Killer whales (Orcinus orca) at Marion Island, Southern Ocean
Killer whales (Orcinus orca) were studied using data obtained on an opportunistic basis between 1973 and 1996 at Marion Island (46°54’S, 37°45’E) in the Southern Indian Ocean. A clear seasonal pattern of occurrence with the main peak between October and December was evident. Most killer whales were observed within 5 m of the shore and adult males typically occurred further offshore than adult females. The distribution of killer whales around the island was not uniform and more than 80 % of all sightings occurred close to the base station. This was probably due to a concentration of search effort in the vicinity of the base station and to an uneven distribution of prey species. Killer whales occurred in groups with an average of 3.56 individuals and a maximum of 28 animals per group; females predominated throughout the study. Dawn-to-dusk surveys of killer whales in the near-shore waters during 1986, 1989 and 1990 showed small, statistically non-significant peaks of sightings in the early morning and late afternoon. Twenty-six killer whales were individually identified from photographs, eleven of which recurred at Marion Island. Modified photogrammetric methods may improve current photogrammetric analysis of cetacean dorsal fins.
Key words: population structure; seasonal patterns, photogrammetry.