Mystery of the Corkscrew Seals

A rash of unusual seal deaths has perplexed scientists along the North Atlantic coast and challenges our desire to find tidy explanations in nature.

It struck at midnight. The two harbor seal pups were swimming in six feet of water, less than 90 feet from shore when it happened. When the animals washed up dead on the beach, their bizarre wounds revealed they were the latest victims of a mysterious seal killer in the waters around Sable Island, Nova Scotia.

The pups were sliced in a corkscrew pattern down and around their bodies, from head to pelvis. It was a clean cut, cleaner than most other predator attacks on seals. Snaggle-toothed great white sharks (Carcharodon carcharias), for example, leave jagged half-moons on seals—as well as the occasional tooth—when they bite. But these wounds were almost as sharp as if they had been made by a knife. And although the corkscrew-patterned wounds penetrated the seals’ skin and blubber, they didn’t reach the layer of muscle below.

This was 1996, and researcher Zoe Lucas had been playing gumshoe detective on the seal-slaying case for more than three years. Lucas’s day job consisted of patrolling the pristine beaches of the mile-wide, 26-mile-long island, looking for oiled birds or any sign of the industrialized ocean beyond this crescent of sand perched out in the North Atlantic, 100 miles from mainland Canada.

Scientists had been studying Sable Island’s seals for decades. But to Lucas, the researchers seemed strangely uninterested in the dead seals with the mysterious corkscrew wounds. She had already seen 463 of them. By 2001, her count would reach a staggering 4,080. Whether these numbers were significant in terms of the local seals population’s stability, she couldn’t say, but she thought the cause was worth investigating.

Although Lucas couldn’t have known it at the time, her initial search for clues was just the beginning of a wildlife mystery that would span two decades and the width of an entire ocean. The investigation has delivered almost as many questions as it has answers. Very little of the evidence produced has enabled wildlife managers to do much to stave off subsequent deaths. And on a more basic level, the mystery has challenged our desire and ability to understand nature—or at least to grasp just what goes on under the surface of the North Atlantic.

Sable Island, Nova Scotia

Lucas could tell exactly where and when the two seal victims from 1996 had been killed—thanks to the time-depth recorders that they carried as subjects of one of the island’s research projects—but she didn’t have many other clues to go on, aside the pattern of wounds on the bodies themselves. One of her first breakthroughs came when she found another seal with wounds that continued under the nylon webbing glued to its fur as an attachment for a time-depth recorder. The recorder was undamaged, which to Lucas meant the seal skin must have been ripped, not cut. “That was the ‘aha’ moment,” she says.

When she tried to imagine what could be doing this to seals on a protected island so far from most of humanity, there seemed to be only one possible answer: sharks.

By 1997, the number of harbor seal (Phoca vitulina) pups born on the island had declined from 625 to just 32 in a scant eight years. Lucas hypothesized that because sharks were attacking adult female harbor seals, which generally weigh less than 220 lbs., the harbor seal colony was losing its ability to reproduce. No adult grey seals (Halichoerus grypus) had been found with the wounds. Lucas still didn’t know what kind of shark was doing the killing, but the size of the prey seemed to make a difference.

Because there was no Internet service on the island then, (phone service is a struggle even today), Lucas took out her envelopes and stamps and started writing to shark experts in every part of the English-speaking world, from South Africa to Australia to California. Most wrote back to say they had never seen anything like the wounds in the photographs Lucas had slipped into the envelopes.

Finally in 2002, Lucas convinced Lisa Natanson, a scientist with the United States’ National Oceanographic and Atmospheric Administration (NOAA) Apex Predators Program, to help her figure out what species of shark might be killing the seals. It was hard for Lucas and Natanson to meet to discuss their list of suspects. Natanson was often at sea researching sharks and Lucas spent most of the year on Sable Island. Still, one by one, they checked out each shark species found near the island.

Black dogfish? Too small. Great white shark? Too cold. The greatest numbers of dead seals were showing up on the island in January and February, when grey seal pups are weaned. Nova Scotian waters during those months aren’t warm enough for great white sharks, or for most other large sharks.

But one large shark species actually prefers these cold temperatures: the Greenland shark (Somniosus microcephalus). Not much is known about these residents of the Arctic and the deepest parts of the ocean. Greenland sharks are certainly large enough to attack a seal. They average 14 feet long and can grow to more than 20 feet, about the same size as a great white. However, part of this shark’s scientific name means “tiny head”—not the ocean’s most menacing designation. But its teeth are as sharp as knives, and appear to be made for pinning and slicing.

After carefully examining one piece of evidence after another, Lucas and Natanson finally announced their conclusion in 2010: The Greenland shark did it. Newspaper articles and a National Geographic documentary trumpeted their finding.

Although the conclusion was based only on circumstantial evidence, there was plenty of that. As the mythical character Sherlock Holmes once said, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” The scientists had examined their evidence closely and eliminated everything as a cause of the corkscrew wounds, except the improbable Greenland shark.

