| The early marine life of Pacific salmon is a critical period when larger size and faster growth have been linked to elevated overall marine survival. In Puget Sound, several salmon populations have experienced recent declines, including Endangered Species Act (ESA) listed Chinook salmon, which are predominately ocean-type and rear extensively in Puget Sound. Between 2001--2007, we sampled nearshore and offshore habitats of Puget Sound to determine the distribution, feeding, growth, and sources of mortality for juvenile Chinook salmon and investigated whether there was evidence for size-selective predation and for environmentally-mediated or competition-based reduction in growth rates that could be linked to overall marine survival trends. In the spring, juvenile salmon represented greater than 50% of the diet of cutthroat trout at nearshore habitats. Although predation by cutthroat trout was likely responsible for a relatively minor amount of the early marine mortality experienced by Chinook salmon, predation pressure was highest on the smallest fish. Assuming other fish predators exhibited similar size-selective predation, this suggested a survival advantage for larger and faster growing fish. Juvenile Chinook salmon initially occupied nearshore habitats (April--June), shifting offshore during July--September. Insects and amphipods were dominant nearshore prey sources, whereas crab larvae and fish were more important in offshore diets. Marine survival rates of Puget Sound Chinook salmon were strongly related to the average offshore body mass in July, with larger sizes associated with higher survivals. Using a bioenergetics model, we found that early marine growth of Chinook salmon was most strongly linked to feeding rates, and less to diet composition and temperature. Spring growth of juvenile Chinook salmon appeared to be food-limited in some years, and low feeding rates corresponded to poor marine survival. In conclusion, we found that Chinook salmon must feed and grow at high rates during May--July of their first marine year to increase overall marine survival. Our findings highlight the importance of local feeding conditions in Puget Sound during spring and summer, and suggest that declines in marine survival since the 1980's may have been caused by reductions in food availability for Chinook salmon during early marine life. |
