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

VALES, D.G.; CARDONA, LUIS; L.R. DE OLIVEIRA; SECCHI, E.; AGUILAR, ALEX; CRESPO, E. A.

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

High levels of fishing have altered the makeup of many ecosystems, depressing the average trophic level of heavily fished ecosystems1. The coastal gillnet fishery of Brazil operate from coastal to offshore waters2, coinciding with the forging areas of South American fur seals (ARCTOCEPHALUS AUSTRALIS)3. Since A. australis is not a boat follower, does not interact with fishing operations and causes no important damage to fishing gear3. On the other hand, an indirect interaction as competition for resources was not discarded. In this scenario what we expected was a drop in the average trophic level of this apex predator through time. Carbon and nitrogen isotopic analyses of skull bone were used to investigate how the development of industrial fishing has potentially affected the diet of A. australis. Skull samples (turbinate bones) were taken from 45 adult male of South American fur seals from the GEMARS and FURG collections. These animals were found dead on shore along the coast of southern Brazil from 1994 to 2009. Samples were dried in a stove at 60ºC for 36h and ground to a fine powder. Lipids and inorganic carbon were both extracted 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. Age determination was made by counting growth layer groups in the teeth. All the individuals were older than 7 years old. No statistical significant difference was found between stranding year and the values of 13C or 15N. We failed to find a change in the average trophic level of A. australis through the period studied, although we do not discard any change in a broader period of time as consequence of human activities in the aquatic environment.