Female Bias in Steelhead Smolts Along a Gradient of Migration Distances

Steelhead and rainbow trout (Oncorhynchus mykiss) are sympatric, mate with one another, and produce offspring that may adopt either a resident or anadromous life history. A frequent observation of returning adult steelhead is that the sex ratio is female-biased. This may be evidence that females and males have different evolutionary tradeoffs associated with anadromy, such that females are more likely to migrate. To investigate female bias among steelhead and to understand how migration distance influences female bias, I obtained smolt samples from eight O. mykiss populations that have a range of migration distances. In one of these rivers, the South Fork John Day, I also measured sex ratios of juvenile and resident O. mykiss to assess corresponding male bias among resident rainbow trout. Female bias in all but one population ranges from approximately 60 to 75% and does not differ by migration distance or latitude. Within the South Fork John Day smolts were approximately 76% female (95% CI: 68, 84%), whereas mature resident fish were 42% female (95% CI: 25%, 59%) and the primary sex ratio was 43% female (95% CI: 29%, 57%). This work provides evidence of possible differences in the life history tradeoffs between males and females.