor (k) may exist but is of negligible influence on the total mortality rate. While the data utilized collectively may be considered as extensive, serious defects in sampling errors and undefined variability were encountered. It is doubted that mortality rates can be more accurately defined from any repetition of experiments used, hence a more direct approach is indicated for solution of this problem.