Research Document - 2012/020

Stock-Dynamic Model for the Northern Hudson Bay Narwhal Population Based on 1982-2008 Aerial Surveys

By M.C.S. Kingsley, P. Richard, and S.H. Ferguson

Abstract

The Northern Hudson Bay narwhal population was previously assessed from aerial photographic surveys of summer aggregations in 1984, 2000, and 2008. The August 2008 survey was intended to provide information necessary for a full assessment of the population, but owing to camera malfunction, sea ice conditions, and poor weather a partial estimate using the 21–22 August survey provided an estimate of only 610 surface-visible narwhals (95% CI: 376–989), suggesting a loss of half of the population since the previous survey in 2000. A stock-dynamic model using Bayesian methods and run on the WinBUGS platform was developed to assess the population size indices updated with the catch history to inform management of this population. First, models were run using all three surveys and updated catch history. Second, the models were run without the 2008 survey. Third, the model was run using all three survey results and adding killer whale predation to understand its possible influence on the time series of population size estimates. The hunting levels of recent years, mean reported landings of 89 narwhals a year since 1998, appear from these results to be sustainable using model results from the model run without the 2008 survey information. The low result of the 2008 survey is not to be explained by the higher catches since 1998. The few survey estimates and the lack of good information on loss rates in hunting limited the ability of model results to estimate the present status of the population. The recent 2008 survey appeared incompatible with earlier survey estimates and reported catches. Model results could match the 2008 survey estimate by introducing predation since the preceding survey in 2000. However, the predation required to fit the survey results was high and would indicate that the population is unlikely to sustain even low catches. The results from this modelling exercise are uncertain and do not provide reliable estimates of future sustainable catches; we conclude that a further survey is required.

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