Research Document - 2012/169
Northwest Atlantic Grey Seal Population Trends, 1960-2012
By Hammill, M.O., Bowen, W.D., and den Heyer, C.
Abstract
A model of Northwest Atlantic grey seal population dynamics was fitted to available pup production data to provide estimates of the Canadian component of the Northwest Atlantic grey seal population from 1960-2012. In previous assessments, the population model estimated a common adult and juvenile mortality rate for the whole population and separate carrying capacities for each herd. For the 2012 assessment, the model was fit to each of the three breeding colonies separately, resulting in separate estimates of pup production, total population size, adult and juvenile mortality and environmental carrying capacity. The 2012 total population of each herd was 262,000 (95% CI 219,000 -332,000), 20,000 (95% CI=17,000-23,000), and 49,000 (95% CI=27,000-102,000), for the Sable, Coastal Nova Scotia and Gulf of St Lawrence herds respectively. In 2012, total pup production was estimated as 75,000 (95% CI=63,000-97,000) animals, with a total population of 331,000 (95% CI=263,000-458,000). Taking into account changes in modeling approaches the population has increased slightly. Total removals should not exceed 36,700 animals if YOY comprise 95% of the harvest or 19,900 animals if YOY comprised 50% of the harvest. Higher harvests are possible, but with increased risk of falling below the limit reference level and subsequent population collapse. Further work is needed on how reproductive rates are incorporated into the herd specific models, as well as the treatment of removals from each area, particularly on the Scotian Shelf area.
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