SOCIAL CHANGES OF SOUTH AMERICAN SEA LIONS AT PUNTA LEON, A PATAGONIAN BREEDING ROOKERY, IN THE CONTEXT OF AN INCREASING POPULATION.

ENRIQUE A. CRESPO; SILVANA L. DANS; GUILLERMO M. SVENDSEN; ALBERTO KOWALYK; BÁRBARA BERÓN VERA; SUSANA N. PEDRAZA

Islas Canarias, España

Congreso; 17th Annual Conference European Cetacean Society; 2003

The sea lion population in northern Patagonia increases annualy at a rate of 4%. Out of 19 rookeries and haul out places, Punta León is one of the most important with 2000 pups (17% of the pups born at Península Valdés). In the early 80’s there were born 900 pups (25% of the total). In addition to the increase of the pups born, it was observed an expansion of the rookery from 4 to 7 km to the south. Social composition in the expanded rookery show different characteristics with regard to those of old breeding 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 calculated separately for zone a and b. The proportions in zone a were 8.4% adult males holding females, 5% solitary adult males, 29.7% females + juveniles and 56.9% pups. In zone b the proportions were 4.5 adult males holding females, 7.5 solitary adult males, 55.5 females + juveniles and 32.5 pups. Zone b holds a larger proportion of solitary males and juveniles between the females. Out of 2,033 pups, 1,592 were born in zone a and 441 in zone b. Zone a showed 2% of pup mortality while zone b 3.6% (p<0,05), which could be explained in part to the absence of the typical breeding structure which gives protection to the pups from solitary males. Finaly, zone a showed a rate of increase of 3% for the period 1983-2002, while zone b increased at a rate of 21% for the period 1990-2002, in agreement with a theoretical decrease in high density conditions.