SEX-SPECIFIC LIFE-HISTORY TACTICS AND RISK-TAKING IN COHO SALMON

Among Pacific salmon (Oncorhynchus spp.) and coho salmon O. kisutch in particular, sexual size dimorphism and sex ratio are not consistent among or within populations. Data here consist of a 16yr 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 16yr, while male size varied inversely with the male/female sex ratio. Females apparently had higher mortality rates than males; 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 2 general population types: 1) males outnumber females, and females are larger at maturity; 2) the sexes are equally abundant and equal in size. The authors 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 nursey areas. Within populations, variation in determined by the interaction of sex-specific foraging strategies with interannual variation in ocean feeding conditions and predator abundance. -from Authors