Integrated Diet Analyses Reveal Contrasting Trophic Niches for Wild and Hatchery Juvenile Chinook Salmon in a Large River Delta

Abstract

Hatchery programs have been used as a conservation tool to bolster declining populations of Chinook Salmon Oncorhynchus tshawytscha along much of the North American Pacific coast. In many watersheds, hatchery stocks are released concurrently with the wild population, thus raising the potential for density‐dependent effects. Competition for prey resources during the critical period for early marine growth and survival may diminish the foraging capacity and growth potential of wild Chinook Salmon, highlighting the importance of a diverse and productive delta habitat mosaic. We used an integrated diet approach with stomach content and stable isotope analyses to evaluate contrasting patterns of habitat use and prey consumption in a fall‐run population of juvenile Chinook Salmon from the Nisqually River delta in Puget Sound, Washington. We examined size‐class and origin‐level differences throughout a gradient of delta habitat types. Wild (unmarked) and hatchery juveniles exhibited distinct habitat use patterns whereby unmarked fish were captured more frequently in tidally influenced freshwater and mesohaline emergent marsh areas, while hatchery fish were caught more often in the nearshore intertidal zone. Consequently, hatchery fish were less likely to consume the energy‐dense terrestrial insects that were more common in freshwater and brackish marshes. Stable isotope signatures from muscle and liver tissues corroborated this finding, showing that unmarked juveniles had derived 24–31% of their diets from terrestrially sourced prey, while terrestrial insects only made up 2–8% of hatchery fish diets. This may explain why unmarked fish were in better condition than hatchery fish and had stomach contents that were 15% more energy‐rich than those of hatchery fish. We did not observe strong evidence for trophic overlap in juvenile Chinook Salmon of different rearing origins, but our results suggest that hatchery juveniles could be more sensitive to diet‐mediated effects on growth and survival.

Freshwater Tidal Forests and Estuarine Wetlands May Confer Early Life Growth Advantages for Delta‐Reared Chinook Salmon

“…Diet and individual consumption varied monthly among habitat types and were driven to some extent by the availability of high‐energy prey (Davis et al. ). Dietary energy density—an important input for the bioenergetics model—was influenced by the proportion of adult insects comprising juvenile Chinook Salmon diets.…”

“…; Davis et al. , ), we grouped P obs by rearing origin, habitat type, and month for calculating habitat‐specific ranges of GRP (Table ).…”

“…; Davis et al. ). These habitat types are rarely studied concurrently with regard to their accessibility, thermal range, prey availability, and potential for conspecific or interspecific competition (Figure ; Simenstad et al.…”

“…; Davis et al. ). Previous studies in the restoring and unaltered tidal marshes have suggested that areas where the terrestrial–aquatic interface is most connected may produce the highest salmon GRP (David et al.…”

Seasonal Use of a Nonnatal Marine Basin by Juvenile Hatchery Chinook Salmon

“…Moreover, the Nisqually estuary provides excellent conditions for feeding and growth compared with nearby Puget Sound (Davis et al. , ; Davis et al. ), which may encourage juvenile salmon to remain for longer periods.…”