Inter- and Intra-Population Variation in the Fecundity of Chinook Salmon (Oncorhynchus tshawytscha) and its Relevance to Life History Theory

Abstract

Chinook salmon (Oncorhynchus tshawytscha) varied significantly in fecundity both between years within populations and between populations throughout their range. Generally less than 50% of the variation in fecundity between individuals within populations could be explained by variation in length. A small additional amount of variation could be attributed to racial differences (e.g. red or white fleshed types) but age, seasonal timing of subpopulations, and stream or ocean type life history pattern did not contribute significantly to variation in fecundity beyond their correlation with length. A great deal of individual variation in fecundity remains to be explained in chinook salmon. The slopes of the regression of fecundity on length for all populations were low in comparison with other fishes, indicating that fecundity increases slowly with increasing size in chinook. The mean age of reproducing females varied among populations, and populations that reproduced at an older age were more fecund at a common length than populations that reproduced at a younger age. The increase in fecundity with increasing age of maturity was consistent with theoretical predictions of the trade-off between fecundity and mortality in fish of reproductive age. The mean age of reproduction within a population, however, was considerably older than the predicted optimum age of reproduction based on the trade-off between increasing fecundity with age and natural mortality. These observations suggest that chinook have sacrificed fecundity for increased size in the allocation of surplus energy, a life history characteristic that is consistent with the survival value of size in anadromous salmon.