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

RICCIALDELLI LUCIANA; NEWSOME SETH D.; GOODALL RNP

Santiago

Encuentro; Reunión del Comité Científico de la Comisión Ballenera Internacional (IWC); 2008

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 (δ13C and δ15N) stable isotope composition. Since the information heldinside bone tissue represents an average of the food taken during the life span of an animal, the stable isotope values of N and Cobtained 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 δ13C and δ15N (Peale’s dolphin) to themost oceanic and southern species with the lowest values (hourglass dolphin and spectacled porpoise) that inhabit waters nearthe Antarctic Convergence. A possible competition between Commerson’s dolphin and Burmeister’s porpoise is suggested bythe great overlap in these values, but differed in both stable isotope values from Peale’s dolphin, indicating that they are not indirect competition for food or habitat with that very coastal species. Southern right whale dolphin and false killer whale stableisotope results suggest that at these latitudes they seem to inhabit waters of the continental slope, with movements principally todeeper waters, but on some occasions toward the coast. Risso’s dolphin, considered as one population in the southwest SouthAtlantic, showed a bimodal distribution of stable isotopes values, so we consider them two ecotypes from different latitudinallocations. 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 muscletissue from possible prey of these small cetaceans to determine the span of food sources and complement previous traditionalfood-habit studies, to better understand the complex ecology of these species.