Comparing naturally occurring stable isotopes of nitrogen, carbon, and strontium as markers for the rearing locations of Atlantic salmon (Salmo salar)

We compared the success of using naturally occurring stable isotopes of N, C, and Sr as markers for the rearing locations of juvenile salmon. We analyzed the isotopic signatures (delta(15)N and delta(13)C in muscle and scales and (87)Sr/(86)Sr in otoliths) of > 200 juvenile Atlantic salmon (Salmo salar) from 12 tributaries of the Connecticut River, USA. Young salmon had distinct N and C signatures 5 weeks after stocking. Signatures were stable over the summer although delta(13)C varied more than delta(15)N or (87)Sr/(86)Sr. Scale and muscle signatures were highly correlated, demonstrating the feasibility of nonlethal sampling using fish scales. Some C (but not N) signature from the hatchery was retained in scales of 3-month-old fish, implicating scale annuli as a repository for past C signatures. The delta(15)N values successfully differentiated fish from tributaries with differences in land use (e.g., agricultural versus forested; 33% of sites); the delta(13)C values differentiated fish from 45% of sites. Based upon a discriminant-function analysis, group membership of individuals was correctly predicted in 44.3% (74 of 167) of cases for which both N and C were analyzed. In combination, N and C isotopes differentiated 73% of study sites, which was close to the success of Sr isotopes in the same system (83%).