Movements and diving behavior of humpback whales in relation to the capelin distribution in the Barents Sea

Abstract

Humpback whales (Megaptera novaeangliae) are a cosmopolitan species, migrating between their mid- and high latitude foraging- and low latitude breeding grounds. Of these, the Northeast Atlantic (NEA-) population cover the longest migration distance of all mammals that last up to half a year. On the foraging grounds they feed extensively throughout the summer until early winter to gain enough energy for their migration. To date, not much is known about their behavior in relation to prey and how it changes within a feeding area over time, individual variation in humpback whale feeding and how dive behavior differs in ‘feeding dives’ and ‘migration dives’. Therefore, in this study we satellite tagged 10 NEA-humpback whales during autumn in the northern Barents Sea. Their behavior was classified using a behavioral index and diving information was compared with capelin biomass, which was estimated using acoustic and trawl data that overlapped both spatially and temporally with the whales. Capelin has a known diurnal vertical migration; therefore, we use sun angle as a proxy for light availability, which in turn is representative of capelin distribution in the water column. Although with some individual variation, we found that humpback whale behavioral movement was influenced by both the horizontal and vertical distribution of capelin. The diving depth of humpback whales followed the diurnal migration of capelin early in the fall when we had data of capelin distribution. Towards the polar night period (shorter days) the differences in depth use of the whales were more even during day and night, this corresponds to reduced diurnal vertical migration in the capelin distribution as a result of decreasing light intensity. Our results also showed that the whales conducted significantly shorter and shallower dives, after leaving the Barents Sea and initiating their southward breeding migration. We conclude that the energy rich and abundant capelin is an important food source that shape the feeding behavior of for the NEA-humpback whales at their main polar feeding ground. Capelin therefore may be their main energy source for the long migration and thus shape humpback whale population dynamics.