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

Killer whale predation on pinnipeds at Sea Lion Island (Falkland Islands)

Killer whales (Orcinus orca, KW) predation on marine mammals has been widely reported in the Southern Hemisphere. Anecdotal reports suggest that KW visiting Falkland Islands include pinnipeds in their diet, but strong evidences are lacking. Here we present the results of a study on KW at Sea Lion Island (SLI), which shelters the main southern elephant seal (SES) breeding colony in the Falklands, and a small southern sea lion (SSL) population. From September 2013 to March 2014 (210 days), we carried out: 1) shore-based surveys (6641 hours; 5542 km walked) of the SES and SSL breeding areas; 2) observations of KW behaviour; 3) regular counts of pinnipeds; 4) necropsies of dead SES and SSL. We observed 71 likely predation events (1.07 per 100 hours), 39 of which were sure. SES were targeted in 79.1% of predations, and SSL in 11.6%. Weaned SES pups were the main prey (38.2% of SES predations). The overall success rate was lower than expected (41.3% of predations). We also observed predations on SES sub-adult males and breeding females. Moreover, apart from this field season, males 6 to 12 years old were killed (but not eaten) by KW during last years. The first event occurred in 2007, and a maximum of 3 breeding males were killed during a season (in 2012). Peculiar injuries were observed in necropsies of breeding males, including significant damages of the ribs. The hunting behaviour of KW was characterised by cooperative strategies and long lasting handlings of preys. Simple energetic models suggest that pinniped preys do not meet KW feeding requirements. Therefore, although SES and SSL predation is probably the main reason of KW presence at SLI, these species should only represent one component of their diet. This suggest unusual generalist hunting habits for this still little known KW population.