Research Document 2019/029

Evaluation of LGL visual aerial survey data for estimating narwhal abundance in Eclipse Sound during the open water season 2013–2015

By Matthews, C.J.D., Hornby, C.A., Ferguson, S.H., and Marcoux, M.

Abstract

The Eclipse Sound narwhal (Monodon monoceros) stock is one of several summering aggregations of the Baffin Bay narwhal population. LGL Environmental Research Associates were enlisted by Baffinland Iron Mines Corporation to conduct aerial surveys of narwhals in Eclipse Sound during the open-water season (early August to mid-October) from 2013-2015 to assess shipping impacts of the Mary River Project on their distribution. Baffinland provided the three years of aerial survey data to DFO to evaluate for the purpose of estimating Eclipse Sound narwhal stock abundance. The spatiotemporal coverage of the surveys differed among the three years, and only the 2014 surveys spanned the entire range of Eclipse Sound narwhals during the early to late August period when narwhals are assumed to be resident to their summering stocks. The 2013 data in particular excluded portions of the Eclipse Sound narwhal summering range during the period of peak occupancy, and were therefore not analysed. Histograms of the number of detections with perpendicular distance revealed a large number of detections were missed within the defined strip width during all survey years. Although distance sampling analysis can account for such declines in detections with increasing distance from the track line, the high proportion of observations missing perpendicular distances posed problems for this analysis. In particular, assumptions that missing distances did not occur on the track line or occurred randomly with respect to perpendicular distance could not be evaluated. Other limitations included the single midline transects of fiord strata, which violated distance sampling assumptions and did not span a sufficient range of covariate values to estimate abundance using density surface modelling. Validity of the abundance estimates for calculating hunt quotas is therefore uncertain. However, estimates may be potentially useful as indices of relative abundance throughout the survey period and among years for the areas that were surveyed consistently.

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