Research Document 2023/078

Grey Seal Pup Production in Canada

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

Abstract

To estimate Grey Seal (Halichoerus grypus) pup production, digital-photographic aerial surveys were conducted of the major Grey Seal breeding colonies in Canadian waters in January 2021. Where there were multiple counts from a colony, the largest count was used to estimate total pup production. A total of 72,209 pups were counted on digital imagery from Sable Island, the largest breeding colony. The pup counts at Brion Island and Pictou Island, the two largest breeding colonies in the Gulf of St. Lawrence, were 5,151 and 3,604 individuals, respectively. Reconnaissance flights identified a small number of pups at new locations in the Gulf of St. Lawrence, but no new colonies along the Atlantic coast of Nova Scotia, New Brunswick and Newfoundland.

Pup developmental stage surveys were undertaken on the ground or from helicopter at the seven largest breeding colonies to describe the birth distribution and correct the pup count for the estimate of pups born after the aerial photographic survey. Two models were used to estimate the proportion of pups born on the day of the survey: the accepted Myers Birth Distribution model, which uses the developmental stage duration estimated from known-age pups sighted daily on Sable Island; and a new Bayesian approach that uses the stage durations estimated from the known-age pups as priors but allows for the model to estimate stage duration for each colony. At all colonies, the pup production estimated by the Myers Birth Distribution was higher than the Bayesian birth distribution model. To maintain the time series, estimates of pup production are provided using the Myers Birth Distribution model. The estimated number of pups born on Sable Island was 76,600 (SE = 2,900), for Coastal Nova Scotia it was 4,700 (SE = 550) and for Gulf of St. Lawrence it was 16,900 (SE = 2,400) for a total production of 98,200 pups (SE = 5,800). For Sable Island, this is the first estimate of pup production since the 1960s that has not been a significant increase. Sable Island accounts for 77.5% of total pup production in Canada, and the change in trend in pup production on Sable Island is reflected in the trend in total pup production. Pup production in the Gulf of St. Lawrence continues to fluctuate with little evidence of trend over the past several decades, while at the more recently-established breeding colonies in southwest Nova Scotia, pup production continues to increase.

Research Document 2023/078 (PDF, 2,088 KB)

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