Pregnant shark eaten off US east coast – could an even larger shark be the culprit?

It really is a shark-eat-shark world.

Scientists have reported the first ever case of a porbeagle shark – a 2.2-metre-long pregnant female – being eaten.

A marine whodunnit investigation suggests that large sharks are eating each other.

Porbeagle sharks are large – reaching up to 3.7 metres and 230 kg. They can live for 65 years and are found in the Atlantic and South Pacific Oceans and the Mediterranean Sea.

Another animal killing and eating such a powerful marine hunter seems unlikely.

“This is the first documented predation event of a porbeagle shark anywhere in the world,” says former Arizona State University graduate student Brooke Anderson, lead author of the scientific investigation published in Frontiers of Marine Science.

Anderson says understanding the circumstances behind the shark’s demise is not just an interesting single case but could have ramifications for the entire porbeagle population.

“In one event, the population not only lost a reproductive female that could contribute to population growth, but it also lost all her developing babies. If predation is more widespread than previously thought, there could be major impacts for the porbeagle shark population that is already suffering due to historic overfishing.”

Porbeagles do not reproduce until they are 13 years old and only produce an average of 4 pups every 2 years.

Anderson’s team captured and tagged porbeagles off Cape Cod in Massachusetts in 2020 and 2022 to research shark migration. Each shark was equipped with satellite tags and transmitters before being released.

Among the tagged porbeagles was the pregnant female.

The female’s satellite tag started to transmit near Bermuda only 158 days after release.

Data from the tag showed the female had been cruising for 5 months 100-200m beneath the surface at night and 600–800m during the day. Water temperature ranged from 6.4°C to 23.5°C.

The tag transmitted only once in that time, confirming that she had remained underwater most of the time.

Suddenly on 24 March 2021, the temperature measured by the tag remained constant at about 22C at depths between 150 and 600 metres. There can be only one explanation for that: the porbeagle shark had been hunted and eaten by a larger predator and the tag excreted about 4 days later when it started to transmit.

What could be large enough to eat a large shark? Another bigger shark.

According to the authors of the new study, only 2 warm-blooded predators are large enough and can be found in the waters where the pregnant porbeagle was hunted: white shark Carcharodon carcharias and shortfin mako Isurus oxyrhinchus.

Shortfin mako sharks can grow to 3.8m. This species is known to feed on cephalopods, bony fish, small sharks, porpoises, sea turtles, and seabirds.

White sharks are the largest living macropredatory fish. These sharks regularly reach more than 4m and the largest reliably measured was nearly 6m, found off the coast of Western Australia. Great whites are known to feed on whales, dolphins, seals, and rays.

Shortfin makos are known to make rapid dives alternating between the surface and deeper depths – a behaviour not registered by the tag.

The porbeagle’s most likely hunter was a great white shark.

“The predation of one of our pregnant porbeagles was an unexpected discovery. We often think of large sharks as being apex predators. But with technological advancements, we have started to discover that large predator interactions could be even more complex than previously thought,” Anderson says.

“We need to continue studying predator interactions, to estimate how often large sharks hunt each other. This will help us uncover what cascading impacts these interactions could have on the ecosystem.”