Ontogenetic dietary changes in South American fur seals in Northern Patagonia, Argentina

VALES, DAMIÁN G; CARDONA, LUIS; GARCÍA, NESTOR A; SVENDSEN, GUILLERMO M; AGUILAR, ALEX; CRESPO, ENRIQUE A

Pto. Madryn

Congreso; XV Reunión de Trabajo de Especialistas en Mamíferos Acuáticos de América del Sur. 9° Congreso SOLAMAC; 2012

Centro Nacional Patagónico (CONICET) y Universidad Nacional de la Patagonia San Juan Bosco (Fac. de Ciencias Naturales; Sede Madryn)

Many eared seals undergo dietary shifts during their lifespan, although nothing is known on this regard in South American fur seals, Arctocephalus australis. This study aims to evaluate ontogenetic changes in the diet of this species in northern Patagonia. Skulls of male (n=42) and female (n=4) fur seals dead stranded in coast of Río Negro and Chubut province (Argentina), from 1980 to 2010, were sampled for isotopic analysis. Male individuals were classified as yearling (around 1yr; n=5), juvenile (from yearling to 4.9yr; n=5), subadult (from 5 to 7.9yr; n=10) and adult (from 8 to 12yr; n=22). Females (yearling n=1, juvenile n=2, adult n=1) were included in the study for comparative purposes, although in this case no statistically tests were performed due to the smaller size of the sample. Age of the individuals was assessed by counting growth layer groups in the teeth. Bone samples were dried in a stove and ground to fine powder, lipids and inorganic carbon were removed, and 1mg of dried sample was weighed into tin capsules and analyzed for carbon and nitrogen stable isotopes ratios. The δ15N of yearling males and females was much higher than that of older fur seals, a pattern already reported for others species of eared seals and interpreted as the persistence of the sucking signal. Accordingly, yearlings were removed for further analysis. Linear regression analysis between stables isotopes ratios and age showed that both δ15N and δ13C significantly increase, indicating a gradual shift in diet with age. When juveniles, subadults and adults males were plotted in the isotopic landscape of northern Patagonia, some juvenile individuals remained out of it, denoting the exploitation of other foraging areas or the consumption of extremely pelagic prey (i.e. myctophid fish). Bayesian mixing model SIAR in subadult and adult categories indicated a reduction in the consumption of Engraulis anchoita and an increase in the consumption of Illex argentinus with age. The δ15N and δ13C values of females appear not to be so different to those of males. Differing from other species of eared seals, in which consumption of benthic prey increases with age as consequence of improved diving skills, South American fur seals remain as pelagic foragers throughout life, though gradually increasing with age the consumption of larger trophic level prey. Research funded by Fundación BBVA and ANPCyT