Local Adaptation in Hood Canal Steelhead? Genetic Variability in Reaction Norms for Early Life History Traits Between Two Populations

Steelhead trout (Oncorhynchus mykiss) span the west coast of North America from California to southeast Alaska and are known to exhibit a wide range of variation in fitness-related life history characteristics.  The recent ESA-listing of steelhead in the Puget Sound basin (USOFR 2007) has emphasized the need to maintain adaptive genetic variation among populations.  The Hood Canal watershed provides an ideal location to examine the potential for variation in adaptive freshwater traits among steelhead trout populations.  The Hood Canal Steelhead Project has collected data suggesting differences in fitness-related life history traits among Hood Canal steelhead populations are associated with physical environmental differences among rivers.  Environmental variation among the rivers, most notably in temperature between the Dewatto and the Duckabush Rivers, may lead to the development of plastic responses and the evolution of adaptive variation in traits among environments.  Phenotypic plasticity reflects the ability of individuals within a population to respond to environmental change.  Genetic variability in reaction norms might account for population differences in the ability to respond to environmental change and may reflect local adaptation.  In this study, reaction norms for early life history traits were compared between two populations of Hood Canal steelhead.  Within this context, we conducted a reciprocal transplant, common garden experiment to explore the degree to which temperature regime alters the shapes of early life history reaction norms between the Dewatto and Duckabush steelhead populations.  Experiments were conducted under two temperature regimes that mimic those experienced by the two populations.  Response variables include fitness-related traits that vary between populations in the wild and that may confer adaptive advantages under local temperature regimes.  Measured traits include instantaneous growth rate, body condition factor, parr size-at-age, and proportion of age-1-smolts and mature males, dorsal fin length, body shape, and coloration.  Results of this study, which will conclude in May 2011, will be presented and will be the first to examine population differences in the life history reaction norms in this species.