Continental-Scale Variability In Feeding and Resource Ecology of Juvenile Chinook Salmon Along the Coastal North Pacific

Pacific salmon are an important resource with significant links to the regional and national economy, and social and cultural heritage of coastal communities. During the last two/three decades, the productivity of Pacific salmon has been declining, which might be linked to large-scale changes in ocean conditions associated with variability in temperature, nutrients, quantity and quality of habitat and food, and predator assemblages. Understanding and modeling the feeding and resource ecology of juvenile salmon on a continental scale have never been done because of the challenges in collecting and integrating samples and following consistent protocols. To achieve this scale of comparative analysis, we collected zooplankton, salmon and forage fish from 8 cruises along the coastal regions off Northern California, Oregon/Washington, Vancouver Island, Southeast Alaska, Eastern Bering Sea and Chukchi Sea from juvenile Chinook salmon during August through October 2007. We examined stable isotope signatures of nitrogen and carbon and concentrations of mercury in the tissue, and stomach contents to understand the continental-scale variability in foodweb dynamics, trophic interactions and resource base of juvenile Chinook salmon.  We found large gradients in the δC-13 signatures along a continental spatial scale that corresponded to regional changes in the diet.  Juvenile Chinook salmon show an ontogenetic increase up to 200 mm in length and then stabilize in most regions. Mercury concentrations in juvenile Chinook tissue appear to be linked to growth patterns, not to large-scale loading or mobilization. Finally, we used the zooplankton and forage fish data from the different regions to estimate baseline isotope signatures to model trophic shifts on a continental scalE.