Research Document 2017/052
Grey Seal Population Trends in Canadian Waters, 1960-2016 and harvest advice
By M.O. Hammill, den Heyer, C.E., Bowen, W.D., and Lang, S.L.C.
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 (GSL) seal herds to provide estimates of the Canadian component of the Northwest Atlantic grey seal population from 1960 to 2016. The model was fit to the pup production estimates for the Scotian Shelf (CNS and Sable Island combined), and the GSL separately to allow for the GSL model to include an influence of ice on mortality. The 2014 assessment model was adjusted to use estimates of survival from mark-resighting data from Sable Island. The mark-resighting analysis of Sable Island indicates that males have higher mortality rates than females, which, assuming constant age structure, would result in a ratio of males to females in the population of 0.69:1, instead of 1:1 as previously assumed. A model incorporating the new sex ratio could not be developed in time for this assessment. Consequently, model estimates of total population were adjusted for the new sex ratio after model runs had been completed. Total grey seal pup production in 2016 estimated by the model was 101,500 (95% CI 80,600 to 121,600). The 2016 sex-ratio adjusted total population is 424,300 (95%CI=263,600 to 578,300). The estimated 2016 total population on the Scotian Shelf was 380,300 (95% CL=234,000 to 517,200), and 44,100 (95% CL=29,600 to 61,100), for the GSL. The models predict that the population size continues to increase at an overall rate of 4.4% per year, due primarily to the increase on Sable Island. The current estimate of abundance is less than that estimated in 2014 because of the adjustments for fewer males in the population, lower adult mortality estimated from the mark-resighting analysis and an increase in the assumed ratio of first year mortality to adult mortality. Total allowable removals depend on the age structure of the harvest and the objective of the removals. In the GSL, harvests of 4,500 and 2,400 animals comprising 95% YOY and 70% YOY, respectively, would meet the current management objective (i.e. have a probability of 0.8 of remaining above the Precautionary Reference Level (N70)). For the Scotian Shelf (combined Sable Island and CNS herds), harvests of 30,000 and 17,000 animals comprising 95% YOY and 70% YOY, respectively, would have a probability of 0.8 of remaining above N70. The number of grey seals in different areas of Atlantic Canada varies seasonally. An illustrative example shows that the number of seals in the southern GSL can vary from 30,500 (SE=3,200) during January to March, increasing to a maximum of 73,100 (SE=8,400) animals during July-September and then declining again as animals leave the area in late fall.
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