These incidents shed light on how a Greenland shark could potentially attempt to take an idle or slow-moving seal. However, easier prey is usually available, and it’s even better if the victim is already dead or requires little effort to consume. A lingering chase and the ensuing battle waste much energy, and this is something the shark needs to avoid. This is one of the reasons why we do not believe that the Greenland shark is associated with the so-called “corkscrew” kills at Sable Island and in the UK.
The corkscrew wound is a long clean cut that spirals around the victim’s body thus exposing the seal’s muscles and organs. Assuming that the Greenland shark – intent on conserving energy – cannot afford to kill for pleasure, why would it abandon its prey without feeding? Photo analysis reveals that many seal carcasses found with the corkscrew wound appear otherwise intact: no parts are missing. It makes no sense whatsoever for a Greenland shark to needlessly expend the fuel it needs to survive if it isn’t going to feed. It will not reduce its energy stores and risk injury unless it plans on consuming most, if not its entire victim. It has to do so in order to make up for the energy spent during the kill and to be more ‘energized’ than it was before it engaged its opponent. Much of the photographic evidence consists of small seal carcasses that could have been completely eaten by a mature Greenland shark. Also, we know from our telemetry research that the Greenland shark travels in large numbers. If a seal actually were too big, other sharks in the vicinity would quickly eat what was left of the remains. Only small bits and pieces would be left for the crabs or to eventually float to shore.
The St. Lawrence Estuary is home to a large population of the Greenland shark. It is also one of the most important seal breeding areas in the world. And yet we have never witnessed or received reports of corkscrew wounds. Seal carcasses occasionally wash up on the northern shore of Prince Edward Island in the Gulf of St. Lawrence, but the lesions are different. Many of the seals are missing their head or flippers and there’s a good chance they are mostly juveniles that died of natural causes in their first year. This leads to the obvious question: Why would the Greenland shark’s feeding behaviour be so markedly different from one location to another? In other words, why would the corkscrew wound only appear at Sable Island and in the UK?
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.
Grey seals at Sable Island. Photo © sleepyorange | Creative Commons
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.
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.
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 Greenland Shark and Elasmobranch Education and Research Group (GEERG). He is also the Editor-in-Chief of the Diving Almanac & Book of Records, and a contributing editor of DIVER Magazine. Jeffrey dove with his first shark, a spiny dogfish, off Halifax (Nova Scotia) in 1991.