Deep-diving by narwhals Monodon monoceros: differences in foraging behavior between wintering areas?

Abstract

Variation in resource selection among sub-populations may elucidate differences in fit- ness and life history strategies. Specifically for top marine predators, differences in movements and behavior may result from responses to variation in a patchy, dynamic environment. Satellite-linked time-depth recorders (SLTDRs) were used to examine differences in narwhal Monodon monoceros diving behavior and habitat selection among 3 sub-populations in Canada and West Greenland (n = 16 individuals). The number of dives to different depths and time allocation within the water column was investigated in 3 seasons, with a focus on 2 discrete wintering grounds in Baffin Bay. Diving parameters were calculated from binned dive data and analyzed using repeated-measures mixed models accounting for temporal autocorrelation and individual variability. The number of surface dives (0 to 50 m) and time at the surface declined between summer and winter. Clear differences were observed between 2 wintering grounds. Whales occupying one wintering ground spent most of their time diving to between 200 and 400 m (25 dives per day, SE 3), confirmed by both depth and temperature recording tags. In contrast, narwhals in a separate wintering ground spent less time at shallow depths and most of their time diving to at least 800 m (13 to 26 dives per day, SE 1 to 3). A model of occupancy time at depth showed that whales making multiple daily deep dives spent over 3 h at > 800 m (SD 0.6) and traveled 13 min (SD 1) per round trip to reach this depth. Whales diving to between 200 and 400 m spent approximately 2.5 h (SD 0.4) at this depth, traveling 5 min per round trip. The observed differences in time allocation and dive behavior indicate local variation between the 2 wintering grounds in the Baffin Bay ecosystem.