Reconstructing marine life-history strategies of wild Atlantic salmon from the stable isotope composition of otoliths

Abstract

Long-term declines in abundance of wild Atlantic salmon Salmo salar, L. have been linked to reductions in marine survivorship, and recent reductions in growth condition (a measure of fish quality) have been correlated to increased mid-winter sea-surface temperature anomalies in the eastern North Atlantic. Establishing a causal link between marine climate and salmon somatic condition is difficult without at-sea measurements of environmental and biological parameters, but electronically tagging these animals to obtain this information is also difficult and costly. The stable isotope composition of oxygen and carbon in the sequential layers of salmon otoliths can provide a basis for retrospective studies of the thermal, metabolic and dietary histories of individual fish. We used high resolution δ18O and δ13C profiles obtained using secondary ion mass spectrometry to compare the marine thermal and metabolic behaviour of 1 sea-winter return-migrant adult salmon. Seasonal and ontogenetic patterns in δ18O and δ13C values dominated isotope variation, although there were marked differences between individual profiles, indicating the potential for large differences in individual migration routes. There was no significant relationship between isotope variation and adult condition. Constructed thermal profiles provided plausible data, but the present uncertainty in otolith thermometry parameters for an openocean fish such as Atlantic salmon preclude firm conclusions based on these estimates. Marine life-history variation in otolith δ13C is likely driven by ontogenetic changes, both in diet and metabolism with size. A marked and rapid decrease in the δ13C values of some fish in the last month(s) of the marine migration could be an indicator of physiological changes occurring during the homing migration. © Inter-Research 2013.