Isotopic analysis reveals changes in the diet of the Patagonian south American sea lion from 1970 to present.

DRAGO, M., CRESPO, E.A., AGUILAR, A., CARDONA, L., GARCÍA, N., DANS, S.L. & GOODALL, N.

Montevideo, Uruguay

Congreso; XIII REUNIÓN DE TRABAJO DE ESPECIALISTAS EN MAMÍFEROS ACUÁTICOS DE AMÉRICA DEL SUR, 7º CONGRESO SOLAMAC; 2008

SOLAMAC

The relative concentration of stable isotopes of carbon and nitrogen in the bone (skull) of South American sea lions from northern Patagonia (30 males and 30 females) was analysed to investigate the potential incidence on the species?f feeding habits of the development of industrial fishing that has occurred since the 70s. Skulls from 21 males from Tierra del Fuego were used as a control, since that neighbouring geographical region had a lower fishing impact. Analysis of potential prey revealed that benthic species are more enriched in 13C and 15N than pelagic. In northern Patagonia, the analysis of sea lion bone showed a steady decrease in ?Â13C from the 70s to present both in males and females, but no apparent variation in ?Â15N. Values from the 30?Ls-50?Ls are similar to present times. Conversely, bone from sea lions from Tierra del Fuego indicated no change neither in ?Â13C nor in ?Â15N. These results indicate a progressive shift to consumption of pelagic prey in northern Patagonia, which occurred in parallel to the development of the fishing industry. This is probably due to the increased availability of Argentine hake discarded from trawlers, an increase in the availability of juvenile hakes, a decrease of hake cannibalism and the expansion and use of more pelagic areas and resources by the population at the time of its increase in numbers. Alternative hypothesis to explain the observed decline in ?Â13C are a change in baseline ?Â13C due to the arrival in the region of the invasive seaweed Undaria pinnatifida, a long term decrease in the primary productivity of the south-western Atlantic, or a long term change in surface ocean carbon reservoirs. These alternative explanations were ruled out by different reasons. Therefore, increased consumption of Argentine hake stands as the only hypothesis consistent with the patterns of isotopic change here reported.