Research Document - 2009/002

Comparison of strip- and line-transect sampling to estimate density and abundance of ringed seals (Phoca hispida) in western Hudson Bay, 2007 and 2008

By M. Chambellant and S.H. Ferguson

Abstract

We conducted aerial surveys of ringed seals (Phoca hispida) hauled-out on the ice in western Hudson Bay at the end of May 2007 and 2008. A 400m transect width was divided in four intervals of 100m each to allow for comparison of density estimates computed by strip- and line-transect analyses. Ringed seal density estimates varied from 0.97±0.06 seals/km² in 2007 to 0.49±0.04 in 2008 and were in general agreement with previously reported density estimates. The right and left observers detected different relative number of ringed seals and their detection functions differed, likely due to differences in experience. Strip-transect analysis computed negatively biased but equally precise ringed seal density estimates relative to line-transect estimates, and was considered a robust and appropriate sampling method under our survey conditions and protocol. However, because line-transect sampling offers the possibility to control for factors likely to affect the seal detectability, this method is better suited to compare density estimates over years and/or across regions. Thus, we recommend future surveys of ringed seals on ice use line-transect sampling and suggest survey design and data analysis improvements.

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