Diving behaviour of narwhals (Monodon monoceros) on their summer grounds

Abstract

Three adult female narwhals from the population that summers in the sheltered waters of northern Baffin Island, Canada, were fitted with satellite-linked UHF radio packages. The units logged and transmitted diving and swim-speed data and revealed the location of the study animals for periods of up to 19 days. Most diving activity was in the top 40 m and bottom 40 m of the water column. Shallow dives were characterized by low rates of descent and ascent (<0.5 m∙s−1), short submersions, and frequent changes of vertical direction. Deep dives were normally to, or near to, the seabed in depths of up to 257 m, and involved both faster rates of vertical movement (1–2 m∙s−1) and longer periods underwater (up to 15.1 min). Midwater dives were intermediate in all respects. Deeper dives were of a characteristic "square" profile and provided an average of 7 min at the chosen depth, representing 57% of the time submerged. No dives were demonstrably longer than the estimated aerobic dive limit for this species. Activity at depth, including rapid swimming and many changes of vertical direction, was indicative of foraging and consistent with a diet of many small prey items. Overall, a relatively small proportion of time was devoted to presumed foraging activity, suggesting either that feeding is a low priority at this time of year or that the whales' nutritional needs could be easily met by the food stocks available.