Decline of Antarctic fur seal (Arctocephalus gazella) population at SSSI No. 32, South Shetlands, Antarctica, during 1997/98: a discussion of possible causes

Little is known of what happened after the exploitation period with the population of Antarctic fur seal (Arctocephalus gazella) on the Antarctic peninsula region, specifically on the South Shetland archipelago. We compiled historical information for Cape Shirreff and San Telmo islets located on the northen tip of Livingston island, where the most important colony of this species on the South Shetland breeds. From 1991 onwards an ecosystem monitoring program started to develop and detailed population data has been obtained as well as other relevant information. Counts have been developed each season diferentially counting sex and age categories to assess total population. We present the up to date logistic model that describes pup population increase from 1965/66 to 1997/98.
During this last season, a notable population decrease was observed and reasons for this occurrence seem unclear since observed pup mortality was lower than previous seasons, and leopard seal (Hydrurga leptonyx) predation was also lower than last year. We hipothesize that large scale fluctuations in physical oceanographic features (like the current El Niño Southern Oscillation event and related sea-ice extent influence) might be changing krill abundance and distribution, in turn affecting local predator reproductive performance. Hucke-Gaete et al. (1998) report that pup growth rates for 1997/98 season do not reflect an El Niño event, although lower mean weights are reported. Nevertheless, we expect to see during the following split season (1998/99), a further decrease in pup production, lower weights in pups, and considerable decrease in total population due to emigration to better foraging areas.