Pregnant Porbeagle Shark Eaten by a Larger Shark Near Bermuda

Scientific sleuthing has revealed a deeply disturbing case of shark-on-shark predation in the waters around Bermuda.

A pregnant porbeagle was killed and eaten, with her fate becoming known only because she was carrying a satellite tag. Unusual temperature readings from that device 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."

Porbeagle sharks face mounting threats

Porbeagles (Lamna nasus) are sizeable mackerel sharks found across much of the world, yet their populations are declining at an alarming rate. They are sought by recreational anglers, targeted by commercial fisheries, and also caught incidentally as bycatch.

The sharks may reach 3.7 metres in length, weigh as much as 230 kilograms, and survive for several decades. Like other mackerel sharks, however, they are oviviparous: their offspring initially develop in eggs within the mother, then hatch while still in the uterus to finish gestation. The pups are born live after roughly eight or nine months.

Female porbeagles do not begin breeding until they are about 13 years old. After that, they produce litters of up to around four pups every one or two years.

Because of this slow reproductive pattern, they are especially susceptible to population losses, particularly when confronted by several threats. To determine how these vulnerable animals can be protected, Anderson and her team tracked the species with satellite tags that document shark movements and behaviour.

Satellite tags reveal an unusual final journey

The researchers used devices known as pop-off tags. For a set period, these collect data including temperature, water depth and an approximate location, retaining the information until they ‘pop off’ - separating from the shark and rising to the surface.

When the tag detects no change in water pressure for three days, it assumes it has become detached from the shark and sends its stored data to the scientists. The shark also carried a fin-mounted tag, which 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 the pop-off tag on 3 April 2021.

The readings were peculiar. Until 24 March 2021, the porbeagle appeared to have been behaving normally. Since late December, she had remained in the North Atlantic Ocean, diving between 100 and 200 metres at night and from 600 to 800 metres during the day, in water temperatures of 6.4 to 23.52 degrees Celsius.

On 24 March, the tag recorded comparable depths in the same area, ranging from 150 to 600 metres. The temperature, though, was markedly warmer - approximately 5 degrees Celsius above that of the surrounding water.

A larger shark was the likely predator

Anderson and her colleagues concluded that the only plausible explanation was that a predator had eaten the porbeagle. The likely culprit was either a white shark (Carcharodon carcharias) or a shortfin mako (Isurus oxyrinchus), both of which are partially endothermic. In other words, heat is generated by the movement of their muscles as they swim.

The tag most likely remained in the predator’s digestive system for a time. Once it was excreted, it floated up 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 behavior, 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."

This is not the first indication that large sharks eat one another, but such events are rarely observed. It is also the first recorded instance involving the predation of a porbeagle. The fact that the shark was pregnant is particularly concerning: in a single event, several sharks rather than one were removed from the porbeagle population. This suggests predation may be an overlooked pressure on the species’ survival.

Anderson said the behaviour is unlikely to be new; instead, it is probably something scientists previously lacked the means to detect. Tagging more porbeagles and other sharks could reveal why porbeagles migrate into the open North Atlantic, how frequently such encounters happen, and what effect they have on porbeagle numbers.

"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 behaviors 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 behaviors like this are going to be revealed."

The research was published in Frontiers in Marine Science.