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 spawning migration along the Exploits River, Newfoundland, in 1996 and analyzed for stable-isotope ratios (delta(13)C and delta(15)N) and tissue concentrations of protein and lipid. Fish became progressively more C-13 enriched during the spawning migration (about -24 to -19 parts per thousand), and the degree 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 thousand). Only liver showed consistent changes in delta(15)N, as overwintering 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). Isotopic enrichment in migrating salmon appeared to result from mobilization, reorganization, and catabolism of stored lipid and protein reserves associated with the cessation of feeding upon entering freshwater. The most significant correlations existed between lipid content and delta(13)C in red muscle (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 carbon and nitrogen in anadromous fishes and that nutritional status should be considered when inferring food web relationships from the isotopic compositions of migrating salmonids.