Ontogenetic Diet Shifts of Juvenile Chinook Salmon in Nearshore and Offshore Habitats of Puget Sound

Abstract

Marine growth and survival of juvenile Chinook salmon Oncorhynchus tshawytscha depend in part on the quality and quantity of prey consumed during this potentially critical life stage; however, little is known about the early marine diet of these fish or factors that affect the diet's variability. We examined the recent (2001–2007) dietary habits of Puget Sound, Washington, Chinook salmon (listed as threatened under the U.S. Endangered Species Act) during their first marine growing season (April–September). Juvenile Chinook salmon initially fed in nearshore marine habitats and then shifted to feed primarily offshore during July–September. Diet composition varied significantly among sampling regions (northern, central, and southern), habitats (nearshore, offshore), years, months, and fish size-classes. At nearshore sites, insects (all months) and gammarid amphipods (July) were dominant prey sources, whereas in offshore diets decapods (primarily crab larvae; July) and fish (September) were most important. Chinook salmon became increasingly piscivorous as they grew and ate fish with fork lengths up to 51% (nearshore) and 52% (offshore) of predator fork length. At nearshore sites, Chinook salmon fed mainly on larval and juvenile Pacific sand lances Ammodytes hexapterus; offshore, they primarily ate juvenile and older Pacific herring Clupea pallasii. Overall, Chinook salmon had more diverse diets and ate higher-quality prey (insects) in northern nearshore and central offshore waters, whereas Chinook salmon caught in the southern nearshore and northern offshore waters had a lower proportion of empty stomachs but ate lower-quality prey (crustaceans). Annual variation in the composition of offshore prey appeared to be determined early in the growing season, suggesting that environmental factors (e.g., climate) affecting marine productivity might produce strong interannual trends in marine survival of Puget Sound Chinook salmon. In addition, the importance of insects as high-quality prey highlighted the terrestrial link to the marine feeding of Chinook salmon and suggests that shoreline development and land use changes will affect feeding opportunities for these fish in Puget Sound.