Shark attack on shark: ‘Unexpected’ solution to sailor’s murder mystery

In what can only be described as a true underwater thriller, scientists believe they have identified the prime suspects involved in the disappearance of a pregnant porbeagle shark that was being tracked off the coast of Bermuda.

The culprit? Either a great white shark or, although less likely, a shortfin mako shark. If confirmed, this would be the first recorded case of shark violence involving species of this size.

Lead author of the study published in The Frontiers of Marine ScienceBrooke Anderson called the event surprising and alarming. “This is the first documented case of porbeagle shark predation in the world,” she said in a statement.

“In one case, the population not only lost a breeding female that could have contributed to population growth, but she also lost all of her developing pups. If predation is more widespread than previously thought, this could have major consequences for the porbeagle population, which is already suffering from historical overfishing.”

Porbeagle sharks, native to the Atlantic and South Pacific Oceans as well as the Mediterranean, are large, powerful creatures that can grow up to 12 feet long and weigh up to 500 pounds.

These sharks are known for their long lifespans, which can reach 65 years, and their slow reproductive cycle. Females do not reproduce until they are about 13 years old and give birth to an average of four pups every one to two years.

Because of this slow reproductive rate, porbeagle populations are particularly vulnerable to threats such as overfishing, whether intentional or accidental, and habitat degradation.

The species is currently classified as endangered in the Northwest Atlantic and critically endangered in the Northeast Atlantic and Mediterranean by the International Union for Conservation of Nature (IUCN).

Anderson and his colleagues tagged the pregnant porbeagle shark involved in this study during a research expedition off Cape Cod, Massachusetts in 2020. The researchers equipped the shark with two satellite tags: a fin-mounted satellite transmitter to track its position and a removable satellite tracking tag (PSAT) to monitor depth and temperature.

After its release, the shark was tracked for 158 days, during which time it initially displayed typical porbeagle behavior, cruising at depths of between 300 and 2,600 feet depending on the time of day. The temperature recorded by the tag ranged from 44 to 74°F.

But on March 24, 2021, PSAT data revealed a sudden and drastic change. For four days, the beacon recorded a constant temperature of 72°F, despite depths of about 500 to 2,000 feet.

According to the team, this meant that the shark had been consumed by a larger predator, which then excreted the PSAT after a few days.

Once the PSAT returned to the surface, it began transmitting data to the team.

The researchers considered two potential predators large enough to take on an adult porbeagle: the great white shark and the shortfin mako shark.

Although both species are known to inhabit the area and could prey on a porbeagle, the data showed that the great white shark was the more likely culprit. Unlike the shortfin mako, which typically exhibits rapid oscillatory diving patterns, the great white shark’s behavior was more consistent with the data recorded by the PSAT.

“The predation of one of our pregnant porbeagles was an unexpected discovery,” Anderson said. “We often think of large sharks as apex predators. But with advances in technology, we’ve begun to discover that interactions with large predators may be even more complex than previously thought.”

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

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References

Anderson, BN, Horstmyer, L., Ballard, KL, Dodd, J., & Sulikowski, JA (2024). First evidence of predation on an adult porbeagle shark equipped with a removable satellite tag in the Northwest Atlantic. The Frontiers of Marine Science. https://doi.org/10.3389/fmars.2024.1406973