Missing pregnant shark may have been eaten by another shark

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

In October 2020, Brooke Anderson, then at Arizona State University, and her colleagues discovered that a pregnant porbeagle shark (Ramnanasus The study was conducted southeast of Cape Cod, Massachusetts, as part of a study on the migration of pregnant sharks.

The researchers used pop-off tags that continuously measured the sharks’ depth and temperature, and when the tags came off the shark and floated to the surface, they transmitted stored data to shore.

Five months after tagging the pregnant shark, the tag resurfaced southwest of Bermuda and researchers received data on the shark’s recent movements.

The tracking device showed that over a five-month period, the shark swam at depths of 600-800 metres during the day and 100-200 metres at night, in water temperatures ranging from 6.4-23.5°C (43.5-74.3°F).

But something changed starting on March 24, 2021. Even though the pregnant shark was swimming at the same depths as before, the surrounding temperature was hovering between 16.4 °C and 24.7 °C (61.5 °F and 76.5 °F). Not only was the temperature changing, but so was her diving pattern.

This indicates that the tag and the shark it was attached to were eaten, says Anderson, who now works for the North Carolina Department of Marine Fisheries. “All of the evidence we have points to the same conclusion,” she says. “It’s clear that our porbeagle was eaten by another shark.”

Great white shark (Carcharodon carchariasThe team concluded that great white sharks were the most likely culprit, as they are the only predators in the area large enough to carry out such an attack, and their diving patterns and body temperatures also matched the data collected by the tags.

“This was a large female shark that was eaten,” says research team member James Sulikowski of Oregon State University, “and probably something much larger than that.”

Anderson says the attack was likely opportunistic. “The predation occurred at a depth of about 300 metres in the open ocean, where conditions can arise sporadically that allow predators to catch prey,” she says. “In this scenario, if successful, a large, pregnant porbeagle would have been a highly valuable meal.”

The endangered porbeagle shark is found in the Atlantic, South Pacific and Mediterranean Oceans and can grow to be up to 3.7 metres long, weigh up to 230 kilograms and live up to 65 years.

Female porbeagle sharks do not begin breeding until about 13 years of age, have a gestation period of eight to nine months, and give birth to about four calves every one to two years.

Shark-to-shark predation is relatively common, Slikowski said, but it’s rare for one shark to go after another large shark in the deep ocean, and this is the first documented case of a porbeagle being eaten by another shark.

If this were happening on a more widespread scale, Anderson says, it could be of concern to conservationists. “In an instant, this population lost not only one of these important reproductive females, but all of her young,” she says. “Predation is a natural phenomenon, but this discovery highlights the need to continue studying predation on porbeagle sharks to determine how frequently it actually occurs.”

But Chris Lowe of California State University, Long Beach, says the paper doesn’t prove that it was sharks that ate the porbeagle sharks. He points out that killer whales have eaten larger sharks before. “There was clearly some predation going on, but without seeing the data I don’t know if we can be sure that it was limited to sharks,” he says.

A year later, another shark tagged by the group died at a similar depth in the same area near Bermuda, though in this case it had sunk to the bottom, Slikowski said. In this case, the shark may have been attacked and only partially eaten, he said.