The case was far from closed, however. Could Greenland sharks, which are considered slow-moving scavengers, also be predators? Can they swim fast enough to catch a seal? Even the experts don’t know for sure.

Meanwhile, across the Atlantic, harbor seal numbers in England, Scotland, and Northern Ireland had begun to crash. Between 2000 and 2008, the population in Great Britain plummeted 50 percent. Scientists were perplexed. Across most of their range on the world’s northern coasts, harbor seal populations were bouncing back from not only disease but also centuries of hunting. The IUCN Red List of Threatened Species lists harbor seals as a species of least concern, the same ranking it gives raccoons and pigeons. But something else was going on just off the coast of the U.K.

In June 2008 two female harbor seals were found dead. They washed up on a popular beach just a lunchtime walk away from the offices and labs of the St. Andrews University’s Marine Mammal Research Unit in St. Andrews, Scotland, where marine biologist David Thompson already had harbor seals on his mind.

Both of the St. Andrews carcasses, found nine days apart, were mutilated in the same bizarre way: a suspiciously clean-edged cut spiraled down the seals’ bodies at about 35 degrees. Since the edge of the wound was clean, but no hairs were cut, Thompson figured that he and his team weren’t looking at the work of a knife or razor, but of a smooth edge, applied with force.

St. Andrews, Scotland

Thompson thought a ship propeller seemed like a logical suspect: It spins in circles and has a smooth edge that moves through the water with great force. But your average ship or boat propeller is well known among marine mammal experts for leaving a single gash, not a spiral pattern.

In the meantime, the dead seals with corkscrew wounds continued to wash up, particularly near Norwich, England; around St. Andrews, Scotland; and near Strangford Loch, on the east coast of Northern Ireland. The bodies were mostly adult female harbor seals and were mostly found in July. In all, Thompson and other British researchers studied more than 70 dead seals.

At one point, it occurred to Thompson that if something held the seal against the propeller, it might create the corkscrew pattern instead of a gash. Some boats do have propellers that spin inside a short tube. These are known as ducted propellers, or sometimes as Kort Nozzles, for the British company that makes some of them. They are typically found on tugboats and other powerful vessels that service oil rigs and offshore wind farms.

Ducted propellers are not particularly common, but when Thompson checked the shipping records against the records of seals washing up with corkscrew wounds, “in almost every case, we could find a ship with the right equipment nearby.”

To test this hypothesis, research assistant Joseph Onoufriou made small, wax models of seals and fed them into a scale model of a ducted propeller. The marks on the wax seals were similar to the wounds on the real seals. They had that same 35-degree angle.

It looked like the St. Andrews team had found its culprit. But just as they were preparing a report on their two years of research, Lucas’s and Natanson’s paper was published. The investigation on the other side of the Atlantic was a surprise to them. Suddenly, they had a new suspect to scrutinize.

Thompson weighed the new paper’s evidence against what he had observed and decided that Greenland sharks couldn’t have caused the wounds he had seen. For one thing, the water around the British Isles in July, which averages 63 degrees F, is 20 degrees warmer than the temperatures Greenland sharks prefer.

The ducted-propeller theory wasn’t perfect—it was hard to imagine what would cause a seal to swim close enough to a noisy propeller to get sucked into the intake, near the ship’s hull. Still, despite the doubts, concern about the harbor seal decline was strong enough that in April 2012 Scotland and the United Kingdom issued official advice to the shipping industry to avoid using ducted propellers near seal conservation areas and during seal breeding season.

Then, on a December morning in 2014, at the grey seal colony on Scotland’s Isle of May, Amanda Bishop, a doctoral candidate at Scotland’s Durham University, saw something that would turn the mystery, and the new shipping policy, on its head: A male grey seal grabbed a recently weaned grey seal pup, dragged it to a nearby freshwater pool, and drowned it. When the pup was dead, the adult male tore at it, ripping its skin and gulping down bits of blubber.

A grey seal colony at pup-weaning time is a chaotic place. Mothers stop protecting their young at weaning, so there are a lot of hungry, vulnerable youngsters trying to get their bearings. It’s also mating season, so there are lots of aggressive males trying to mate with females newly free of their maternal responsibilities.

Still, Bishop, an expert on male grey seal aggression, had never seen anything like it. The other seal researchers on the island, some with decades watching seals, had never seen anything like it, either.

Later that December day, when the male was elsewhere, researchers retrieved the pup’s body from the pool. It was a bloody, tattered mess, but when it was cleaned up, it showed the telltale corkscrew pattern in the tearing of its skin. Onoufriou, who had investigated the ship propellers, was one of the researchers working with Bishop that day and, even to him, these new observations cast serious doubt on his earlier conclusions.

