Long-term studies on stable isotopes reveal no change in the diet of South American fur seals along the southern Brazilian coast

VALES, DAMIÁN GUSTAVO; CARDONA, LUIS; OLIVEIRA, LARISSA ROSA DE; SECCHI, EDUARDO R.; AGUILAR, ALEX; CRESPO, ENRIQUE ALBERTO

Galway

Conferencia; 26th European Cetacean Society Conference; 2012

Irish Whale and Dolphin Group (IWDG); Galway-Mayo Institute of Technology (GMIT), Marine Biodiversity Research Group (GMIT)

<!-- /* Font Definitions */ @font-face {font-family:"Cambria Math"; panose-1:2 4 5 3 5 4 6 3 2 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-1610611985 1107304683 0 0 415 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:ES; mso-fareast-language:ES;} .MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; font-size:10.0pt; mso-ansi-font-size:10.0pt; mso-bidi-font-size:10.0pt;} .MsoPapDefault {mso-style-type:export-only;} @page WordSection1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.WordSection1 {page:WordSection1;} --> High levels of fishing have altered the structure of many ecosystems, depressing the average trophic level of heavily exploited ecosystems. The coastal fisheries of southern Brazil operate utilizing diverse gears from coastal to offshore waters and currently many commercial stocks are overexploitated or fully exploited. South American fur seals, Arctocephalus australis, do not breed in southern Brazil, but migrate seasonally to forage. This species is not a boat follower and hence do not interact directly with fishing operations, but competitive interactions cannot be discarded. In this scenario, the intense exploitation of marine resources in southern Brazil might have resulted in a drop of the average trophic level of this high trophic level predator through time. Stable isotopes of carbon and nitrogen were analysed to test this hypothesis. Skull samples (turbinate bones) were taken from 45 adult male of South American fur seals from two scientific collections at southern Brazil. These animals were found dead on shore along the coast of southern Brazil from 1994 to 2009. Age determination was made by counting growth layer groups in the teeth and only adult males older than seven years old were used for further analysis. Bone samples were dried in a stove at 60ºC for 36h and grounded to a fine powder. Lipids and inorganic carbon were both removed with a chloroform/methanol (2:1) solution and hydrochloric acid (0,5N), respectively. Dried samples were weighed into tin capsules and analyzed with a continuous flow isotope ratio mass spectrometer. No statistical significant correlation was found between stranding year and the values of d13C or d15N, which were rather constant through time. This result reveals no major change in the diet of fur seals since 1994, although changes over a broader time span, as consequence of human activities, cannot be ruled out.