Sable Island lies 300 kilometres off the coast of Nova Scotia, Canada, near the edge of the eastern continental shelf. A windswept, crescent-shaped sandbar 43 kilometres long and barely one kilometre wide, it has long been known as the graveyard of the Atlantic, with over 350 ships having foundered on its shores since 1583. The treacherous waters around the island prompted the establishment of a rescue station in 1801, which over the following 150 years saved the lives of thousands of shipwrecked sailors.

Today Sable Island hosts a weather station run by the Canadian government, manned year-round by a small group of researchers. An important wildlife preserve, the island is home to a population of 550 wild Sable Island Horses – descendants of livestock released onto the island in the late 18th Century – and large breeding colonies of seabirds as well as Grey, Harp, Hooded, Ring, and Harbour Seals. But in 1993, Sable Island became the site of a gruesome murder mystery, prompting a decades-long hunt for the strange and elusive killer.

In January 1993 Canadian researcher Zoe Lucas was conducting regular patrols of the island’s beaches, searching for oil-covered birds and other signs of environmental damage, when she stumbled across a number of seal carcasses washed up on the beach. The seals had been horribly mutilated, with bizarre corkscrew wounds spiralling head-to-tail. The edges of the wounds were clean, as if they had been cut with a sharp instrument, and cut right through the seal’s skin and blubber while leaving the muscle beneath untouched. Lucas had never seen anything like it.

While at first this gruesome discovery seemed to be an isolated incident, soon more and more carcasses began turning up. By 1996 over 400 had been discovered on the island, while by 2011 the total had reached over 4,000. Meanwhile, across the Atlantic, dead seals began washing ashore near Norwich, England, St. Andrews, Scotland, and Strangford Loch, Northern Ireland, all bearing the same distinctive corkscrew wounds. These discoveries corresponded with a sharp decline in the birthrates of Grey and Harbour seals, as the victims were nearly all young pups or pregnant mothers.

But what on earth could be inflicting such strange injuries? As in any good murder mystery, Zoe Lucas began by rounding up the usual suspects: human action, pack ice, boat propeller strikes, and attacks by Orcas. But all these culprits were quickly ruled out. As a Federally-controlled territory, access to Sable Island is strictly controlled, making it highly unlikely that a human assailant could have snuck ashore and mutilated the seals. Pack ice also does not form within 200 kilometres of Sable Island, and most of the Harbour Seal deaths had occurred in June and July. While at first ship propellers appeared to be the most plausible culprit, the corkscrew wounds bore little resemblance to the propeller-strike injuries found on other sea creatures like manatees; nor did they resemble injuries from entanglement in fishing lines or nets. Furthermore, many of the seals were still warm or even alive when discovered, indicating they had been attacked just offshore, in water too shallow for most ships to travel. Finally, attacks by Orcas were ruled out as the wounds were inconsistent with the tooth shape and hunting behaviour of those particular predators. Lucas quickly found herself back at square one.

But soon two tantalizing pieces of evidence turned up. The first was the carcass of a seal which had been fitted with a time-depth monitor as part of an environmental study. To Lucas’ surprise, the corkscrew wound ran beneath the webbing used to secure the monitor to the seal’s skin, but the webbing itself had not been damaged. This seemed to indicate that the skin and blubber had not been sliced, but rather torn. The second piece of evidence was the fact that while 98% of the examined carcasses exhibited the distinctive corkscrew wounds, a small fraction bore markedly different injuries such as missing heads and flippers, linear slash marks, and crescent-shaped bites. To Lucas this could only mean one thing: the seals were the victims of shark attack. But what kind of shark? To find out, Lucas wrote letters to shark experts in South Africa, Australia, and California, enclosing photos of the unusual spiral wounds. While none of the experts she contacted had ever seen anything like it, in 2002 Lisa Natanson, a scientist with the United States’ National Oceanographic and Atmospheric Administration Apex Predators Program, agreed to help Lucas track down the culprit.

