Feeding Habits of Outmigrating Juvenile Salmon in the Fox River Delta, Kachemak Bay, Alaska

While Alaska’s salmon populations are generally strong, habitat degradation and uncertainties from climate change may bring challenges for their continued success. As a defining ecotone between river and ocean environments, estuaries have been shown to provide vital habitat for juvenile salmon during their outmigration. However, comparatively little is known about how juvenile salmon interact with Alaska’s diverse estuarine environments, especially those of glacial watersheds.  To address this lack of information, in 2009 we initiated studies on the outmigrant timing, habitat use, and foraging of juvenile salmonids in the glacially-derived Fox River delta, located at the head of Kachemak Bay in southcentral Alaska. This high latitude northern estuary (latitude 59°) is fed by glacial rivers, and has an extreme (5.5 m) tidal range.  

Our preliminary results on juvenile salmon outmigration timing and estuarine habitat use are presented orally by Hoem-Neher et al.; corresponding results from juvenile salmon diet and prey availability studies are presented in this poster.  In 2010, three estuarine distributary channels were sampled monthly from May through September.  Juvenile salmon diet composition was quantified by stomach contents analyses of fish sacrificed from systematic sampling with three-pass seining between block nets. Insect fallout traps on the marsh surface and epibenthic tows in the tributary channels were used to sample available prey.  Stomach content samples were analyzed for numerical and gravimetric composition, and frequency of occurrence. 

 Juvenile salmon were predominantly feeding on aquatic insects, particularly dipterans, but terrestrial insects, especially beetles, were also prominent in their diet composition.  The majority of the prey organisms in the stomach contents were undigested, indicating that fish had fed recently in the marsh channel habitats. Fallout traps provided a corresponding indication of prey availability.  Taxa richness increased progressively until late June and then stabilized at 30-35 taxa.  Mean total densities varied between 150 and 500 m^-2 with maxima in early May and late June. Taxa composition corresponded to diet spectra of both sockeye and coho.

Estuarine diets of juvenile sockeye and coho salmon are seldom so dominated by aquatic and terrestrial insects with the exemption of amphipods, mysids and other benthic/epibenthic crustaceans. The impressive taxa diversity of the insects may reflect the higher elevation marsh habitat with less frequent inundation.  Among other expansions of these studies, we propose to evaluate further quantitative aspects of juvenile foraging in the Fox River delta, including consumption rates and bioenergetic costs and benefits of these unique diet spectra.