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Research Document - 2014/037

Grey Seal Population Trends in Canadian Waters, 1960-2014

By M.O. Hammill, C.E. den Heyer and W.D. Bowen

Abstract

A model of grey seal population dynamics was fitted to pup production estimates for the Sable Island, Coastal Nova Scotia (CNS) and Gulf of St Lawrence (Gulf) seal herds to provide estimates of the Canadian component of the Northwest Atlantic grey seal population from 1960-2014.  As with the 2012 assessment, the model was fit to each of the three herds separately, resulting in separate predictions of pup production and total population size by estimating initial population size (α), adult mortality (M) and environmental carrying capacity (K).  Grey seal pup production in 2014 is estimated to be 93,000 (95% CI=48,000 to 137,000) animals, with a total population of 505,000 (95% CI=329,000 to 682,000). Sable Island production is estimated to account for about 77% of the estimated total number of pups born in 2014. The estimated 2014 total population of each herd was 394,000 (95% CI 238,000 to 546,000), 13,800 (95% CI=9,300 to 27,300), and 98,000 (95% CI=54,000-179,000), for the Sable, Coastal Nova Scotia and Gulf of St Lawrence herds respectively.  The model predicts that the overall population has continued to increase. To assess the impact of potential future removals on the population, the 2014 population was projected forward for 30 years under different harvest scenarios. To maintain an 80% probability of staying above the precautionary reference level (N70) and assuming that young of the year (YOY) comprise 95% of the catch, then harvests of 39,200 animals (Sable:33,000; CNS:1,200; Gulf:5,000) could be taken. If YOY comprise 90% of the catch then the total harvest would be 36,600 (Sable=31,000; CNS: 1,100; Gulf: 4,500). If YOY comprise 70% of the catch, then the total harvest would be 28,200 (Sable: 24,000; CNS: 700; Gulf: 3,500). Higher harvests are associated with increased risk of falling below the limit reference level and subsequent population collapse. Based on this assessment of the population, harvest levels at 50,000 grey seals or more, even with a harvest comprising 95% pups, have a moderate to high probability of resulting in a population decline below N70 and N50. The risk increases with the level of harvest and with the percentage of the harvest of seal age 1 and older.  Further work is needed to estimate age and sex-specific adult mortality and reproductive rates and to incorporate change in these rates into the herd-specific models, to determine the strength and nature of density-dependent changes in vital rates. Density- dependent changes in vital rates have a large impact on estimates of carrying capacity and the sustainability of future removals. Estimates of population size and trends will become increasingly uncertain until there is new information on pup production for each of the herds. A reassessment of the stock should be completed when new pup production estimates are available or be triggered by evidence of considerable changes in vital rates, including mass mortality of adult seals.

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