Narwhal Abundance in the Eastern Canadian High Arctic in 2013

Abstract

In summer, narwhals (Monodon monoceros) migrate from Baffin Bay to northeastern Canada and northwest Greenland, where they are hunted by Inuit for subsistence. To prevent localized depletion, management of narwhals is based on summer stocks. The High Arctic Cetacean Survey (HACS), conducted in August 2013, was the first survey to estimate abundance of all 4 Canadian Baffin Bay narwhal summer stocks, as well as putative stocks in Jones Sound and Smith Sound, in the same summer. Narwhal abundance was estimated using a double-platform aerial survey. Distance sampling methods were used to estimate detection probability away from the track line. Mark-recapture methods were used to correct for the proportion of narwhals missed by visual observers on the track line (i.e., perception bias). We used a data-driven approach to identify single and duplicate sightings, using 4 covariates to compare differences in sightings made by front and rear observers based on: time of sighting, declination angle, group size, and species identity. Abundance in fjords was estimated using density surface modelling to account for their complex shape and uneven coverage. Estimates were corrected for availability bias (narwhals that are not available for detection because they are submerged when the aircraft passes overhead) using a new analysis of August dive behaviour data from narwhals equipped with satellite-linked time depth recorders. Corrected abundance estimates were 12,694 (95% CI: 6,324–25,481) for the Jones Sound stock; 16,360 (95% CI: 3,833–69,836) for the Smith Sound stock; 49,768 (95% CI: 32,945–75,182) for the Somerset Island stock; 35,043 (95% CI: 14,188–86,553) for the Admiralty Inlet stock; 10,489 (95% CI: 6,342–17,347) for the Eclipse Sound stock; and 17,555 (95% CI: 8,473–36,373) for the East Baffin Island stock. Total abundance for these 6 stocks was estimated at 141,908 (95% CI: 102,464–196,536). Sources of uncertainty arise from the high level of clustering observed, in particular in Admiralty Inlet, Eclipse Sound, and East Baffin Island, as well as the difficulty in identifying duplicate sightings between observers when large aggregations were encountered.

Mapping Arctic cetaceans from space: A case study for beluga and narwhal

“…The Baffin Bay narwhal population is the largest narwhal population in the world (> 140,000 narwhals; [ 32 ]), whose range spans Canadian and Greenlandic waters. They are known to conduct long-distance migrations in large groups, spending winters in dense pack-ice and summers in fiords and bays of the Canadian Arctic Archipelago [ 27 , 33 – 36 ].…”

Distribution and Abundance of the Eastern Canada – West Greenland Bowhead Whale Population Based on the 2013 High Arctic Cetacean Survey

“…Therefore, we provide only a brief description of the methodology here, with a focus on aspects that were specific to bowhead whales. Additional details on survey design, allocation of effort, data collection and management, and analytical approaches are available in Doniol-Valcroze et al (2020).…”

“…We estimated the proportion of visible bowhead whale sightings that were missed by observers using MRDS methods, which required the identification of between-platform duplicate sightings. This was much more straightforward for bowhead whales than for narwhals, which often occurred in large aggregations (due to their higher densities and grouping behaviour), resulting in many sightings over a short period of time (Doniol-Valcroze et al, 2020). In contrast bowhead sightings tended to be more isolated in time, making the identification of duplicates unequivocal in most cases.…”

“…Unlike the case for narwhal (Doniol-Valcroze et al, 2020), there is no experimental evidence to suggest how deep in the water column a bowhead whale can be detected from an aircraft, making it difficult to determine which depth bins to use to calculate an instantaneous availability bias correction factor. To account for this uncertainty, we have combined multiple depth bins (0-2, 0-4 and 0-6 m bins), giving equal weight to each bin.…”

“…No aerial survey has covered the full extent of the bowhead whale summer distribution in the Eastern Canadian Arctic in a single year. While the primary target species of the High Arctic Cetacean Survey (HACS) conducted in August 2013 was the narwhal (Monodon monoceros;Doniol-Valcroze et al, 2020), the summer distribution range of narwhals overlaps to a considerable extent with that of EC-WG bowhead whales.…”