Science Advisory Report 2008/061

Science Advice on Harvesting of Northwest Atlantic grey seals (Halichoerus grypus) in 2009

Summary

• Grey seals form a single genetic population that can be divided into three groups based on the location of breeding sites. Most pups (81%) are born on Sable Island, 15% are born in the Gulf of St. Lawrence and 4% are born along the Eastern Shore of Nova Scotia. This distribution has changed over time, with a decline in the fraction of the population born on the ice compared to on small islands, and an increase in the proportion of animals born on the Eastern Shore, compared to the Gulf.
• Using aerial surveys, total production of Northwest Atlantic grey seals in 2007 was 67,500 (SE=1,400). This includes 54,500 (SE=1,300) pups born on Sable Island, 3,000 (SE=40) along the eastern shore of Nova Scotia, and 9,900 (SE=600) in the Gulf of St. Lawrence.
• Incorporating this information into a population model results in a total population of 300 000 animals (95% CL 240,000-370,000).
• Removals from the population during the last five years include animals taken in the commercial harvest, scientific collections, nuisance seal removals and incidental catches in commercial fisheries. No information is available on incidental catches, and data on number of seals killed as nuisance seals are limited.
• Eight Total Allowable Catch (TAC) scenarios with different age compositions (50% Young of Year [YOY], 50% 1+ animals; 90% YOY, 10% 1+ animals) and four catch levels (15,000, 30,000, 40,000 and 50,000 animals) were examined.
• Model results indicated that seven out of eight scenarios would respect the management objective that the predicted L20 remains above 210,000 during the next five years. The model predicted that L20 would decline below 210,000 with a harvest of 50,000 animals, and an age composition of 50% YOY and 50% 1 + animals.
• Dividing a TAC of 15,000 animals among the three regions would result in a harvest allocation of 11,941, 2,378 and 666 animals for Sable Island, Gulf of St. Lawrence and Eastern Shore, respectively.
• For a TAC of 30,000 animals, the allocation would be 23,882, 4,756, and 1,332 animals for the Sable Island, Gulf of St. Lawrence and Eastern shore herds, respectively.
• 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.

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