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Diet habits of five Pacific salmon species caught in the marine waters of the eastern and central regions of the Gulf of Alaska (GOA) were analyzed for spatial, interannual, seasonal, and ontogenetic differences. By making comparative analysis of diet variability over several years and marine conditions, between the eastern and central GOA ecosystems, during summer and fall, and between juvenile and adult salmon, we add to the understanding of the role of salmon in the GOA ecosystem. Diet composition differences were significant between all salmon/age-class pairs except for juvenile pink and sockeye salmon (no diet difference). The diets with the strongest separation (difference) were between either piscivorous salmon (Chinook or coho) and any planktivorous salmon (chum, sockeye or pink). Interannual differences in diet were also prevalent (all tested pairs were significant), followed by size-based ontogenetic diet changes between juveniles and adults, seasonal differences, and regional differences (eastern vs. central GOA). Lower and upper trophic level productivity in the GOA varied over the study period which influenced the type and amount of prey available to both piscivorous and planktivorous salmon. The year 2011 was an anomalously low production year in the GOA and this was reflected in poor feeding rate (stomach fullness) and condition factor. In contrast, foraging conditions during 2013 allowed for a positive condition factor for all juvenile salmon across the GOA even with low stomach fullness. Juvenile salmon in 2012 and 2014 had average feeding rates and condition factor. Interannual differences in the type of prey consumed, feeding rate, and condition factor often co-varied across region. These findings suggest that juvenile, immature, and maturing salmon growth and condition can be influenced by bottom-up forces in the ocean which may ultimately affect run timing and survival rate. |
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