Research Document 2023/053

Grey Seal Abundance in Canadian Waters and Harvest Advice

By Hammill, M.O., Rossi, S.P., Mosnier, A., den Heyer, C.E., Bowen, W.D., and Stenson, G.B.

Abstract

Here we introduce a new integrated population model (IPM) to provide harvest advice for the Gulf of St. Lawrence (Gulf), Coastal Nova Scotia (CNS) and Sable Island grey seal herds, and compare model outputs with estimates from a deterministic model used in previous assessments. The IPM was fit to the pup production estimates for the Scotian Shelf (CNS and Sable Island combined) and the Gulf. As with the previous assessment model, the new model was fit to both pup production and pregnancy rates, and includes an index for ice‑related pup mortality in the Gulf. The new model includes both density‑dependent and density‑independent pup mortality, and fits to sighting histories of individually marked seals at the breeding colony on Sable Island to estimate sex- and age‑specific survival and recruitment to the breeding colony. The model estimated that total pup production increased slightly from 92,300 (95% CI = 86,700-100,100) in 2016 to 99,300 (90,900–107,700) in 2021, while total abundance increased slightly from 339,400 (317,900–361,500) in 2016 to 366,400 (317,800–409,400) in 2021. The rate of growth of the population has continued to slow, declining from approximately 4% during the last assessment period, to 1.5% per year between 2016 and 2021. The updated population estimate from the previously accepted deterministic population model was 363,600 (298,700-450,000) for 2021, which is very similar to the estimate of abundance generated using the IPM. Although the population continues to grow, the current estimate is below that presented during the 2016 assessment. The difference is due to changes in the structure of the new population model and higher estimates of juvenile mortality produced by the model fit to the 2021 pup production estimates. Additional information on juvenile survival and how it responds to changes in abundance (density‑dependent) and environmental (density independent) variation is needed as it represents a significant gap to our understanding of the dynamics of this population and of large marine mammals in general. Total allowable removals depend on age structure of the harvest and whether the harvests are conducted in winter at the breeding colonies, or at other times of the year when animals from all herds are mixed. Using an integrated model incorporates many of the inputs in a unified framework that allows for uncertainty to be propagated throughout the analyses.

Keywords: grey seal, population model, abundance, harvest

Research Document 2023/053 (PDF, 2,132 KB)

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