Movement and diving of killer whales (Orcinus orca) at a Southern Ocean archipelago

Abstract

Eleven satellite tagswere deployed on 9 killerwhales at the Prince Edwards Islands in the Southern Ocean. Statespace switchingmodelswere used to generate position estimates fromArgos location data,while two behavioural modes were estimated from the data. Individuals were tracked for 5.6–53.2 days, duringwhich time they moved 416–4470 km (mean 82.7 km day−1) but 69% of position estimates were within the 1000 m depth contour around the islands (b35 km from the tagging site). Killerwhales showed restricted behaviour close to the islands, particularly inshorewhere they can effectively hunt seals and penguins, and at seamounts to the north of the islands. Generalised linear mixed effect models were used to explore the relationship between environmental variables and behavioural mode. The best model included depth, sea surface temperature, latitude, sea surface height anomaly and bottomslope, but killerwhales did not clearly target features such as fronts and apparentmesoscale eddies. Killer whales showed restricted behaviour in shallow water, at high latitudes and low sea surface temperature — the conditions characterising the archipelago. Dive data fromtwo individuals largely revealed shallow dives (7.5–50mdeep), but deeper dive bouts to around 368mwere also recorded. Diveswere significantly deeper during the day and maximumdive depthswere 767.5 and 499.5 m, respectively. This suggests that killer whales might also prey on vertically migrating cephalopods and perhaps Patagonian toothfish. Three individuals made rapid and directed long-distance movements northwards of the islands, the reasons for which are speculative.