Research Document - 2008/078

Assessment of Population Consequences of Harvest Strategies for the Northwest Atlantic grey seal population.

By L. Thomas, M.O. Hammill, and W.D. Bowen

Abstract

We used the outputs of a Bayesian analysis of the population dynamics of the Northwest Atlantic grey seal population between 1977 and 2007 as the basis for an investigation of the consequences of a range of potential future harvest strategies.  We simulated populations using the posterior distribution of model states and parameters from the fitted model, and then projected these populations forward stochastically for 20 years under different harvest regimes. The management objective was to find harvest levels that have an 80% probability of maintaining the population at above 70% of its largest population estimate to date, i.e., above 210,000. We found that this objective could be achieved with harvests as high as 45,000 animals per year when looking over a 5-year window, or 25,000 animals when looking over a 20-year window for a harvest that was 50% young of the year and 50% older animals. Quotas specifying a higher proportion of young of the year could sustain higher total harvest levels.

These results are preliminary, and more discussion of potential harvest strategies and management goals are needed. Note also that are results are dependent on the adequacy of the population dynamics model used. For example, we make no allowance for any behavioural response of seals to increased hunting levels. Also the nature and extent of density dependence in vital rates is poorly understood and may change over time. How density dependence acts on vital rates will have an impact on sustainable harvest scenarios.

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