In what can only be described as a real-life underwater whodunnit, scientists think they have identified the primary 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, though less likely, a shortfin mako. If confirmed, this would represent the first recorded case of shark-on-shark violence with species of this size.
Lead author of the study published in Frontiers in Marine Science, Brooke Anderson, described the event as both surprising and alarming. “This is the first documented predation event of a porbeagle shark anywhere in the world,” she said in a statement.
“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.”
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Porbeagle sharks, native to the Atlantic and South Pacific Oceans as well as the Mediterranean, are large, powerfully built creatures that can grow up to 12 feet long and weigh as much as 500 pounds.
These sharks are known for their long lifespan, which can extend up to 65 years, and their slow reproductive cycle. Females do not reproduce until around 13 years of age, and they give birth to an average of four pups every one or two years.
Because of this slow reproductive rate, porbeagle populations are particularly vulnerable to threats such as overfishing, whether purposeful or by accident, and habitat degradation.
The species is currently listed 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 her colleagues tagged the pregnant porbeagle involved in this study during a research expedition off Cape Cod in Massachusetts in 2020. The researchers equipped the shark with two satellite tags: a fin-mount satellite transmitter to track its location and a pop-off satellite archival tag (PSAT) to monitor depth and temperature.
After its release, the shark was tracked for a total of 158 days, during which it initially exhibited typical porbeagle behavior, cruising at depths between 300 and 2,600 feet depending on the time of day. The temperature recorded by the tag flicked between 44 and 74°F.
But on March 24, 2021, the PSAT data revealed a sudden and drastic change. For four days, the tag recorded a constant temperature of 72°F, despite being at depths of around 500 to 2,000 feet.
This, the team suggested, meant that the shark had been consumed by a larger predator, which had then excreted the PSAT after a few days.
Once the PSAT returned to the surface, it began transmitting data back to the team.
James Sulikowski
The researchers considered two potential predators large enough to predate on a mature porbeagle: the great white shark and the shortfin mako shark.
While both species are known to inhabit the area and could feasibly hunt a porbeagle, the data pointed to the great white as the more likely culprit. Unlike the shortfin mako, which typically exhibits rapid, oscillatory diving patterns, the great white’s behavior aligned more closely 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 being apex predators. But with technological advancements, we have started to discover that large predator interactions could 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 what cascading impacts these interactions could have on the ecosystem.”
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References
Anderson, B. N., Horstmyer, L., Ballard, K. L., Dodd, J., & Sulikowski, J. A. (2024). First evidence of predation on an adult porbeagle equipped with a pop-off satellite archival tag in the Northwest Atlantic. Frontiers in Marine Science. https://doi.org/10.3389/fmars.2024.1406973