Comparison of Ocean Growth and Mortality of Sockeye Salmon During Their Last Two Years

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Abstract
Average instantaneous rates of growth in weight of sockeye (Oncorhynchus nerka) of two stocks are computed for their last two growth-years, from scale measurements by Barnaby and Foerster. Values from 0.081 to 0.156 per month are obtained. These rates greatly exceed a maximum estimate of instantaneous natural mortality rate, 0.038 per month, for the same period. The latter is based on the total rate of ocean mortality of large smolts, divided by the number of months they are at sea. On this basis, oceanic capture of sockeye during the year prior to their maturation yields less than 35% to 50% of the catch that would be taken from the same fish by a shore fishery the following year. Minimum estimates of loss from high-seas capture of major growth types of sockeye during the year of their maturation vary from 6% to nearly 40%, depending on how early in the season the fishing is done and the particular stock involved. While an increase in mortality rate as sockeye approach shore cannot be excluded, any resulting decrease in the estimate of loss among currently-maturing fish must be accompanied by an increase in the estimate of loss from capture in the year before maturity. In any event, a high-seas fishery that takes any significant fraction of non-maturing sockeye must markedly decrease the total yield from the stocks involved.