Satellite Tag Reveals Pregnant Porbeagle Shark Eaten by Larger Shark

Scientific detective work has revealed a harrowing case of one shark eating another in the waters around Bermuda.

A pregnant porbeagle was killed and consumed, and her fate came to light only because she had been fitted with a satellite tag. Its unusual temperature readings pointed overwhelmingly towards a single explanation.

"I couldn't believe it when I got the data from our pregnant porbeagle shark's satellite tag," marine biologist Brooke Anderson of Arizona State University told ScienceAlert.

"I kept trying to think of alternative explanations for the increase in temperature at depth, but all signs pointed to the same conclusion…that our pregnant female porbeagle had to have been eaten by an even larger shark."

Why porbeagle sharks are vulnerable

Porbeagles (Lamna nasus) are large mackerel sharks found across much of the world, but their numbers are declining at a worrying rate. They are sought by recreational and commercial fishers and are also caught as bycatch in commercial fisheries.

The sharks can reach 3.7 metres in length, weigh as much as 230 kilograms and live for several decades. Like other mackerel sharks, they are ovoviviparous: their young develop first in eggs inside their mothers, hatch while still in the uterus and complete their gestation there. The pups are born live after roughly eight or nine months.

Female porbeagles begin reproducing at around 13 years old and subsequently have litters of up to roughly four pups every one or two years.

Such a slow reproductive cycle leaves them especially susceptible to population decline, particularly when several threats are involved. To work out how best to protect these vulnerable animals, Anderson and her colleagues used satellite tags to monitor their movements and behaviour.

How the porbeagle's satellite tag revealed her fate

The researchers used devices known as pop-off tags, which collect and store information including temperature, water depth and approximate location. After a period of time, a tag detaches from the shark and floats to the surface.

If water pressure remains unchanged for three days, the tag assumes it is no longer attached and sends its stored data to the scientists. A separate tag mounted on the shark's fin provides more precise location information whenever the fin breaks the water's surface.

The pregnant porbeagle, which measured 2.23 metres, was tagged on 28 October 2020. Researchers began receiving information from her pop-off tag on 3 April 2021.

The readings were unusual. Until 24 March 2021, the shark appeared to be behaving fairly normally. From late December, she had spent her time in the North Atlantic Ocean, diving to depths of 100 to 200 metres at night and 600 to 800 metres during the day. Temperatures ranged from 6.4 to 23.52 degrees Celsius.

On 24 March, the tag recorded comparable depths in the same location, between 150 and 600 metres. The temperature, however, was markedly higher: about 5 degrees Celsius above that of the surrounding water.

Anderson and her colleagues concluded that the only possible explanation was that a predator had eaten the porbeagle. The likely culprit was a white shark (Carcharodon carcharias) or a shortfin mako (Isurus oxyrinchus). Both are partially endothermic, meaning their muscle movements produce warmth as they swim.

The tag probably spent some time inside the predator's digestive tract before being excreted. It then rose to the ocean surface and began transmitting 3.5 days later.

"It's highly unlikely that the temperature increase was due to the shark moving into a warmer water body based on all the other data we have, such as the change in diving behaviour, the premature release of the pop-off satellite tag, and the fact that we never received any other locations from the finmount satellite tag," Anderson explained.

"All this other evidence still points to a predation being the most likely fate of our shark."

What the attack means for porbeagle sharks

There is previous evidence of large sharks eating one another, although such events are rarely observed. This is the first recorded encounter involving a porbeagle being preyed upon. The fact that she was pregnant is also concerning: several sharks, rather than just one, were lost from the porbeagle population in a single event. That suggests predation may be an overlooked pressure on the species' survival.

Anderson believes the behaviour is unlikely to have developed recently; instead, researchers have lacked the tools to detect it until now. Tagging more porbeagles and other sharks could help explain why porbeagles migrate into the open North Atlantic, how frequently these encounters happen and what effect they have on the porbeagle population.

"Uncovering the mysteries of the open ocean has always been challenging, and it's been the development and advancement of satellite tagging technologies in recent decades that has finally allowed us to track and uncover novel behaviours in marine species such as highly mobile sharks," she told ScienceAlert.

"I think the more of these animals we can tag and track, the more behaviours like this are going to be revealed."

The research was published in Frontiers in Marine Science.