In 2010, shark researchers Jeffrey Gallant and Chris Harvey-Clark were featured in two television documentaries: Predator CSI: Corkscrew Killer (National Geographic Channel) and The Seal Ripper (Nature Shock). These productions highlighted a lingering enigma: were the Greenland shark or the white shark truly responsible for the mysterious “corkscrew” seal mortalities? Consistent with the views they expressed in those programs, we have never believed that either species was the cause.

Beginning in the early 1990s, reports of mutilated seal carcasses on Sable Island, and later in the North Sea, sparked intense controversy over the possible role of the Greenland shark. Many of the victims presented a corkscrew-like cut twisting around the length of the seal’s body. Similar reports began to emanate out of England and Scotland in the summer of 2010. At first glance, the spiral wounds looked predatory, but on closer examination, they did not match known Greenland shark feeding wounds.

There is no doubt that the Greenland shark does occasionally predate live seals [1]. However, the victim would likely be an inexperienced pup or be asleep, distracted, injured, in an ice hole, or visibility would have to be so poor as to offer the seal no warning of the incoming shark’s presence. The Greenland shark is much more likely to feed on an already dead seal, which requires no energy output and offers no risk of injury. As Gallant has emphasised, “It makes no sense for a Greenland shark to needlessly expend the fuel it needs to survive if it isn’t going to feed.” In contrast, the white shark leaves very different, unmistakable bite marks that are now commonly observed throughout the Gulf of St. Lawrence with the resurgence of the white shark population [2].

Because shark predation did not fit the evidence, an alternative explanation was sought. The initial hypothesis pointed to a mechanical cause: dynamic, ducted propellers—azimuth positioning thrusters—used by vessels engaged in offshore drilling or construction in the immediate vicinity of where the carcasses had washed up at Sable Island and in Scotland. These systems can generate powerful suction, pulling curious seals into the blades and producing spiral lacerations. This hypothesis was proposed by Scottish scientists investigating similar mortalities in the UK [3], and early assessments leaned toward thrusters as a cause. Subsequent reviews and a 2015 update from Scotland’s Sea Mammal Research Unit documented the extent of spiral injuries and weighed candidate mechanisms, including thrusters and biological predation.

Grey seals (Halichoerus grypus) on Brion Island, in the Magdalen islands. Photo © ORS | Davy Hay Gallant

Grey seal (Halichoerus grypus) cannibalism event on Brion Island, in the Magdalen islands. Drone image © ORS | Jeffrey Hay Gallant

However, detailed behavioural and pathological work has since highlighted a more direct, biological culprit. Grey seals (Halichoerus grypus) themselves have been documented killing and feeding on other seals and porpoises. In March 2018 on Helgoland (German Bight), a subadult male grey seal was observed catching, killing, and feeding on a juvenile grey seal. The carcass showed severe cutaneous lacerations starting in the head region and following a helical, corkscrew-like course around the trunk, with detached skin and broadly manipulated blubber, lesions that mirror the “corkscrew” pattern that sparked the controversy [4].

Parallel investigations in Scotland supported this interpretation: grey seal infanticide and cannibalism can produce spiral lacerations indistinguishable from many historical “propeller” cases, leading those authors to propose that most UK carcasses with spiral lesions were caused by grey seal predation [5].

Our own field observations support this hypothesis. During our 2023 white shark expedition at Brion Island in the Magdalen Islands, we witnessed and filmed grey seal-on-grey seal cannibalism, one seal killing and consuming another. This was only the second documented case of such behaviour in Canadian waters.

These incidents shed light on how a grey seal could take an idle or slow-moving conspecific and peel back skin in helical strips to access high-energy blubber with relatively brief bursts of effort and considerable stealth, behaviour consistent with observations from Helgoland and elsewhere.

By contrast, inferring Greenland shark predation from external mutilation patterns alone is problematic. Photographic evidence can be confounded by post-mortem scavenging and transport, and “missing heads or flippers” is not a diagnostic sign of Greenland shark feeding. What the literature and observations do support is that the only reliably described Greenland shark bite trace is circular “plug-type” removal, not a clean, helical laceration [6].

