MARCO CASOLI1, FILIPPO GALIMBERTI2, SIMONA SANVITO2
1 Dipartimento BiGeA, Università degli Studi di Bologna, Bologna, Italy
2 Elephant Seal Research Group, Sea Lion Island, Falkland Islands, www.eleseal.org

BEHAVIOURAL ECOLOGY OF KILLER WHALES (Orcinus orca) AT SEA LION ISLAND, FALKLAND ISLANDS

Killer whales (Orcinus orca, KW) are the only social top predators of the oceans. They are a charismatic species, and have a worldwide distribution, a complex matrilineal social organization, a features rich communication system and elaborated predation strategies. One of the main KW hot spots in the southern hemisphere is the Atlantic coast of South America. Although the presence of KW along the coast of Patagonia is well documented, their status in the nearby Falkland Islands is uncertain. Preliminary evidences suggested the presence of KW at Sea Lion Island (SLI), the main breeding site of elephant seals (Mirounga leonina, SES) in the Falklands. From September 2013 to March 2014 (210 days) two to seven observers carried out a study of KW at SLI, that included: a) shore-based observations (6641 hours, 5542 km walked), b) photo-identification to recognise individuals, 3) regular counts of potentials preys, SES and southern sea lions (Otaria byronia, SSL) and, 4) necropsies of dead seals and sea lions. We identified a small community of 16 KW, 56% of which were regularly observed during the whole field season. Some of these KW have already been photographed at SLI in the past and, therefore, show site fidelity. Sightings occurred from 15/09/2013 to 06/03/2014, with one main peak in February (1.31 sightings per 10 hours; SES moulting and SSL breeding season) and a lower one in November (0.86; end of SES breeding and peak presence of SES weaned pups). KW activities were concentrated in areas of high density of weaned SES pups and in front of the SSL breeding colony. The likelihood of sightings was higher with low winds, null or moderate swell and calm sea close to the shoreline. The social organization of KW was complex and dynamic. The basic social unit (pod) was the association of mother and 1-2 calves. Different pods showed a variable level of aggregation during the season, apparently related to the distribution of preys. We witnessed 71 predation events, the majority against elephant seal weaned pups. Although predation success was somehow lower than expected, the temporal and spatial distribution of sightings and predation attempts suggest that SES may represent an important prey in KW feeding strategies.