The environmental and behavioural evidence put forward also does not concur with our own findings that are based in part on telemetry data and on actual observations of the Greenland shark. The waters around Sable Island, a 44-km-long sand bar about 150 km east of mainland Nova Scotia, are shallow and the bathymetry is not that which is usually associated with this species. Other environmental factors, which will be detailed in a new science article by GEERG in 2011, might also make Sable Island seasonally inhospitable to the Greenland shark.
Research conducted by GEERG in the St. Lawrence would indicate that the preferred temperature range for the Greenland shark is from -1˚C to 6˚C. The warmer conditions found off Norfolk (UK) are not unheard of for the Greenland shark. However, they are at the extreme range for this species that repeatedly retreats to much deeper and colder conditions on a daily basis. With an average depth of 94 m, the North Sea itself is not ideal habitat for the Greenland shark.
Grey seals at Sable Island. Photo © sleepyorange | Creative Commons
As far as we know, the Greenland shark has never been observed alive at either location although the odd dead shark could certainly wash up on shore at Sable at any time of the year. This doesn’t mean it isn’t out there. The Greenland shark could be present in large numbers in winter when the water is colder and the days are shorter, to feed on seal pups that drift out to sea, dead or alive. Seal carcasses that wash up on Sable Island without the distinctive corkscrew wound may have indeed been scavenged or killed by the Greenland shark patrolling offshore. However, whether the shark is present or not may ultimately prove to be irrelevant, for the secret lies in the bite.
Missing heads and flippers are typical of a Greenland shark feeding, but the only confirmed bite wound pattern for the Greenland shark is circular. The shark bites into its victim and then twists itself repeatedly until a “plug” of flesh is torn out. It may even be possible for a large seal – or even a cetacean – to survive such an attack. The corkscrew wound simply does not fit with the known feeding pattern of the Greenland shark: a pattern that has been observed and filmed.
(Right) The Pacific sleeper shark (Somniosus pacificus), which is virtually identical to the Greenland shark (Somniosus microcephalus), feeding on a whale carcass (BBC Earth | YouTube).
The actual cause of the corkscrew wound is probably mechanical. If this is the case, the culprits are almost certainly dynamic – azimuth – positioning thrusters used by vessels associated with offshore drilling or construction. Such operations are present off all sites reporting corkscrew wounds (see below). Seals are curious creatures often seen diving near shipwrecks and other man-made objects. The powerful suction effect produced by a thruster would easily overpower a seal that got too close. Unlike regular ship propellers that run continuously while a ship is at sea, thrusters operate on a need-only basis and thus turn on and off sporadically.
A curious seal inspecting the intake side of this odd tunnel-like object would have no chance if the power were suddenly turned on. Being sucked into the blades would either slice the hapless seal to death or produce the horrific wounds witnessed at Sable Island and in the UK. Some of the butchered seals may even survive and swim back to the beach to die.
The Greenland shark does leave a trademark wound on its victims but this most certainly isn’t it. We therefore believe that corkscrew fatalities at Sable Island and in the UK are in fact unrelated to the Greenland shark.
Human activity is yet again the likely cause for these needless deaths. Who knows how many lifeless bodies didn’t actually make it to shore? Life is dangerous enough for seals without having to deal with giant underwater food processors. If I were a seal, I’d choose the shark. I would at least have a fighting chance to survive, and if I were killed, my death would serve to sustain a fellow creature of the sea.
Azimuth thruster on a ship in Antwerp. Belgium. Photo by Alfvanbeem (Creative Commons)
Note: Some of the more mangled dead seals that wash up at Sable Island may have been killed by the white shark, Carcharodon carcharias, which appears to be making a comeback in the North Atlantic.
UPDATE: A 2010 report¹ by the Sea Mammal Research Unit (University of St Andrews, Scotland) determined that “corkscrew” wounds in the UK were consistent with seals being drawn through a ducted propeller or Azimuth thruster, and concluded that “Greenland shark predation are difficult to reconcile with the actual observations and, based on the evidence to date, seem very unlikely to have been the cause of these mortalities.”
The updated report² in 2015 led to the same conclusion: “There does not appear to be any direct evidence from Sable Island that Greenland sharks are the principal cause of spiral lacerations to seals.” “There is no evidence at all that Greenland shark bites produce such wounds on carcasses of seals and no plausible mechanism for them to inflict such wounds.”
¹ Gallant, Jeffrey J., Marco A. Rodríguez, Michael J. W. Stokesbury, and Chris Harvey-Clark. 2016. Influence of environmental variables on the diel movements of the Greenland Shark (Somniosus microcephalus) in the St. Lawrence Estuary. Canadian Field-Naturalist 130(1): 1-14.
² ³ Thompson, D., Bexton, S., Brownlow, A., Wood, D., Patterson, A., Pye, K., Lonergan, M. & Milne, R. (2010) Report on recent seal mortalities in UK waters caused by extensive lacerations. Report to Scottish Government, Sea Mammal Research Unit, University of St Andrews, St Andrews.
4 Thompson, D., Onoufriou, D., Culloch, R. & Milne, R. (2015) Current state of knowledge of the extent, causes and population effects of unusual mortality events in Scottish seals. Updated Report to Scottish Government, Sea Mammal Research Unit, University of St Andrews, St Andrews.
Jeffrey Gallant, M.Sc., is the Scientific Director of the St. Lawrence Shark Observatory (ORS) and of the Greenland Shark and Elasmobranch Education and Research Group (GEERG). He is also the author of the Diving Almanac and a contributing editor of DIVER Magazine. Jeffrey dove with his first shark, a spiny dogfish, off Halifax (Nova Scotia) in 1991.