The Feeding Ecology of Postsmolt Salmon Along Near-Shore Salinity Gradients during Years of Contrasting River Discharge

Transitions through ephemeral estuary-ocean interfaces are significant survival challenges for early marine-phase salmonids; individuals must adapt to and compensate for fluctuating thermal, salinity, and prey regimes, among other challenges. We compared survival indices and diets of postsmolt Atlantic and yearling Chinook Salmon migrating through the Penobscot and interior Columbia River basins along near-shore salinity gradients during years of contrasting river discharge to gain insight into salmonid foraging dynamics. Within each system, annual salinity gradients and thermal habitats were vastly different and strongly influenced by river discharge. Stomach fullness indices decreased with increased with river discharge. During early migration, Atlantic salmon consumed insects, Chinook salmon consumed Corophium sp. and insects, and neither species consumed much fish. In marine waters, both species consumed high amounts of fish, slightly more in high than in low discharge years. Atlantic salmon consumed Atlantic herring, while Chinook Salmon had a varied diet consisting of sandlance, sculpin, and flatfish. Overall diet variability was low between years. River discharge correlated positively with two sea-winter Chinook Salmon returns but not Atlantic salmon returns, suggesting factors unrelated to early-marine resource acquisition regulated survival trends. Contrasting postsmolt salmonid dynamics from different systems provides insights into underlying mechanisms that influence survival.