LAUSR

Anadromous fishes need to store sufficient energy at sea to migrate and reproduce, but the energetic demands of freshwater migration distance have been difficult to distinguish from the demands of fasting in fresh water prior to spawning. In addition, differences in stored energy may result from differences in metabolic storage or marine diet. We estimated somatic lipids (SL) and stable isotopes of N and C (as indices of marine trophic position and feeding location) in adult wild and hatchery steelhead (Oncorhynchus mykiss) of summer (stream-maturing) and winter (ocean-maturing) runs that migrate similar distances to spawn. We found that SL at return depended on several factors. Summer steelhead had twice the SL as winter steelhead, and within each run SL decreased with arrival date, converging on about 1% SL. In summer steelhead, females had 19% more SL than males, and wild fish had 21% more SL than hatchery fish. In winter steelhead, females had 27% less SL than males, and there was no difference betwee...