Effects of the spawning migration on the nutritional status of anadromous Atlantic salmon (Salmo salar): insights from stable-isotope analysis

Anadromous Atlantic salmon (Salmo salar) were collected during their spawni ng migration along the Exploits River, Newfoundland, in 1996 and analyzed f or stable-isotope ratios (delta(13)C and delta(15)N) and tissue concentrati ons of protein and lipid. Fish became progressively more C-13 enriched duri ng the spawning migration (about -24 to -19 parts per thousand), and the de gree of enrichment was dependent on tissue type (red muscle: +4.1 parts per thousand, liver: +2.6 parts per thousand, white muscle: +1.3 parts per tho usand). Only liver showed consistent changes in delta(15)N, as overwinterin g kelts (13.5 +/- 0.8 parts per thousand) were about +2 parts per thousand more enriched than upstream migrants (11.4 +/- 1.5 parts per thousand). Iso topic enrichment in migrating salmon appeared to result from mobilization, reorganization, and catabolism of stored lipid and protein reserves associa ted with the cessation of feeding upon entering freshwater. The most signif icant correlations existed between lipid content and delta(13)C in red musc le (r(2) = 0.67) and protein content and delta(15)N in liver (r(2) = 0.32). This study shows that fasting affects the stable-isotope ratios of both ca rbon and nitrogen in anadromous fishes and that nutritional status should b e considered when inferring food web relationships from the isotopic compos itions of migrating salmonids.