Proximate Composition and Energy Density of Stream‐Maturing Adult Steelhead during Upstream Migration, Sexual Maturity, and Kelt Emigration

In anadromous salmonids, muscle tissue provides the primary source of energy to support spawning migrations and spawning. We examined changes in the lipid content, protein content, and energy density of white muscle collected from stream‐maturing Snake River–Columbia River steelhead Oncorhynchus mykiss during several phases of reproduction. At a basinwide scale, we estimated that between early freshwater entry and postspawning (kelt) emigration, the lipid content of white muscle was reduced by 94% to levels less than 1% of wet tissue weight. Lipid was depleted more rapidly than protein during the reproductive cycle and afterward provided the only remaining somatic energy source for the postspawning migration. We found that protein content was consistently higher in sexually mature male steelhead than in females, suggesting that energy allocation prior to reproduction varies between the sexes. In kelts, the lipid content, protein content, and energy density of white muscle were significantly higher in good‐condition individuals than in poor‐condition fish. Fork length was positively correlated with both protein content and energy density in kelts, suggesting that larger kelts have higher somatic energy than smaller kelts. We found no evidence of significant interannual variation in lipid, protein, and energy density of sexually mature steelhead and steelhead kelts. Postspawning survival of steelhead is likely limited by the low energy density of kelts, and our data lend support to observations of low iteroparity rates in this and other populations of inland stream‐maturing steelhead.