Differences in social structure, pup mortality and rate of increase between traditional and new occupied areas in south american sea lion (Otaria flavescens) at Punta León, northern Patagonia.

MARTÍN SVENDSEN, GUILLERMO; ENRIQUE CRESPO; SILVANA DANS; BERÓN VERA, BÁRBARA; SUSANA PEDRAZA; SANTIAGO AMEGHINO

San Sebastián-Donostia, España

Congreso; 21th Annual Conference European Cetacean Society; 2007

European Cetacean Society

The population of South American sea lions in northern Patagonia is in increasing at a rate close to 5.7 % annually, fact which is associated to new breeding areas where juveniles are the main social component. Punta León is a traditional breeding rookery, expanded to new beaches in the last 15 years giving place to old and new areas. The objective of this work was to test the null hypothesis that some variables remain constant when comparing old breeding areas (zone a: north) and new ones (zone b: south). These variables included: a) age composition, b) pup mortality during the breeding season and c) rate of increase. Age composition was studied by Principal Component Analysis (PCA) for 2002, 2003 and 2004 breeding seasons. PCA results showed that zone b presents a larger proportion of juveniles and subadult males among adult females, while zone a was characterized by a larger proportion of pups. For 2002, 2003 and 2004 breeding seasons zone a showed 1.9-2.8% of pup mortality, while zone b 3.6-5.6% (p<0,05). This in part could be explained by the absence of the typical breeding structure which gives protection to the pups from solitary males, or in addition to younger females with lesser experience in nursing pups. Finaly, zone a showed a rate of increase of 3% for the period 1983-2004, and not statistically different from 0 for 1994-2004 while zone b increased at a rate of 17% for the period 1994-2004, in agreement with a theoretical decrease in high density conditions. Why females breed and nurse their pups in areas where pup survival is lower is still to be understood.