Over the next week, Bishop and Onoufriou staked out the male. They observed him killing four more seal pups. They took video of him committing the act three times and photographed him another. Each time, he tore into the pup on or near its head and worked his way down. In one video, a single claw of the adult male catches in the pup’s wound, ripping it further. What looks accidental to the researchers at first happens again and again.

In all, 14 seal pup carcasses were found near the male. The researchers radio-tagged the male the day after his first attack. He left the breeding colony six days after the scientists witnessed the killing, and researchers later tracked him to another grey seal breeding colony in Germany.

The researchers brought eleven of the pup carcasses back to the mainland so that a veterinarian could examine them. When Thompson saw these carcasses, he marveled at how similar the wounds on these pups were to the wounds he had seen on the earlier seal carcasses.

A little over a month after they first witnessed the male seal killing a pup, Thompson, Onoufriou, and Bishop released a draft report to the Scottish Government describing the new evidence. The government promptly withdrew its advice to the shipping industry to keep ducted propellers away from seal colonies.

Thompson says his research team is still investigating all possibilities, including ship propellers, but Onoufriou, who saw some of the killings first-hand, is convinced that the culprit in the mystery of the corkscrew seals has been identified.

“It’s an interesting lesson,” says Onoufriou. “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.”

Yet, while the mystery again seemed solved, perhaps for the final time, loose ends still remained. “Mystery over seal ‘corkscrew’ deaths sparks row between conservationists and government,” read the headline in The Herald, one of Scotland’s leading newspapers, in March 2015. “Fury as seal killings are blamed on cannibal seals,” blared The National, another paper, in April.

Twenty conservation groups, led by the organization Whale and Dolphin Conservation (WDC) protested the quick withdrawal of the government’s advice to the shipping industry. They had seen the preliminary report on seal cannibalism, and they thought it was too soon for the government to let the shipping industry off the hook. They wanted shippers to continue to avoid seal conservation areas.

“We lobbied quite hard,” says Sarah Dolman, WDC’s Northeast Atlantic program manager.

Last December, a year after Bishop first observed the grey seal killings, Thompson traveled to San Francisco to present the team’s findings at a marine mammal conference. There was one scientist in the room he was keen to convince: Susan Wilson of Tara Seal Research in Northern Ireland. Wilson had yet another theory for the corkscrew seal deaths, at least those on Northern Ireland’s east coast: a tidal turbine, a propeller-like underwater power plant that generates electricity from the ebb and flow of the tides.

A few months after the conference, Bishop was putting the final touches on a scientific paper, scheduled for release this June, that will provide more details on the Isle of May attacks than were given in the Sea Mammal Research Unit report. As the journal staffers arranged the maps and photos on the page, they also posted a video of the male grey seal attacking the seal pup on the journal’s website. It changed everything.

The video convinced Doman and others at the NGOs that the grey seal was the cause of the corkscrew seal deaths. While the conservation organizations involved still think that ship strikes are an issue for seals, Doman says, the fight became a much lower priority when the new evidence came to light.

In a wildlife mystery, much as in a court case, “who done it” matters. When nature is to blame, wildlife managers and conservationists rarely interfere unless extinction is on the line. When humans are guilty, even if only as accessories to the crime, we—as a society—generally want to step in and put a stop to it.

With the release of the video, and the forthcoming scientific paper, it would seem that the jury was in on the case of the corkscrew seals. But each theory – shark, ship, and seal – still has its advocates and its critics.

Wilson, for one, wasn’t persuaded by Thompson’s talk, the video, or by a report from Germany that a male grey seal had been observed killing a young harbor seal. “I haven’t actually changed my mind on this,” she says. “There is no evidence at all, that I know of, of male grey seals causing corkscrew injuries to adult seals.” The new evidence only explains the strange wounds on young seals, she says, and believes that ships with ducted propellers should continue their precautions.

Then there’s Lucas, who defends her shark predation theory, but less ardently than she used to, having seen so many new twists and turns in the mystery. She has come to believe that perhaps all three theories have merit. “I don’t think what we’re seeing in Europe is inconsistent with what we see at all,” says Lucas.

Whether it’s sharks, ships, or seals, it’s the seal’s skin ripping in a corkscrew pattern that remains consistent. For Lucas, that pattern ties the mysterious seal deaths together. Maybe if you tear at a seal, the way it comes undone isn’t the result of what has attacked it, but how its body is put together.

It’s possible that the mechanical sameness of the seals’ dead bodies across decades and an ocean points, not to the nature of the mysterious killer that strikes in six feet of water at midnight, but to the nature of the seals themselves.

Isle of May landscape photo by Amanda Bishop.

Behavioral observation photo by Sean Twiss.

Madeline Bodin

Madeline Bodin is a Vermont-based freelance science journalist who has written for The Magazine, Wildflower,, Popular Mechanics, and other publications. She was a COMPASS journalism fellow at the 2015 Society for Marine Mammalogy international conference in San Francisco, where some of the reporting for this article occurred.

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