Research Document - 2015/059

Spatial modelling of narwhal density in fiords during the 2013 High Arctic Cetacean Survey (HACS)

By T. Doniol-Valcroze, J.-F. Gosselin, D. Pike and J. Lawson

Abstract

Narwhal stocks in Baffin Bay, Jones Sound and Smith Sound were surveyed in the 2013 High Arctic Cetacean Survey (HACS). Previous studies have shown that narwhals spend time inside narrow inlets and fiords on their summer distribution range. Thus, any surveying effort must include these areas to provide a credible abundance estimate. Estimating abundance in fiords, however, creates logistical and statistical difficulties because of their narrow complex shapes and high cliffs, preventing the use of conventional distance sampling based on systematic transects. To address these issues, we used a two-stage cluster sampling design in which fiords designated as primary sampling units were selected in a way that maintained equal probability and systematic coverage. Within each fiord, we estimated density and abundance of narwhals using spatial density modeling. Density surface models do not require track lines to be designed according to a formal survey sampling scheme, and accommodates both non-random and unequal coverage. Moreover, the resulting variance of the abundance estimate incorporates both the variance from the detection function and that of the spatial model. Because no observations were made in West Ellesmere fiords, no abundance estimate was produced. Sightings of narwhals in the other fiords during HACS were highly variable. After expanding the abundance estimates to unsurveyed fiords, total (surface) abundance estimates were 45 for Jones Sound fiords (CV 94%), 1,916 (CV 45%) for Smith Sound fiords, 143 (CV 85%) for Admiralty Inlet fiords, 1,135 (CV 19%) for Eclipse Sound fiords, and 3,799 (CV 35%) for east Baffin Island fiords. Abundance estimates for the fiord strata will be added to other strata estimated via conventional distance sampling.

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