Influence of Migration Distance on the Expression of Anadromy in Steelhead and Rainbow Trout

Steelhead and rainbow trout are two life history types of the partially migratory species, Oncorhynchus mykiss. Partial migration describes the phenomenon where only a portion of the population migrates, which in the case of O. mykiss broadly divides populations into anadromous or “steehead” and resident or “rainbow trout” individuals. From an evolutionary perspective, partial migration can be understood as the result of a tradeoff between growth and survival to maximize fitness. Steehead 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, rainbow trout may enjoy higher survival and can reproduce at a younger age, but attain smaller body size.  Increased body size associated with anadromy should be most beneficial to females because fecundity is closely tied to size. In contrast, males benefit less from size because they are able to adopt alternative mating tactics, which are not as related to size. These cost:benefit tradeoffs are influenced by local environmental conditions, such as migration difficulty. This is a primary prediction from recent models of life history expression.  As the costs of migration increase, the proportion of the population exhibiting anadromy should decline. My project will focus on how an increase in migration difficulty, measured as distance and migration time, influences the relative proportion of males and females that adopt an anadromous life history. Sex ratios will be used as a proxy for the relative proportion of the population that migrates. I will compare smolt sex and age of steelhead smolts in 2-4 coastal Oregon streams with relatively short migration distances and 2-4 interior Columbia streams with relatively long migration distances. I anticipate that the sex ratio of smolts in the interior Columbia streams will be female-biased, indirectly indicating a greater degree of residency. In coastal streams I predict equal proportions of male and female smolts, indicating little development of residency. These results will provide a test of the hypothesis that sex and cost of migration influence expression of life history in O. mykiss at broad scales.