What killed and ate a really big shark? An even bigger shark, scientists say

As It Happens6:05What killed and ate a really big shark? An even bigger shark, scientists say

When marine biologist James A. Sulikowski started monitoring the migration and mating habits of porbeagle sharks, he didn't expect he would end up investigating a murder mystery.

But when Penelope, a 2.4-metre-long porbeagle, dropped off his research team's radar, he and his colleagues ultimately concluded that she met a grisly fate as someone's dinner.

Their primary suspect? Another, even bigger, shark. 

"Penelope was a big shark, and to think that something large ate her is kind of scary," Sulikowski, director of Oregon State University's Big Fish Lab, told As It Happens host Nil Köksal.

"The fact that this occurred ... is mind blowing to us."

Sulikowski and his colleagues have published a study in the journal Frontiers in Marine Science detailing what they say is the first evidence that porbeagle sharks have natural predators, and a rare example of one large adult shark hunting another. 

Scientists who were not involved in the research say the study's conclusions are plausible, but more research is needed to say, without a doubt, that Penelope was the victim of shark-on-shark snacking. 

'Something really strange happened'

Porbeagles, Sulikowski says, are fast-moving sharks that can regulate their own body temperature, allowing them to survive in both cold and temperate waters. They can grow as big as 3.5 metres and, until now, were believed to have no natural predators.

They are, however, hunted by humans for their meat, and sometimes end up as bycatch from longliners fishing for tuna and swordfish. They're listed as a vulnerable species worldwide by the International Union for Conservation of Nature.

That's why Sulikowski and his colleagues have been trapping, tagging and tracking porbeagles, specifically pregnant females, to better understand their mating and migration routes in the name of conservation planning. 

They tagged and released Penelope off the coast Cape Cod in 2020. The satellite tags, which measure depth and temperature, are meant to last a year, Sulikowski said. But Penelope's came off after just five months.

"They don't normally jettison unannounced like that," he said. "When we started looking at the data, it was like, wow, something really strange happened."

The data showed that Penelope was swimming 600 metres beneath the surface when suddenly the temperature spiked from 15 C to 27 C. It stayed that way for several days, while moving "up and down cyclically in the water," he said. 

That means the device likely ended up "in the belly of the predator," Sulikowski said. 

That predator, he said, would have to be bigger than Penelope. Based on the depth and location of the tag, that leaves only two reasonable suspects: a great white shark or a shortfin mako shark.

'Another murder'

Shark biologist Alison Towner, who was not involved in the research, told CBC the study's conclusions are "not just intriguing, but also quite plausible."

"White sharks have a diverse diet, and while satellite data often shows patterns like oscillatory diving in open ocean environments, it's not always clear what they're hunting," Towner, a postdoctoral fellow at South African International Maritime Institute, said in an email. 

"In this case, the temperature increases observed fit well with what we'd expect from a white shark predation event."

Not everyone is convinced. Megan Winton of the Atlantic White Shark Conservancy in Chatham, Mass., told Science News it's possible another shark ate the tag, but not Penelope.

Sulikowski says it's unlikely.

"When a shark goes to eat, like, it's not nibbling. It's taking a big chunk," he said. "The tag that she was carrying was mid way on her body. So that event probably either ripped her in half or took a chunk of her body off."

What's more, he said "another murder" occurred at the same location, a year after Penelope.

In that case, he says another porbeagle they were tracking appeared to suddenly nosedive bottom of the ocean, suggesting it was killed, and what remained of its body sank — tag and all. 

"You put those two together, it probably comes to the conclusion that something ate both these sharks," he said. 

Sharks eat sharks, not people

If the authors are correct, it would not be the first example of shark-on-shark predation, says Toby S. Daly-Engel, director of the Florida Tech Shark Conservation Lab.

Big sharks are known to prey on smaller shark species, she said, or even smaller individuals of their own species.

Nevertheless, Daly-Engel, who was not involved in the research, said: "The study is cool."

"The novel thing about it is that the shark that was eaten was an adult, so it must have been something big that ate it," she told CBC.

Asked whether the grisly findings paint a negative picture of the very sharks he's trying to protect, Sulikowski says this is actually good news for those who fear getting gobbled up at sea. 

"Sharks are on a shark's menu, OK? We, as human beings, are not on a shark's menu. If we were, there would be a lot more people disappearing from the ocean, that's for sure," he said.

"We're a lot slower than porbeagles and we like to splash … so we'd be easy pickings."