This gruesome discovery of nearly 150 dead turtles is a warning for the future

It started with the mysterious discovery of more than a hundred turtles, some with their shells smashed, others dismembered, all of them dead. It ended with a potential warning for the future.

It was Gregory Bulté who found the turtles. The biologist from Carleton University was out on the water of eastern Ontario's Opinicon Lake in April 2022 when he saw a dead northern map turtle — so-called because its shell resembles the contour lines of a map. As he bent to pick it up from the shallows, he saw another.

Bulté raced home, fetched his wetsuit and snorkel and got into the frigid water — where winter ice had recently melted — to collect the bodies. It quickly became clear the deaths were widespread. He kept finding more piles of dead turtles; he filled buckets with them. “I was like, ‘Whoa, when is this going to end?’” he said.

When it eventually did, he had nearly 150 dead turtles, many of which Bulté knew from his two decades of monitoring work at the near-pristine, forest fringed lake. It was a devastating blow, wiping out roughly 10% of the lake’s population.

The deaths were a puzzle for Bulté. It was clear from the turtles’ damaged bodies that this was a predator attack, and only one animal was likely strong enough to have done it: the river otter.

But the bigger question was why this had happened in the first place. It was the first mass mortality event Bulté had seen at the lake, and protecting the turtles in the future meant understanding why so many had died.

Northern map turtles are fascinating, Bulté said. They have adapted to survive the harsh Canadian winter by spending it clustered together, submerged underwater beneath a thick layer of ice. They stay there for months, moving only slightly, keeping their body temperature near freezing and their metabolism slow.

The turtles have other interesting quirks, too. Females are much bigger than males and about 10 times heavier, with stronger jaws that allow them to eat mollusks, while males tend to stick to insects and snails. Their size advantage doesn’t always protect them, however. Bulté has documented females moving to deeper water and burying their bodies in the sediment to escape unwanted, relentless attention from over-amorous males during mating season.

Northern map turtles are fairly abundant in parts of the United States, but in Canada, home to an estimated 10,000, they are designated a species of “special concern” because of the threats they face.

They rely on “big water,” said Jacqueline Litzgus, an ecologist at Laurentian University, meaning the vast lakes and rivers that are often heavily used by humans. This puts them at risk of boat strikes, being disturbed when they venture out of the water to bask and nest and being caught in fishing nets.

Northern map turtles are also vulnerable because of their long lives. They take years to reach maturity and their hatchlings have low survival rates. “The loss of even a few adults can cause a population to collapse,” said a spokesperson for Ontario Waterways, part of the Canadian national park system.

History shows the risks with a different species of turtle. Over three winters in the late 1980s, otters killed around 50% of the snapping turtles in Algonquin Park, Ontario. More than two decades on, “the population still has not recovered,” Litzgus said. Recent analyses show it is still declining, she added, “suggesting it may have reached a tipping point of not being able to recover.”

To solve the mystery of what happened to the turtles of Opinicon Lake back in 2022, Bulté went through a process of elimination.

He started with what he knew: The turtles are vulnerable in the winter because they cluster in big groups and sit exposed on the lakebed; they don’t burrow into the mud and sediment.

Anything that can get to them “has an all-you-can-eat buffet of turtles,” Bulté said. In this case, it was river otters, whose populations have rebounded, in part because their pelts have become less lucrative to trappers. They typically eat fish, but won’t say no if turtles are on offer.

The big question was how these otters had breached the lake’s thick winter ice. Bulté ruled out people damaging the ice because the overwintering site is away from the shore and far from any human infrastructure.

So, he turned to temperature. “Maybe it got a little bit warmer, it melted along the shoreline and (otters) were able to sneak in,” he said. They may have got under the ice from borrows or cavities along the shoreline.

If temperature is the key to the puzzle, the implications could be worrying. As human-driven climate change warms the planet, it could put these turtles at increasing risk.

It’s not yet possible to know if climate change played a role in this specific case, Bulté cautioned. “We would need to have been able to document several of these events over a long period of time,” he said. So far, there has only been one.

But what is clear, he added, is that the way these turtles spend the winter — in large groups, in the same spots each year — makes them vulnerable.

The otter attacks are a “cautionary tale,” Bulté said, and a sign of how vital it is to protect the turtles’ overwintering sites across Canada and beyond, many of which are in busier and less pristine lakes than Opinicon.

Northern map turtles are increasingly vulnerable as shorelines develop, trees are felled, and motorboats, with their deadly propellers, proliferate. Lakefront homes are also increasingly using “bubblers,” which push bubbles into the water to stop it freezing around docks and boathouses and could provide another entry point for predators.

Longer term, climate change poses another big risk. Bulté has already noticed some extremely early springs in recent years. “We're going to have less reliable ice. So, we are certainly wondering if this is going to affect opportunity for predations in the future,” he said.

The future remains uncertain for the northern map turtles. If there was just one problem affecting them, “maybe this is something they can live with,” Bulté said. But they face so many threats, he added. “It’s death by a thousand cuts.”

Images supported by funding through National Geographic Society's Preserving Legacies project.