In the Gulf of St. Lawrence, home to a large population of Greenland sharks and one of the world’s most important seal breeding areas, we have never witnessed or received reports of corkscrew wounds. Seal carcasses do occasionally wash up on the northern shore of Prince Edward Island, but the lesions differ, and many of the dead are likely juveniles that died of natural causes in their first year. More recently, in Prince Edward Island, the Magdalen Islands, and the Gaspé Peninsula, many carcasses show clear evidence of white shark attacks. In this context, it is relatively straightforward to distinguish between shark-inflicted wounds and signs of natural mortality. The contrast is thus between the wounds attributed to Greenland sharks and the circular lesions observed elsewhere. Why would a single shark species display such radically different feeding signatures from one location to another? The more plausible interpretation is that corkscrew lesions are not a Greenland shark signature.

Environmental context also argues against routine Greenland shark involvement at Sable Island and in parts of the North Sea. The waters around Sable Island are shallow, with bathymetry and seasonal conditions that are not typically associated with the Greenland shark’s preferred, colder habitat. The North Sea’s average depth and temperatures further limit ideal conditions. If Greenland sharks are present seasonally, that does not change the fundamental mismatch between their known feeding traces and the “corkscrew” pattern.

While sporadic mechanical injuries from azimuth thrusters may occur, recent behavioural observations, necropsy evidence, and subsequent analyses indicate that spiral, corkscrew-like lacerations are far more likely the result of grey seal predation, including cannibalism. The so-called “Corkscrew Killer” is not a Greenland shark or a white shark. The culprit is hiding in plain sight: another grey seal.

REFERENCES
(1) Leclerc, L.-M. E., Lydersen, C., Haug, T., Bachmann, L., Fisk, A. T., & Kovacs, K. M. (2012). A missing piece in the Arctic food web puzzle? Stomach contents of Greenland sharks sampled in Svalbard, Norway. Polar Biology, 35(8), 1197–1208. https://doi.org/10.1007/s00300-012-1166-7
(2) Bastien, G., Barkley, A., Chappus, J., Heath, V., Popov, S., Smith, R., Tran, T., Currier, S., Fernandez, D. C., Okpara, P., Owen, V., Franks, B., Hueter, R., Madigan, D. J., Fischer, C., McBride, B., & Hussey, N. E. (2020). Inconspicuous, recovering, or northward shift: Status and management of the white shark (Carcharodon carcharias) in Atlantic Canada. Canadian Journal of Fisheries and Aquatic Sciences, 77(10), 1666–1677. https://doi.org/10.1139/cjfas-2020-0055
(3) Sea Mammal Research Unit. (2015). Report on spiral lacerations in seals. University of St Andrews.
(4) van Neer, A., Gross, S., Kesselring, T., Wohlsein, P., Leitzen, E., & Siebert, U. (2019). Behavioural and pathological insights into a case of active cannibalism by a grey seal (Halichoerus grypus) on Helgoland, Germany. Journal of Sea Research, 148-149, 12–16. https://doi.org/10.1016/j.seares.2019.03.004
(5) Brownlow, A., Onoufriou, J., Bishop, A., Davison, N., Thompson, D., & Crocker, D. E. (2016). Corkscrew Seals: Grey Seal (Halichoerus grypus) Infanticide and Cannibalism May Indicate the Cause of Spiral Lacerations in Seals. Plos One, 11(6). https://doi.org/10.1371/journal.pone.0156464
(6) Jena E. Edwards, Elizabeth Hiltz, Franziska Broell, Peter G. Bushnell, Steven E. Campana, Jørgen S. Christiansen, Brynn M. Devine, Jeffrey J. Gallant, Kevin J. Hedges, M. Aaron MacNeil, Bailey C. McMeans, Julius Nielsen, Kim Præbel, Gregory B. Skomal, John F. Steffensen, Ryan P. Walter, Yuuki Y. Watanabe, David L. VanderZwaag, & Nigel E. Hussey. (2019). Advancing Research for the Management of Long-Lived Species: A Case Study on the Greenland Shark. Frontiers in Marine Science, 6. https://doi.org/10.3389/fmars.2019.00087