Pregnant shark that disappeared may have been eaten by another shark

A pregnant porbeagle shark may have been eaten by a great white shark near Bermuda, in the first recorded incident of its kind.

In October 2020, Brooke Anderson, then at Arizona State University, and her colleagues tagged a pregnant porbeagle shark (Lamna nasus) south-east of Cape Cod in Massachusetts as part of research into pregnant shark migration.

The researchers used pop-off tags, which continuously measure the depth and temperature of the shark. When the tag detaches from the shark and floats to the surface, the stored data is pinged back to shore.

Five months after the pregnant shark was tagged, the tag resurfaced south-west of Bermuda and the researchers received data on its recent movements.

The shark had spent five months cruising through waters at a depth of 600 to 800 metres during the day and 100 to 200 metres at night, with water temperatures varying from 6.4 to 23.5°C (43.5 to 74.3°F), according to the tracker.

But from 24 March 2021, something changed. The ambient temperature around the pregnant shark remained between 16.4°C and 24.7°C (61.5°F and 76.5°F), despite it swimming at a similar depth to before. As well as the change in temperature, there was a shift in diving patterns.

It suggests the tag – and the attached shark – was eaten, says Anderson, who now works at North Carolina Division of Marine Fisheries. “All our evidence points to the same conclusion,” she says. “It’s clear that our porbeagle shark was eaten by another shark.”

A great white shark (Carcharodon carcharias) is the most likely culprit, the team concluded, as it is the only predator in the area large enough to mount such an attack. The diving patterns and body temperature of a great white also match the data collected by the tag.

“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.”

Anderson says the attack was most likely opportunistic. “The predation occurred about 300 meters deep in the open ocean where there may be sporadic pulses of prey available for predators,” she says. “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.”

Porbeagle sharks, which are endangered, are found in the Atlantic Ocean, the South Pacific Ocean and the Mediterranean Sea. They grow up to 3.7 metres long, can weigh up to 230 kilograms and live for up to 65 years.

Female porbeagle sharks don’t start breeding until they are about 13 years old, and give birth to around four pups every one or two years, with a gestation period of 8 to 9 months.

Shark-on-shark predation is relatively common, says Sulikowski, but it is rare for a shark to target another large shark in deep water. This is the first documented case of a porbeagle being eaten by another shark.

If it is happening more widely, it could be a worry for conservationists, says Anderson. “In one instant, the population not only lost one of these important reproductive females, but also all of her babies,” she 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.”

However, Chris Lowe at California State University, Long Beach, says the paper doesn’t prove it was a shark that ate the porbeagle. There have been instances of orcas eating large sharks, he points out. “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,” he says.

A year later, another shark tagged by the group also died at a similar depth, in the same area near Bermuda, says Sulikowski. In that case, though, the shark sank to the bottom of the ocean. Sulikowski believes this was a case where the shark was attacked and may have only partially been eaten.