Trophic ecology of small cetaceans of the southwestern-most South Atlantic as revealed by stable isotope analysis; preliminary results

RICCIALDELLI L.; NEWSOME S.D.; GOODALL R.N.P.

Santiago de Chile

Otro; IWC Scientific Committee; 2008

International Whaling Commission (IWC)

To investigate the trophic ecology of eight species of small cetaceans of the Subantarctic waters off Tierra del Fuego, Argentina, bone samples from six delphinid (Risso’s dolphin, hourglass dolphin, southern right whale dolphin, false killer whale, Commerson’s dolphin and Peale’s dolphin) and two phocoenid species (spectacled porpoise and Burmeister’s porpoise) (n=371) were collected and analyzed for carbon and nitrogen (d13C and d15N) stable isotope composition. Since the information held inside bone tissue represents an average of the food taken during the life span of an animal, the stable isotope values of N and C obtained represent the long term diet of each species and indicate its main trophic behaviour. We found a trophic continuum, as revealed by these isotopes, from the most coastal species with the highest values of d13C and d15N (Peale’s dolphin) to the most oceanic and southern species with the lowest values (hourglass dolphin and spectacled porpoise) that inhabit waters near the Antarctic Convergence. A possible competition between Commerson’s dolphin and Burmeister’s porpoise is suggested by the great overlap in these values, but differed in both stable isotope values from Peale’s dolphin, indicating that they are not in direct competition for food or habitat with that very coastal species. Southern right whale dolphin and false killer whale stable isotope results suggest that at these latitudes they seem to inhabit waters of the continental slope, with movements principally to deeper waters, but on some occasions toward the coast. Risso’s dolphin, considered as one population in the southwest South Atlantic, showed a bimodal distribution of stable isotopes values, so we consider them two ecotypes from different latitudinal locations. The samples that were the most depleted in 13C and 15N came from the hourglass dolphin and the spectacled porpoise, values that indicated that both species feed far from shore in cold oceanic waters. We continue to analyse bone and muscle tissue from possible prey of these small cetaceans to determine the span of food sources and complement previous traditional food-habit studies, to better understand the complex ecology of these species.