Fisheries development does not affect the diet of South American sea lions (Otaria flavescens) in southern Brazil.

ZENTENO, L.; CRESPO, ENRIQUE A.; L.R. DE OLIVEIRA; CARDONA, L.; SECCHI, E.; BORRELL, A.; AGUILAR, A.

Galway

Congreso; 26rd. ANNUAL CONFERENCE OF THE EUROPEAN CETACEAN SOCIETY; 2012

Although incidental bycatch is often considered a main negative impact of fisheries on marine mammals, depletion of food resources due to overfishing is also of major concern. Industrial bottom trawling in southern Brazil has developed since 1990 and has resulted in a significant reduction in the average individual size of the exploited fish species, which in turn resulted in a reduction in biomass of species situated at higher trophic level. Stable isotopes of carbon and nitrogen have been used to asses whether these changes have affected the diet of the South American sea lions, Otaria flavesces, foraging in the region. Male sea lions prevail in souther Brazil and are thought to disperse from the breeding colonies in Uruguay. Bone samples were collected from the skulls of 47 malesfrom this species that were found dead stranded in southern Brazil from 1986 to 2010. Bone samples were grinded and the resulting powder was soaked in 2:1 chloroform: methanol solution to extracted lipids. Bone samples were later treated with hydrochloric acid to remove inorganic carbon. Finally, samples were analysed using a mass spectrometer. Male age was assessed by counting growth layers in the dentine of the canines. No statistically significant correlation was found between stranding year and the values of δ13C or δ15N, either for the whole sample or for subsamples including only males younger than 7 years (immature males) and older than 6 years (adults). Furthermore, no statistically significant differences were observed in the δ13C and δ15N of males of any age class stranding before and after the development of the fishery (1990). These results suggest that the development of the industrial fishery has not caused relevant changes neither in diet composition nor in patterns of habitat use by South American sea lions, even if changes in food availability have occurred.