- The Columbia River Basin historically supported abundant populations of Pacific salmon (Oncorhynchus spp.) but, largely due to anthropogenic influence, many populations are now listed as threatened or endangered under the U.S. Endangered Species Act. Habitat restoration efforts have been a critical component of salmon recovery plans. However, although the importance of shallow-water wetland habitats is well documented for sub-yearling Chinook Salmon (O. tshawytscha), or individuals that spend <1 yr. in freshwater before migrating to the ocean, their importance is less clear for yearling Chinook Salmon, or individuals that migrate more rapidly and at larger sizes after 1 yr. in freshwater. Therefore, we need a better understanding of the importance of wetland habitat for yearling Chinook Salmon. The overall goal for this thesis was to determine if yearling Chinook Salmon rely on wetland-derived prey during their ocean-ward migration and whether their foraging habits change as they migrate through the Lower Columbia River and Estuary (LCRE), which extends from the lowermost mainstem
dam (Bonneville) to the mouth of the estuary. Therefore, I examined stomach fullness (relative indicator of feeding success), diet composition, and stable isotope signatures (13C & 15N) of prey supporting migrating yearling Chinooks.
. Yearling Chinook were collected in April and May, 2016 and 2017, from three riverine sites and at the mouth of the estuary during peak yearling Chinook migration. Yearling Chinook collected in 2017 had greater stomach fullness and were in better condition than those collected in 2016. Additionally, mean condition and stomach fullness decreased as fish moved closer to the ocean. Yearling Chinook diet composition varied across sites and between years, but yearling Chinook consumed insects (primarily wetland-derived prey) more frequently in 2016, when dipterans occurred in 60-100% of the diets, than in 2017 (dipteran mean = 12%). In contrast, amphipods (benthic prey) were consumed more frequently in 2017, when they occurred in 85-100% of diets compared to 2016 (amphipod mean = 55%). δ13C values of prey from 2016 diets were more reflective of the natural variation observed in prey collected in the field compared to 2017. These differences could be related to flow since 2017 was a higher flow year than 2016.
Based on biomass, benthic prey were identified as most important in yearling Chinook salmon diets, contributing 4x more than terrestrial prey taxa, which are primarily wetland-derived. However, based on energy density (kJ/g of fish meal), benthic and predominantly wetland-derived prey were equally important. Thus, the use of energy density to represent diet data provides an informative metric to evaluate
wetland habitat subsidies, especially when some taxa have caloric values three times greater than others.