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

Abstract

Anadromous Atlantic salmon (Salmo salar) were collected during their spawning migration along the Exploits River, Newfoundland, in 1996 and analyzed for stable-isotope ratios (δ13C and δ15N) and tissue concentrations of protein and lipid. Fish became progressively more 13C enriched during the spawning migration (about -24 to -19‰), and the degree of enrichment was dependent on tissue type (red muscle: +4.1‰, liver: +2.6‰, white muscle: +1.3‰). Only liver showed consistent changes in δ15N, as overwintering kelts (13.5 ± 0.8‰) were about +2‰ more enriched than upstream migrants (11.4 ± 1.5‰). 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 δ13C in red muscle (r2 = 0.67) and protein content and δ15N in liver (r2 = 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.