Research Document - 2011/007

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 2010 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 244,230. We found that this objective could be achieved with harvests as high as 30,000 animals per year when looking over a 20-year window, given a harvest that was 50% young-of-the-year and 50% older animals, assuming that mortality was distributed among ages, sexes and regions in proportion to relative abundance. Quotas specifying 95% young-of-the-year and 5% older animals could sustain higher total harvest levels, up to 70,000 animals per year, and still meet the management objective. Higher quotas could be sustained over shorter time periods, but for a long-lived species such as grey seal, it is debatable whether even 20-years is a long enough time window to judge long-term sustainability.

These results are preliminary, and more discussion of potential harvest strategies and management goals are needed. Note also that 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.

Complete document (PDF, 73 KB)

Accessibility Notice

This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.