Research Document 2019/030
Effect of the 2015 narwhal (Monodon monoceros) entrapment on the Eclipse Sound narwhal stock
By Watt, C.A., Marcoux, M., Dunn, J.B., Hodgson, R., Moore, R., and Ferguson, S.H
Abstract
A narwhal (Monodon monoceros) entrapment event of at least 249 whales occurred near the community of Pond Inlet, Nunavut in November 2015. We evaluated how this particular entrapment event may impact the Total Allowable Landed Catch (TALC) for Pond Inlet for future years, and modelled how these events may impact the Eclipse Sound narwhal stock over the next 100 years. To determine entrapment impacts on the TALC we modelled different magnitudes of entrapment, from 249 narwhals (minimum), up to 1,000 narwhals (maximum). Satellite tagging data in recent years have shown some mixing of the Admiralty Inlet and Eclipse Sound narwhal stocks. As a result of this potential mixing of narwhals, we modelled scenarios where 100% of the entrapment included whales only from Eclipse Sound, and another scenario where only 76% of the individuals were from Eclipse Sound, assuming the other 24% came from Admiralty Inlet (proportion of the stock that left Admiralty Inlet and entered Eclipse Sound in the fall based on 4/42 satellite tags). Potential biological removal (PBR) was calculated assuming the entrapment event exceeded mortality levels already included in the base level of natural mortality by varying proportions (20%, 40%, 60%, 80%, and 100%). A population model was also used to investigate the impact of frequent entrapments (every 3, 5, and 10 years) that range in magnitude anywhere from 200–1,000 narwhals to investigate the long term impact these events may have on the Eclipse Sound narwhal stock. Models were run with no error term and with an arbitrary process error of 0.05. Results from the PBR model indicated that in a worst case scenario, where the entrapment included 1,000 individuals all from Eclipse Sound, and where mortality is considered above the natural mortality already included in the PBR calculation, the TALC need only be reduced by 13 animals, from 134 to 121. When no error term was included in the population model, the model indicated that entrapment events of the magnitude previously seen in Eclipse Sound would not cause the stock to go extinct if the events occur every 3 years. The lower 90% confidence intervals, however, do suggest an extinction possibility if entrapments occur every 3 years, and indicate declines in stock size if the events occur every 5 or 10 years. Similar results were seen when an error term was included in the population model, except that entrapment events happening every 3 years would result in stock extinction within 100 years, and the median stock size declines even with entrapments only occurring every 5 or 10 years. In the event of a future entrapment, monitoring of the event and research on narwhal behaviour would aid in improving our understanding of the impacts of these events on narwhal stocks.
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