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Research Document 2017/056

Pup Production at Scotian Shelf Grey Seal (Halichoerus grypus) Colonies in 2016

By den Heyer, C.E., Lang, S.L.C., Bowen, W.D., and Hammill, M.O.


To estimate grey seal (Halichoerus grypus) pup production, we conducted digital-photographic aerial surveys of breeding colonies on Sable Island and smaller islands along Atlantic coast of Nova Scotia in January 2016. A total of 74,889 pups was counted on the mosaic image from Sable Island. Given the high quality of the imagery, no correction to the count was applied for pups that could not be seen on the image. Small corrections were made for pups that were missed by the image reader (0.022) and for pup mortality (0.051) prior to the survey. To estimate the proportion of pups born at the time of the survey, a transition model was fit to the stages of known-age pups and the distribution of the pup stages collected weekly in the colony between December 14, 2015, and January 29, 2016. We estimated that 97% (SE=1%) of pups had been born by the time of the Sable Island survey. The estimated total pup production for Sable Island in 2016 was 83,600 with 95% confidence limits of 63,600 to 103,500. Pup production on Sable Island has continued to increase, but at a slower rate than it did in the late 1990s and early 2000s. Three digital-photographic aerial surveys were completed at Hay Island. Snow cover and the rocky shore of Hay Island made counting pups more challenging, but there was reasonable agreement between the multiple whole island counts (1,833, 2,002, 2,301). The model of the distribution of pupping was fit to the limited pup stage transect data available for these islands using the Sable Island pup stage durations. We estimated that 100% of pups had been born by the time Hay Island was surveyed by drone on January 28. The Sable correction for pre-survey mortality was also applied to the count. The estimated number of pups born on Hay Island was 2,500 with 95% confidence limits of 1,700 to 3,200. This is similar to pup counts in 2007 (2,600) and 2010 (2,500), suggesting that this breeding colony may have reached carrying capacity. A total of 1,849 pups was counted on four small islands (Round, Mud, Noddy and Flat) in the vicinity of Seal Island, southwest Nova Scotia. This was the first aerial photographic survey for these colonies. After applying corrections for mortality and the proportion of pups born, the total pup production estimate for the four islands in southwest Nova Scotia was 2,100 with 95% confidence limits of 1,800 to 2,400. This is about a 5-fold increase from visual counts from helicopter in 2010 (417). A helicopter survey of the coastline from Yarmouth to Cape Breton was used to obtain visual counts of pups born at other coastal Nova Scotian colonies. The total pups along the coast east of Halifax, not including Hay Island, has increased from 50 in 2010 to 78 in 2016. In total, 4,600 pups were produced by Coastal Nova Scotia breeding colonies, which accounts for 5.1% of the 88,200 pups born on the Scotian Shelf.

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