Somatic Lipid Content and Marine Foraging Patterns in Summer-Run and Winter-Run Steelhead, Oncorhynchus mykiss, Returning to the Kalama River, Washington

We estimated  percent somatic lipid content and stable isotopes of nitrogen (indicator of marine trophic position) and carbon (indicator of coastal versus open-ocean foraging) in adult steelhead (winter and summer runs; wild and hatchery origins) to test hypotheses regarding differences in stored energy and marine trophic ecology related to run type, arrival date, origin, and sex. Steelhead (n = 764) were sampled in the Kalama River, Washington for one year, encompassing nearly the entire return for each run type. Results revealed multiple factors associated with lipid levels. Percent lipid within each run decreased as a function of arrival date, converging on about 1% fat. Summer-run steelhead entered with nearly twice the percent lipid as winter-run steelhead.  In winter-run steelhead, females had 30% less stored lipids than males, and there was no difference in energy storage between origins. In summer-run steelhead, females had 20% more stored lipids than males, and wild steelhead had 20% more stored lipids than hatchery steelhead. In contrast to differences in lipid levels, we detected no differences in δ¹⁵N or δ¹³C. Taken together, the results indicate very different patterns of energy storage among and within steelhead runs, despite no apparent differences in trophic position at sea.