Natanson drew up a list of six large shark species commonly found in the waters off Sable Island, but once again the peculiar circumstances of the seal slayings seemed to rule out most of them. The Bluntnose Sixgill Shark and the Tiger Shark had only ever been reported in the area as juveniles, and these were too small to prey upon adult seals. Ditto for the Blue Shark, while the needle-like teeth of the Shortfin Mako Shark did not match the spiral wounds. The Great White Shark was known to frequent the area and was likely responsible for at least some of the killings, but this did not account for the large number of Grey Seal deaths in January and February, when the waters off Sable Island are too cold for Great Whites. This left only one, highly unusual candidate: the Greenland shark.

The Greenland shark is a mysterious and elusive creature. A member of the Somniosidae or “sleeper sharks,” they typically inhabit the cold, dark depths of the Arctic Ocean, where the temperature rarely rises above freezing. As a result, the Greenland shark lives life in the slow lane, with one of the slowest metabolisms of any shark. This, in turn, gives it exceptional longevity, with certain individuals estimated to have lived for nearly 500 years. The Greenland shark is a generalist predator and scavenger, feeding on a variety of sealife like fish, squid, crabs, jellyfish, and the carcasses of any land animals that happen to sink to the seafloor. Indeed, one specimen was discovered with an entire reindeer in its stomach. Strangely, nearly all Greenland sharks are infected with a type of crustacean called a copepod, which anchors itself in the cornea the shark’s eyes, effectively rendering them blind.

At first glance the Greenland shark’s slow, scavenging lifestyle would seem to make it an unlikely culprit in the Sable Island seal killings, but the closer Lucas and Natanson looked, the more the facts seemed to fit together. While commonly associated with arctic waters, Greenland sharks actually migrate widely and have been found as far south as North Carolina. Indeed, they are known frequent the waters around Sable Island in the winter and even to come into shallower waters to feed. While at first it seemed unlikely that such a slow-moving shark could catch a fast and agile seal, Lucas and Natanson speculated that the Greenland sharks hunted by ambush, attacking inexperienced pups in murky water or while they were sleeping. But the key piece of evidence was the shark’s particular anatomy and feeding style. The teeth of the Greenland shark differ dramatically between its upper and lower jaw, with the upper teeth being needle-like for gripping and the lower teeth blade-like for slicing. Though capable of brief bursts of speed, when ambushing fast prey the Greenland typically feed by suction, quickly opening its buccal cavity to draw the prey in its mouth. With the prey firmly clamped in its jaws, the shark then shakes its head violently side-to-side to further slice it up before swallowing it. According to Lucas and Natanson, this combination of gripping and shaking  would cause the seal’s skin and blubber to split and tear, the unusual corkscrew pattern being accounted for by two factors. First, the collagen fibres in the seal’s skin wind around its body in a spiral fashion, and it is along these lines that the skin splits; and second, a seal’s instinctive reaction when grabbed is to spin around its body axis in a bid to free itself – a reflex which, ironically, worsens the damage inflicted.

Lucas and Natanson’s findings, published in 2010, were picked up by dozens of scientific journals and formed the basis of a National Geographic documentary on the case. The pair were widely celebrated as having finally solved the mystery of the Corkscrew Killer.

…or had they? According to Jeffrey Gallant of the St. Lawrence Shark Observatory, Lucas and Natanson’s theory just doesn’t add up. Firstly, most of the seal carcasses found in the UK washed up in July, when the waters around the British Isles are a full 20 degrees warmer than Greenland sharks typically prefer, making them unlikely culprits in those killings. Secondly, the Greenland shark theory failed to account for the sudden appearance of the corkscrew corpses, which had never been reported prior to 1993. But most damningly, the seals’ injuries were entirely inconsistent with Greenland shark behaviour. As Gallant explains:

“There is no doubt that the Greenland shark does occasionally predate live seals. However, the victim would probably be an inexperienced pup or be asleep, distracted, asleep, injured, in an ice hole, or the 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 that offers no risk of injury, for even small seals have sizeable teeth with which to defend themselves.

[Furthermore] 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.”

