Alternative Male Reproductive Phenotypes Affect Offspring Growth Rates in Chinook Salmon

Abstract

AbstractMale age at maturity in Chinook salmon Oncorhynchus tshawytscha is a heritable trait in which the physiological “decision” to mature depends on an individual's exceeding a body size or condition threshold at critical developmental periods. In Chinook salmon, high juvenile growth rates promote the so called “jack” male life history. Jack males mature 1 year earlier than the youngest females in a population and are substantially smaller than older (hereafter, “adult”) males from the same population. We tested the hypothesis that the offspring of jack Chinook salmon males grow faster than the offspring of adult males, consistent with the heritability for age at maturity already demonstrated for this species. We controlled for maternal effects by artificially spawning individual jack and adult males with the same female, incubating the eggs, and testing offspring growth and survival in a common, quasinatural stream environment. Paternal life history had a significant effect on final length and weight over all stream channel sections combined. The mean difference in body size between the offspring of jack and adult males ranged from essentially none to 31% among the eight stream channel sections. The size differences we detected could significantly affect which fish mature and thereby suggest that paternal effects on juvenile growth rate constitute a plausible proximate mechanism in maintaining life history diversity in Chinook salmon. Maternal body size was positively correlated with offspring growth rate and egg size was negatively correlated with growth rate, suggesting that fairly complex mechanisms regulate the growth rate through early ontogeny. The effect of sire life history on offspring growth is consistent with estimates of high heritability for age at maturity in Chinook salmon and a genetic effect of alternative mating tactics on offspring performance.