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Shark Predation on Sable Island Seals (March 2003) |
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Shark predation on pinnipeds - seals, sea lions and fur seals - is a well- documented phenomenon. Shark-inflicted mortality may be a significant factor in the biology and behaviour of many pinniped species, influencing, for example, diving patterns, timing of breeding season, length of suckling period, and population dynamics. Reported shark predation on pinnipeds most often involves great white shark Carcharodon carcharias (e.g. California and South Africa) and tiger shark Galeocerdo cuvier (e.g. Hawaii).
During the last twenty years actual shark attacks on Sable Island seals have been witnessed only a dozen times, most often from seismic boats working near the island. The observers were not able to identify the shark species involved. However, based on observations and specimens collected since 1993 - wound patterns, score marks on bone and tooth fragments - it appears that several species are responsible, including great white shark.
However, of the various wound patterns seen in the seal carcasses washed ashore on Sable Island, by far the most common is a peculiar clean-edged `cut' (termed "corkscrew") which is quite unlike wounds characteristic of attacks by great white, tiger and mako Isurus oxyrinchus sharks recorded elsewhere. Also, occurrence of the corkscrew wound peaks during winter - a season when such sharks are unlikely to be in the area. Thus, of the several shark species involved in predation on seals at Sable Island, at least one of them is not among the "usual suspects".
During the first year of the study it became apparent that the corkscrew wound was not consistent with what was then known of the predatory behaviour of various shark species. Photographs of the wounds were sent to seal and shark researchers in many countries including Norway, Scotland, Australia, New Zealand and South Africa, and to numerous scientists on the east and west coasts of North America. The amount of time spent in such `investigation' almost equaled the amount of time spent collecting data on the beach. Although none reported seeing similar wounds, many people shared information about predation and wound patterns observed elsewhere - all of which suggested that the nature and scale of shark predation at Sable Island comprised a unique, albeit perplexing, phenomenon.
As the study proceeded, consideration of the jaw and tooth structure of various sharks suggested that the species responsible for the corkscrew wound might be the Greenland shark Somniosus microcephalus. Discussions with Jack Casey and Wes Pratt (Apex Predators Group, NMFS) and review of historical documents supported this theory, and interviews with Nova Scotian fishermen confirmed that Greenland sharks were certainly in the Sable Island area. While much other `circumstantial evidence' has also implicated this species, until actual attacks are witnessed and documented, the role of the Greenland shark in predation on seals around Sable Island will be a matter of speculation, albeit well-founded speculation. |
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