So if not sharks, then what else could have inflicted such strange, spiral wounds? This was the question plaguing David Thompson of the St. Andrews University Marine Mammal Research Unit in Scotland, one of the first researchers to investigate the UK seal carcasses. Like Lucas and Natanson, at first Thompson dismissed boat propellers as the culprit as the seals’ wounds did not match known boat-strike injuries. But one day it occurred to him that if some structure held the seals against a propeller, it would create a corkscrew-shaped wound instead of the typical series of straight gashes. And as luck would have it, certain ships featured just such a structure. Known as positional thrusters or Kort Nozzles, these devices consist of a propeller housed in a short, swivelling duct. They are typically found on tugboats, oil rigs, and drill ships and are used to maintain the vessel’s position over the seafloor. Unlike a regular propeller, positional thrusters are only operated intermittently in short bursts, meaning that an inquisitive seal could easily swim up to one only to be sucked in when the propeller is activated. Poring through shipping records for the areas where seal carcasses were found, Thompson discovered that in nearly every case, a vessel with positional thrusters had been nearby. Experiments conducted by Thompson’s research assistant, Joseph Onoufriou, seemed to confirm the theory. Onoufriou constructed small wax models of seals and fed them into a scale model of a positional thruster. The models emerged bearing the same corkscrew wounds as the real seals, cut at the same 35-degree angle. These findings, published at the same time as Lucas and Natanson’s, prompted politicians to take action to prevent further seal deaths. In April 2012, the UK government issued official advice to the shipping industry to avoid using positional thrusters near seal conservation areas and during seal breeding season.

According to Jeffrey Gallant, Thompson and Onoufriou’s discovery turned the case of the Sable Island Seals from a curious natural mystery to a tragedy of environmental degradation:

“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.”

But just as the mystery appeared to have been solved, once again a new discovery came along and blew the case wide open. In December 2014, Amanda Bishop, a doctoral candidate at Durham University in Scotland, was observing a colony of Grey seals on the Isle of May when she witnessed a shocking incident. As she watched, a male seal grabbed a newly-weaned pup, dragged it to a nearby freshwater pool, and drowned it. Once the pup was dead, the male began tearing into it, ripping up and eating pieces of skin and blubber. Such behaviour is common in many animals as a means of reducing genetic competition, but what caught Bishop’s eye was the manner in which the male seal attacked the pup’s carcass. As the male worked his way along the pup’s body, the skin tore along the spiral collagen lines, creating a distinctive corkscrew wound. Over the following week, Bishop, along with Joseph Onoufriou, watched the male commit infanticide three more times, in every case inflicting wounds nearly identical to the Sable Island carcasses. In a 2016 paper, Bishop, Onoufriou, and their colleagues posited that cannibalistic infanticide by male seals, and not sharks or positional thrusters, was the true cause of the corkscrew deaths. As for why this kind of spiral wound had never been reported prior to 1993, the team speculated that most seal colonies are located far from shore or on ice, making carcasses unlikely to wash up on inhabited beaches; or that the wounds had previously been misattributed to other causes like entanglement fishing lines and nets. In particular, they pointed to an incident in 1998 where a large number of mutilated seal corpses washed up on the beaches of Prince Edward Island, resulting in widespread speculation that they had been deliberately mutilated by fishermen or sealers or become trapped and torn up by sea ice. According to Onoufriou, the discovery of infanticide among Grey Seals presents an valuable lesson in the power of preconception in science:

“I firmly believe that if grey seals had been witnessed attacking grey seal pups or harbor seals, killing them and creating corkscrew lesions before any of these other seals had washed up, nobody would have thought of ship propellers.”

However, this discovery has done little to bring the case of the Corkscrew Killer to a satisfying conclusion, and many researchers remain convinced that the true culprit is still Greenland sharks or positional thrusters or even something completely different like tidal turbines used to generate electricity. Some even argue that all four could be responsible, each accounting for a portion of the total deaths. But one thing is certain: when dealing with nature, nothing is ever simple as it seems.

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