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Research Document - 2013/031

Estimating total allowable removals for walrus (Odobenus rosmarus rosmarus) in Nunavut using the potential biological removal approach

By R.E.A. Stewart and J.W. Hamilton


DFO Science was asked for advice on sustainable harvest levels for seven stocks of walrus in the Canadian Arctic. No data are available for one stock (south and east Hudson Bay stock) but recent surveys allow calculation of total allowable removal (TAR) levels for the other six stocks. The Potential Biological Removal (PBR) method has been adopted by DFO Science to recommend sustainable levels of removal from marine mammal populations for which there is little information, termed data poor. The results of recent walrus surveys included estimates of the Minimum Counted Population (MCP), as well as adjusted abundance estimates derived using different factors for availability and detectability. The MCP and adjusted abundance estimates were used to estimate levels of TAR (Total Allowable Removal) using the PBR method, noting that all of the estimates are considered negatively biased. The resulting TAR estimates were then compared to recorded landed harvest levels over the last 25 years (about one walrus generation) without adjustments for reporting accuracy or struck-and-lost estimates. For the high Arctic walrus population, it was not possible to partition harvest to the three component stocks but the overall estimated TARs exceeds currently reported landed harvests in Canada. For the central Arctic walrus population, the TAR estimates for the Foxe Basin stocks straddle the lower 95% confidence limit of recent harvest levels, indicating a need for better survey coverage in estimating abundance, and better information on current removals from all sources. Only a small portion of the range of the Hudson Bay-Davis Strait stock has been surveyed, and calculated TAR levels suggest that the local harvest is sustainable. The central Arctic population as a whole lacks sufficient data for a meaningful population estimate and subsequent advice on TARs. Stocks within both the high Arctic and the central Arctic walrus populations are shared with Greenland, and continued collaboration and exchange of harvest data is warranted.

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