Research Document - 2014/038

Assessing Harp Seals and Providing Advice in a Multiyear Framework

By M. O. Hammill and G.B. Stenson

Abstract

The commercial harvest of harp seals in Atlantic Canada is managed under the Atlantic Seal Management Strategy (ASMS) using renewable 5 year management plans. The ASMS identifies two approaches to providing advice depending upon our understanding of the population dynamics of the population. Harp seals are classified as being ‘Data Rich’ and advice can be provided through an appropriate population model. Therefore, scientific advice on catch levels that will meet the management objectives are provided using a population model that incorporates data on annual reproductive rates, age specific removals (reported catches in Canada and Greenland, bycatch and estimates of the numbers of seals killed but not landed or reported), ice associated mortality of young and periodic estimates of pup production. Reproductive rates are obtained every year but major assessments are only carried out every 4-5 years when new pup production estimates are available.  Under the ASMS, if the most recent estimates of pup production or reproductive rates are more than 5 years old, the population would be considered ‘Data Poor’ and a more conservative approach adopted for providing catch advice.

Because the assessments are carried at 4-5 year intervals, it is important that the uncertainty associated with multi-year assessments is incorporated in the provision of advice. The vast majority of seal are killed as young while the main indicator of population trends is pup production. Since seals are not sexually mature until, on average, 6 years of age, any proposed harvest must continue to respect the precautionary reference level over a period of at least 15 years. The projection period is extend to allow the impact of taking primarily (>95%) young of the year to be propagated through the population structure. Also, the precautionary reference level (N70) must be set to ensure that probability that the population will remain above the limit reference level remains high (~95%). To account for an increase in uncertainty in our population projections as the time since the last survey increases, proposed harvest levels are required to maintain an 80% probability that the population would remain above precautionary reference level, rather than the mean or medium of the runs. A multiyear Total Allowable Catch (TAC) was successfully used for harp seal in the past. Simulation studies show that up to 20% of the average quota can be transferred to other years as long as the total removals over the time of the management plan remain the same.

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