Poster: The Influence of Migration Distance on the Expression of Anadromy in Male and Female Steelhead

Steelhead and rainbow trout are two life history types of the partially migratory species, Oncorhynchus mykiss. From an evolutionary perspective, partial migration can be understood as the result of a tradeoff between growth and survival to maximize fitness. Anadromous steelhead attain greater body size, which likely results in greater fitness, but anadromy also has a cost: a lower probability of survival to reproduction. In contrast, resident rainbow trout may enjoy higher survival and can reproduce at a younger age, but attain smaller body size. The increased body size associated with anadromy should be most beneficial to females because fecundity is closely tied to size, wherease males are able to adopt alternative mating tactics that are not as related to size. These cost:benefit tradeoffs are influenced by local environmental conditions, such as migration difficulty.  My project will test the hypothesis that sex and cost of migration influence expression of life history in O. mykiss at broad scales. I will compare steelhead smolt sex and age demographics in 4 coastal streams with short migration distances and 4 interior Columbia streams with long migration distances. I anticipate that the sex ratio of smolts in systems with difficult migrations will be female-biased, indirectly indicating a greater degree of residency by males. In systems with easy migrations, I predict equal proportions of male and female smolts, indicating little development of residency in either sex. Under these predictions coastal populations may have less overall life history diversity than interior populations, which may represent differences in overall resiliency between the two population types.