Hatchery programs have been used as a conservation tool to bolster declining Chinook salmon (Oncorhynchus tshawytscha) populations throughout much of the Salish Sea. In many watersheds, hatchery fish are released concurrently with the natural-origin population, thus raising the potential for density dependent effects via depleted prey resources, territorial behavior, and movement into sub-optimal habitats. Competition during the critical period for early marine growth and survival might have detrimental effects for wild Chinook salmon populations, highlighting the potential importance of a productive delta habitat mosaic. We used an integrated diet approach with stomach content and stable isotope analyses to evaluate differential patterns of habitat use and prey consumption in a fall run population of juvenile Chinook salmon from the Nisqually River Delta in Puget Sound. We examined size class and origin-level differences throughout the out-migration gradient, from freshwater riverine to nearshore habitat. Natural- and hatchery-origin smolts exhibited distinct habitat use patterns, whereby hatchery-origin individuals were captured less frequently in forested and transitional habitats, and more frequently in the nearshore. Consequently, hatchery-origin juveniles were less likely to consume terrestrial insect drift that was almost twice as energy rich as nearshore crustacean prey. Stable isotope signatures from muscle and liver tissues corroborated this finding, showing that while natural-origin Chinook salmon derived 24–31% of their diets from terrestrially sourced prey, terrestrial insects only made up 2–8% of hatchery-origin diets. This may have explained why natural-origin fish were in better condition and had stomach contents that were 15% more energy-rich on average than hatchery-origin fish. We did not observe strong evidence for trophic overlap in natural- and hatchery-origin juvenile Chinook salmon, but our results suggest that hatchery fish are less likely to take advantage of the terrestrial-aquatic interface, and could suffer behaviorally-mediated consequences to early marine growth and survival
