Initial density estimates of humpback whales Megaptera novaeangliae in the inshore waters of the western Antarctic Peninsula during the late autumn

Abstract

In the Southern Ocean, humpback whales Megaptera novaeangliae were depleted by commercial whaling operations during the 20th century, but many populations now appear to be recovering. Previous surveys of whale distribution along the western Antarctic Peninsula (WAP) suggested that humpbacks feed on krill swarms over the continental shelf during the sum- mer, but little is known about their movements and densities during autumn, when krill begin to seek inshore refugia for overwintering. Here we present estimates of humpback whale densities in some inshore regions of the WAP during the late autumn. We surveyed 653.9 km of track line in the Gerlache Strait and adjacent bays during 26 April to 1 June 2009. We detected 371 groups of humpback whales in a distance sampling framework that allowed us to calculate estimates of whale density along track lines in open and enclosed habitats within our study area. Density esti- mates along track lines ranged from 0.02 to 1.75 whales km �2 ; the highest densities were found along track lines in the enclosed regions of Wilhelmina Bay, the Errera Channel, and Andvord Bay. These results provide preliminary insight into the density and distribution of WAP hump- backs and indicate that large numbers of whales remain in Antarctic feeding grounds late into autumn. This study also provides details on the difficulties in estimating density of whales in the inshore regions of the WAP using traditional line transect/distance sampling methods, and pro- vides direction for future studies including the use of model-based approaches to estimating whale densities in this region.