During mid-December through late January, researchers with DFO Science (Fisheries & Oceans Canada) and Dalhousie University carry out field studies on grey seals Halichoerus grypus. This population has been researched since the early 1960s when there were about 200-300 grey seals born on the island - since then their numbers have increased. Sable Island is now the largest grey seal colony in the world - DFO Science estimated that about 50,000 grey seal pups were born during the winter 2003-04 pupping season.

Until 1990, pup counts were done by tagging all pups born ("complete cohort marking"). As the population increased, and the breeding colonies spread over more of the island - inland on vegetated dunes, as well as on the beach, from one end of the island to the other - the tagging count became impractical. To achieve a reliable pup census, aerial photography is now used. For aerial counts done in 1993 and 1997, a fixed-wing aircraft served as the platform. In January 2004, a helicopter was used. Helicopter services were provided by Four Seasons Aviation Ltd., and aerial photography was done by Airborne Sensing Corporation, both companies based in Toronto. Airborne Sensing also provided the photography for the 1993 and 1997 surveys. For a detailed account of population increase and census methods see Bowen et al. 2003.


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Pup counts will be done from 9x9 inch colour positives of the aerial photos. First a mosaic will be prepared to remove areas of overlap, then pups will be marked and counted on a gridded clear overlay. It is expected that an experienced counter from the DFO Science lab in Newfoundland will analyze the imagery, a process which will take several months. The accuracy of the aerial photographic counts is determined by comparing counts from the aerial imagery with counts done on the ground by DFO Science and Dalhousie field workers at the time of the survey. Corrected counts are then further adjusted - using a birth distribution model - to account for the fact that all pups were not born at the time of the survey during second week of January. Grey seal pupping continues through to late January and early February. The model is fit to data collected on Sable Island during the course of the season.

Reference:

Bowen, W.D., J. McMillan & R. Mohn. 2003. Sustained exponential population growth of grey seals at Sable Island, Nova Scotia. ICES Journal of Marine Science 60: 1265-1274.

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