Stochastic recruitment alters the frequencies of alternative life histories in age‐structured populations

Abstract

Alternative reproductive tactics, where members of a population exhibit divergent mating approaches, are widespread in fishes. The maintenance of such polymorphisms is often explained using game-theoretic concepts such as frequency-dependent selection, which typically assume populations are at equilibrium. However, fish populations exhibit stochastic variability that is inconsistent with the assumption of equilibrium dynamics. We explored how stochastic population dynamics interact with sexual and harvest-induced selection to shape the frequencies of alternative male phenotypes in Pacific salmon (Oncorhynchus spp., Salmonidae). Males of several salmon species are composed of typical "hooknoses" that compete on the spawning grounds for mating opportunities, and rarer "jacks" that mature younger and at smaller sizes to attempt reproduction by "sneaking." Using stochastic simulations, we found that realistic levels of recruitment variation could increase the prevalence of jacks relative to equilibrium expectations. This result is attributable to the "cohort mismatch" phenomenon, whereby differences in age at maturity between male phenotypes cause jacks from strong recruitments to spawn in the same year as hooknoses from weaker cohorts, elevating the proportion of jacks among spawners. Simulations were supported by data from major populations of sockeye salmon (O. nerka, Salmonidae) throughout their North American range, which indicated a positive association between recruitment variability and jack prevalence in some regions. Moreover, we found that frequency-dependent mating dampened population responses to fisheries-induced selection. Our findings emphasize that stochastic ecological processes are important to the maintenance of alternative life histories and interact with selection to produce emergent dynamics that have not previously been recognized.