Movements and Densities of Narwhals in the Offshore Winter Pack Ice - Impacts of Climate Change and Other Factors

The offshore pack ice is one of the most important habitats for narwhals (Monodon monoceros), yet few data are available to quantify ecological relationships. Winter movements of narwhals (n=34) satellite-tagged between 2003 and 2005 on Baffin Island were combined with data on distribution and abundance collected from a double-platform visual aerial survey on the wintering grounds conducted in April 2008. Continuous high-resolution digital photographic sea ice images (n= 2,685) and downward-looking video (5 hours) were also collected on the survey tracklines facilitating a detailed description of the habitat. A fully corrected abundance estimate of 17,239 narwhals (cv=0.58) was calculated for the 9,500 km² area. Within this region, narwhals had access to only 233 km² of open water (2% of the surveyed area), resulting in a density of ~73 narwhals per km². Spatial pattern analysis of lead size, length, and shape on digital images, together with satellite images of sea ice concentration, provided the first detailed quantification of the extreme and dynamic habitat used by this species. Differences in narwhal home range and velocity coincided with variability in annual sea ice conditions. In heavy ice years (2003-04 and 2005-06) the overwintering area used by tagged whales was 128,000 and 153,000 km², respectively, while in light ice years (2004-05) whales used only 21,000 km². No significant differences in mean wintering ground latitude were found across the three years, however median daily velocity (km/day) was significantly different (p=0.002). Narwhals ranged most widely and had the highest velocities in years with the most dense sea ice cover, but remained stationary over their preferred foraging grounds in years with low sea ice cover. This may suggest heavy sea ice requires whales to conduct compensatory movements to keep up with leads and cracks that move up to 25 km/day. Understanding narwhal habitat use in the pack ice is critical to this species given climate change induced sea ice loss rates of 9% decade in Baffin Bay.