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

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 delta O-18 and delta C-13 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 delta O-18 and delta C-13 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 open-ocean fish such as Atlantic salmon preclude firm conclusions based on these estimates. Marine life-history variation in otolith delta C-13 is likely driven by ontogenetic changes, both in diet and metabolism with size. A marked and rapid decrease in the delta C-13 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.