Male Age-At-Maturity in Chinook Salmon Affects Reproductive Behavior, Breeding Success, and Offspring Growth Rates Under Experimental Conditions

There is a strong relationship between male size and age-at-maturity in Chinook salmon (Oncorhynchustshawytscha). Jack males mature one year earlier than the youngest females in a population and are much smaller than older ‘adult’ males. Frequency-dependent selection during reproduction has long been proposed as an important mechanism in maintaining alternative male reproductive phenotypes in Pacific salmon. We tested the hypothesis that mating success of both phenotypes is consistent with the frequency-dependent selection model, which in simple terms predicts a fitness advantage for the rare form. By holding male density constant and varying the frequency of adult and jack males in eight separate breeding groups, we found that adult male access to females, participation in spawning events, and adult-to-fry reproductive success increased with their decreasing frequency in a breeding group. Jack males exhibited the same pattern (increasing success with decreasing frequency) although the relationships were not as strong as for adults. Overall, jack and adult males mated with a similar number of females but jacks sired only 20% of all offspring. Fertilization success in individual spawning events was strongly correlated with order of nest entry at the time of spawning and indicated that adult males benefited from sperm precedence. In a companion experiment, subyearling offspring of jack males grew faster in a quasi-natural stream channel than offspring of adult males and attained a significantly greater mean body size by May (final fork length, P = 0.009; final body mass, P = 0.013). The size differences suggest that offspring of jack males may have a greater tendency to attain body size thresholds for early maturation than offspring of adult males.  As such, paternal effects on juvenile growth rate constitute a plausible proximate mechanism maintaining diversity of alternative male reproductive phenotypes in Chinook salmon. The effect of sire phenotype on offspring growth is consistent with estimates of high heritability for age-at-maturity in Chinook salmon and a genetic effect of alternative male phenotypes on offspring performance.