Sex‐Specific Life History Tactics and Risk‐Taking in Coho Salmon

Differences in size and abundance between males and female at sexual maturity are common in vertebrates. Evolutionary explanations for these differences generally assume a consistent pattern within a species. Among Pacific salmon (Oncorhynchus spp.) and coho salmon (O. kisutch) in particular, however, sexual size dimorphism and sex ratio are not consistent among or within populations, making extant explanations unsatisfactory. In this paper we describe the variation size dimorphism and sex ratio within and among coho salmon populations and propose a model to account for this variation. Our data consist of a 16—yr study of coho salmon in carnation Creek, British Columbia, and shorter term studies of 30 other coho populations ranging from California to Alaska. Among mature coho salmon in Carnation Creek males outnumbered females but females were larger, and the male/female sex ratio varied directly with the female/male size ratio. Mean female size was constant over 16 yr, while male size varied inversely with the male/female sex ratio. Females apparently had higher mortality rates than males, and the difference in mortality rates appeared to increase as the animals neared maturity. Sex— and size—ratio data from other coho salmon populations indicate that there are two general population types. In one type males outnumber females, and females are larger at maturity. In the other type the sexes are equally abundant and equal in size. We propose that the range of size for successful reproduction in coho salmon is set by conditions in the breeding environment, and that the range differs for males and females because of the different roles that the sexes play in reproduction. The sex—specific constraints on reproductive size lead to sex—specific foraging strategies and risk—taking. Interpopulation differences in size and sex ratio are determined by differences in physical characteristics of the breeding environment, spawning population density, and the relative riskiness of ocean nursery areas. Within populations, variation is determined by the interaction of sex—specific foraging strategies with interannual variation in ocean feeding conditions and predator abundance.