Habitat, trophic levels and migration patterns of the short-finned squid Illex argentinus from stable isotope analysis of beak regions

Illex argentinus is an ecologically and economically important species, assumed to be restricted to the Patagonian Shelf and around the subtropical convergence. Beaks found in diet samples from black-browed albatrosses Thalassarche melanophris during chick rearing initially suggested that it may also inhabit Antarctic waters until it was appreciated that I. argentinus is used as fishing bait by commercial longliners within albatrosses foraging areas. Here, we applied a new methodology involving stable isotope analysis (δ13C and δ15N) in two regions [tip of the rostrum (juvenile) and wing (adult)] of lower beaks obtained from diet samples of black-browed albatrosses breeding at Bird Island (South Georgia). The aims were to (1) assess if I. argentinus could inhabit Antarctic waters somewhere in the life cycle (2) determine the trophic ecology of I. argentinus, and (3) discuss possible migration patterns of I. argentinus and whether its distribution may change in the future. Values of δ13C (proxy for habitat) were − 18.4 ± 0.7‰ and − 17.1 ± 0.4‰ during the juvenile and adult life stages, respectively, indicating a northwards ontogenetic shift, and that this species exclusively inhabits waters north of the Antarctic Polar Front. Values of δ15N was lower in juveniles (+5.9 ± 1.1‰) than adults (+8.4 ± 1.3‰), indicating an increase of one trophic level throughout the squid’s life, suggesting a diet shift from zooplankton to fish and squid. Based on predicted effects of climate change, the distribution of I. argentinus may become more restricted as the northern limit moves southwards because of warming ocean temperature.