Research Document - 2015/060

Abundance estimates of narwhal stocks in the Canadian High Arctic in 2013

By T. Doniol-Valcroze, J.-F. Gosselin, D. Pike, J. Lawson, N. Asselin, K. Hedges , and S. Ferguson

Abstract

In summer, narwhals of the Baffin Bay population migrate to the fiords and inlets of northeastern Canada and northwest Greenland. Numerous Inuit communities across Nunavut hunt narwhals on their summering grounds or along their fall migration routes for subsistence. To prevent localized depletion, management of this population is based on summering aggregations. Abundance estimates for most of these stocks were dated while others were totally lacking or known to be incomplete. DFO conducted the High Arctic Cetacean Survey (HACS) in August 2013 to estimate abundance of all four Canadian Baffin Bay narwhal summer stocks as well as putative stocks in Jones Sound and Smith Sound. This is the first survey to count all of the narwhal stocks in the Canadian High Arctic during one summer. This document presents the results of the survey and new abundance estimates for the stocks, as well as updated estimates of Potential Biological Removals (PBR).

Narwhal abundance was estimated using a double-platform aerial survey. Three aircraft were used simultaneously to cover the vast survey area within a short time frame. Each stock range was divided in several strata, based on geographic boundaries as well as observed densities of narwhals from past surveys. Distance sampling methods were used to estimate detection probability away from the track line. Mark-recapture methods were used on the sighting data from two observers on each side of the aircraft to correct for the proportion of narwhals missed by visual observers (i.e., perception bias). Abundance in fiords was estimated using density spatial 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 plane passes overhead) using a new analysis of satellite-linked time depth recorders transmitting information on the diving behaviour of narwhals in August.

Fully corrected abundance estimates were 12,694 (Coefficient of Variation [CV] 33%) for the Jones Sound stock, 16,360 (CV 65%) for the Smith Sound stock, 49,768 (CV 20%) for the Somerset Island stock, 35,043 (CV 42%) for the Admiralty Inlet stock, 10,489 (CV 24%) for the Eclipse Sound stock, and 17,555 (35%) for the East Baffin Island stock. Sources of uncertainty arise from the high level of clustering observed, particularly in Admiralty Inlet, Eclipse Sound and East Baffin Island, as well as the difficulty in identifying duplicate sightings between observers in large aggregations.

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