Influences of Freshwater and Marine Growth on the Egg Size–Egg Number Tradeoff in Coho and Chinook Salmon

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Abstract
How to partition a finite amount of energy into progeny is an important issue in evolutionary biology. Salmonids produce a small number of large eggs compared with other teleost fishes, and there are positive correlations between female size and both the size and number of eggs produced. We examined the temporal variation in reproductive investment (gonad mass, fecundity, and egg size) of coho salmon Oncorhynchus kisutch and Chinook salmon O. tshawytscha at the University of Washington hatchery over the last three to five decades. During this period, fish growth rates in freshwater and at sea varied greatly, allowing us to test the hypotheses that rapid early growth rate results in high fecundity and that hatchery populations evolve high fecundity over time. In the coho salmon, reductions in length, condition factor, and size‐adjusted fecundity over the past decades combined to reduce overall average fecundity to only about half its former level. In Chinook salmon, age at maturity decreased as smolt size increased, so average size decreased over time but size at a given age did not decline. Reproductive investment (gonad size, corrected for body size) decreased for the coho salmon but not for the Chinook salmon. Egg size (adjusted for variation in fish size) did not vary with growth rate for either species or environment. Size‐adjusted fecundity did not increase with increasing freshwater growth for either species, but it did increase with marine growth for the coho and age‐3 Chinook salmon. These results contradict the prediction that females adjust egg size depending on the perceived quality of the juvenile habitat. Egg size is probably determined much later in life, corresponding to the number of remaining eggs and the energetic constraints at maturity. The results are also inconsistent with the hypothesis that hatchery populations will evolve small eggs and high fecundity as a result of a relaxed selection for large fry.