In what can only be described as a real underwater whodunnit, scientists think they have identified the main suspect behind the disappearance of a pregnant porbeagle shark 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 a species of this size.
The lead author of the study published in Frontiers in Marine ScienceBrooke Anderson, described the event as both surprising and alarming. “This is the first documented predation event of a porbeagle shark anywhere in the world,” he said in a statement.
“In one event, the population not only loses reproductive females that can contribute to population growth, but also loses all the babies that develop. It has suffered from historical 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 and weigh up to 500 pounds.
These sharks are known for their longevity, which can reach 65 years, and their slow reproductive cycle. Females do not reproduce until they are 13 years old, and on average give birth to four offspring every one or two years.
Because of this slow reproduction rate, the porbeagle population is particularly vulnerable to threats such as overfishing, whether intentional or unintentional, and habitat degradation.
This species is currently listed as endangered in the North Atlantic and critically endangered in the North Atlantic and Mediterranean by the International Union for Conservation of Nature (IUCN).
Anderson and his colleagues tagged a pregnant porceagle participating in this study during a research expedition on Cape Cod in Massachusetts in 2020. The researchers equipped the shark with two satellite tags: a fin satellite transmitter to track its location and a pop-off satellite. archival tag (PSAT) to monitor depth and temperature.
Once released, the shark was tracked for a total of 158 days, during which it initially exhibited typical porceagle behavior, cruising at depths between 300 and 2,600 feet depending on the time of day. The temperature recorded by the tag fluctuates between 44 and 74°F.
But on March 24, 2021, the PSAT data showed a sudden and drastic change. For four days, the tag recorded a constant temperature of 72°F, despite being at a depth of about 500 to 2,000 feet.
This, the team suggests, means that the shark has been consumed by a larger predator, which then releases the PSAT after a few days.
After the PSAT returned to the surface, it began sending data back to the team.
James Sulikowski
The researchers considered two potential predators large enough to predate on adult porbeagles: the great white shark and the shortfin mako shark.
While both species are known to inhabit the area and may hunt porbeagles, the data suggests the great white is the bigger culprit. Unlike the shortfin mako, which usually shows a fast and oscillatory diving pattern, the great white’s behavior is closer to the data recorded by the PSAT.
“The predation of one of the pregnant porbeagles was an unexpected discovery,” Anderson said. “We often think of large sharks as apex predators. But with advances in technology, we are beginning to discover that large predator interactions may be more complex than previously thought.
“We need to continue to study predator interactions to estimate how often large sharks hunt each other. This will help us discover what may be driving these interactions in the ecosystem.”
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Reference
Anderson, BN, Horstmyer, L., Ballard, KL, Dodd, J., & Sulikowski, JA (2024). First evidence of predation in pop-off satellite-tagged adult porbeagle in the North Atlantic. Frontiers in Marine Science. https://doi.org/10.3389/fmars.2024.1406973
