Scientist Witness Great White Shark Eating a Rival Shark For The First Time

Scientists had placed a tracker on a large porbeagle shark near Bermuda to trace its migration. That is how they accidentally documented the event of shark cannibalization. The scientists theorize the porbeagle shark had been eaten by a great white in a paper published in the journal Frontiers in Marine Science. Porbeagle sharks were once thought of as apex predators, but there may be more predation going on in the ocean than once thought.

The Victim: A Pregnant Porbeagle Shark

Porbeagle sharks live in the Atlantic Ocean, South Pacific, and Mediterranean Sea. They can grow up to 3.7 m/12 feet and weigh 227 kg/500 pounds. Because of their massive size, researchers once assumed they had no predators, although they are vulnerable to extinction. Female porbeagle sharks begin to reproduce at age 13 and bear an average of four pups after a gestation period of eight or nine months. This is considered a slow reproduction rate that can’t keep up with their numbers dying from overfishing, bycatch, and habitat loss.

Because of the species’ threatened status, Brooke Anderson, then at Arizona State University, and her colleagues tagged a pregnant porbeagle shark near Cape Cod in October 2020. They were researching pregnant shark migration. “We really want to understand what habitats are important to them and where they might be going to give birth, in order to help identify areas that could be used to protect that super important part of the population,” says Anderson.

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A Change in Depth and Diving Patterns

The researchers used pop-off satellite archival tags that measure the sharks’ depth and temperature. That tag is designed to detach after a year and rise to the surface, which is when the data is sent back to the researchers. 

Five months after the tagging, the tag appeared south-west of Bermuda and the researchers received the data. They found out the shark had swam at a depth of 600–800 m during the day and 100–200 m at night. The water temperature ranged from 6.4–23.5°C or 43.5–74.3°F.

However, something changed from March 24, 2021, and on. The temperature around the shark stayed between 16.4°C and 24.7°C (61.5°F and 76.5°F), although it swam at the same depths as before. There was also a change in diving patterns. The researchers puzzled over this anomaly until they came to a startling realization. “All our evidence points to the same conclusion,” says Anderson, who now works at the North Carolina Division of Marine Fisheries. “It’s clear that our porbeagle shark [and the tag] was eaten by another shark.”

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The Attacker: A Great White Shark?

The most likely culprit is a great white shark, a local predator large enough to do the job. Additionally, the new diving patterns and body temperatures measured by the tracker are in line with the habits and biology of great whites. “This was a big female shark that got eaten,” says team member James Sulikowski at Oregon State University. “So something probably larger than it had to have attacked it.” Another suspect is a shortfin mako shark, which is large enough to attack a porbeagle but its swimming patterns don’t align as well with the recorded data.

Great whites are not known cannibals, but Anderson believes the attack was opportunistic. “The predation occurred about 300 meters deep in the open ocean where there may be sporadic pulses of prey available for predators. In this scenario, if you can pull it off, a large pregnant porbeagle shark would be a lot of bang for your buck in terms of a meal.”

Shark Cannibals

Sharks can prey on each other, but rarely does it happen among large shark in deep water. Even more so, this is the first documented indication of a shark consuming a porbeagle. It may be an amazing discovery in terms of better understanding these creatures, but it is bad news for conservationists trying to help this vulnerable species. “In one instant, the population not only lost one of these important reproductive females, but also all of her babies,” Anderson says. “While predation is a natural event, this discovery highlights the need to continue studying predation of porbeagle sharks and to determine how often it really occurs.”

Other Unusual Suspects

However, not everyone agrees with the conclusion that a great white shark consumes porbeagles. “There was clearly some predation event, but [I’m] not sure without seeing the data that I could be convinced that it was limited to lamnid sharks,” says Chris Lowe at California State University, Long Beach. Orca whales are also known to attack sharks; however, they are not typically found near Bermuda where the tag was located.

Similarly, shark scientist Megan Winton of the Atlantic White Shark Conservancy points out other possibilities. “There’s no question that something ate the tag,” Winton says. “But it’s hard to tie that necessarily to the mortality of the porbeagle.” For example, she says a predator could have eaten the tag but not the shark itself. Additionally, the attacker may have been another porbeagle shark, not a great white.

“We do know that white sharks eat other shark species, including other large shark species, so it’s definitely a potential candidate for what consumed this tag,” Winton says. “But even if it was just the tag being bit off, other than then, you know, another shark eating that shark, it’s still a very interesting interaction between species.”

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