Science Advisory Report 2022/018

Stock assessment of Northwest Atlantic grey seals (Halichoerus grypus) in Canada in 2021

Summary

• Grey seals form a single population that is divided into two herds in Canada for management purposes based on the location of breeding sites: Scotian Shelf (Sable Island and Coastal Nova Scotia) and Gulf of St. Lawrence (Gulf). 
• As done previously, pup production was assessed through aerial surveys in 2021. Total pup production after accounting for pre-survey mortality and availability bias was 98,200 (95% Confidence Interval (CI)= 86,800–109,700), with 81,300 (95% CI= 74,500–88,100) born on the Scotian Shelf, and 16,900 (95% CI= 12,300–21,500) born in the Gulf. Sable Island was the largest colony with 76,600 (95%= 70,800–82,300) pups (78% of the total). Pup production estimates on the Scotian Shelf were slightly less than 2016 estimates, while the reverse was observed in the Gulf, although these differences were not significantly different from the previous estimates.
• The 2021 survey marks the first time in 60 years that the estimate of pup production has decreased on Sable Island.
• A new integrated population model (IPM) was used to convert pup production estimates to total population size using reproductive and survival rates, and anthropogenic removals. This model combines data from various sources in a single model that better captures both what is known about grey seal biology and the level of uncertainty about the dynamics of the population.
• The IPM was fit with four sets of parameters that reflect uncertainty in our understanding of factors affecting juvenile mortality. The combined results were used to develop the advice. The estimated total abundance for grey seals in 2021 was 366,400 (95% CI= 317,800–409,400, rounded to the nearest 100). For the Scotian Shelf, the total abundance estimate was 310,700 (95% CI= 263,200–351,500); the Gulf was 56,000 (95% CI= 48,600–64,600).
• The new model provided a 2016 abundance estimate of 339,400 (95% CI= 317,900–361,500) grey seals that was within the range of the previous assessment estimate of 424,300 (95% CI= 263,600–578,300), and shows the same overall population trajectory. The difference between assessments is due to changes in the structure of the new model and the higher estimates of juvenile mortality produced by the model fit to the 2021 pup production estimates.
• The rate of growth of the population has continued to slow. Total abundance increased at a rate of 1.5% per year between 2016 and 2021.
• For a winter harvest with 95% young of year (YOY), a total harvest of 77,300, with 68,600 allocated to the Shelf and 8,700 allocated to the Gulf, would respect the current management objective. As the proportion of 1+ in the harvest increases, the allowable harvest declines. For example, total for both herds of a harvest of 50% YOY would be 24,200 seals, with 22,500 allocated to the Shelf and 1,700 allocated to the Gulf.
• Pupping begins earlier on the Scotian Shelf than in the Gulf. Assuming that lactation lasts for 20 days, an estimated 50% of pups were weaned on 6 January on the islands in southwest Nova Scotia, 10 January on Sable Island, 16 January on Hay Island, 18 January on Brion Island, 21 January on Henry Island, and 24 January on Pictou and Saddle Islands.
• Changes in timing of pupping will impact aerial survey design and timing of harvests. Birth dates have shifted in the last 30 years, with pupping on Sable Island advancing by 15 days. There has also been a shift in birth dates of 10-15 days over the past 15 years in the Gulf, but the inter-annual variability is greater than on Sable Island.
• Juvenile mortality is an important driver in our estimates of abundance and provision of harvest advice. However, the level of juvenile mortality and how it changes over time is poorly understood. 

This Science Advisory Report is from the November 15-19, 2021 National Marine Mammal Peer Review Committee (NMMPRC) Meeting on Results of 2021 Northwest Atlantic grey seal pup production survey, variation in timing of reproduction, and sustainable harvest advice. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2022/018 (PDF, 903 KB)

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