Abstract:In partially migratory species, such as Oncorhynchus mykiss, the emergence of life history phenotypes is often attributed to ﬁtness trade-offs associated with growth and survival. Fitness trade-offs can be linked to reproductive tactics that vary between the sexes, as well as the inﬂuence of environmental conditions. We found that O. my kiss outmigrants are more likely to be female in nine populations throughout western North America (grand mean 65% female), in support of the hypothesis that anadromy is more likely to beneﬁt females. This bias was not related to migration distance or freshwater productivity, as indicated by latitude. Within one O. my kiss population we also measured the resident sex ratio and did not observe a male bias, despite a high female bias among out migrants in that system. We provide a simulation to demonstrate the relationship between sex ratios and
the proportion of anadromy and show how sex ratios could be a valuable tool for predicting the prevalence of life history types in a population.