Unexpected discovery: Large sharks are hunting each other

In a marine science mystery reminiscent of a game of Clue, researchers from the United States set out to investigate the death of a pregnant porbeagle shark in the open waters southwest of Bermuda. The evidence points to a larger shark, equipped with its deciduous triangular teeth, as the most likely culprit.  

Predation of a porbeagle shark 

“This is the first documented predation event of a porbeagle shark anywhere in the world,” said Brooke Anderson, the study’s lead author and a former graduate student at Arizona State University.

“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.”

Endangered porbeagle sharks

Porbeagles are formidable sharks found in the Atlantic, South Pacific Ocean, and the Mediterranean. These sharks are large, active, and robustly built, growing up to 3.7 meters in length and weighing as much as 230 kilograms. 

They are also long-lived, with lifespans that can reach 30 to even 65 years. Female porbeagles have a slow reproductive cycle, not reaching reproductive maturity until around 13 years old. When they do reproduce, they give birth to an average of four pups every one or two years after a gestation period of eight to nine months.

Due to this slow reproduction rate, porbeagle populations struggle to recover from threats such as overfishing, recreational fishing, bycatch, and habitat loss and degradation. 

These pressures have led to the porbeagle being listed as endangered in the Northwest Atlantic on the IUCN Red List of Threatened Species, with populations in the Northeast Atlantic and the Mediterranean considered critically endangered.

Porbeagles tagged off Massachusetts 

As part of a broader research effort to study shark migration, Anderson and her colleagues tagged porbeagles off the coast of Cape Cod, Massachusetts, in 2020 and 2022. 

Each shark was fitted with two satellite tags: a fin-mount satellite transmitter and a pop-off satellite archival tag (PSAT). 

The fin-mount tag sends the shark’s location to satellites whenever the fin surfaces, while the PSAT records depth and temperature data continuously, storing it until the tag detaches and floats to the surface to transmit its collected data.

Unexpected turn of events 

One of the tagged porbeagles was a pregnant female measuring 2.2 meters in length. Anderson and her team hoped that tracking this female would provide valuable insights into important habitats for porbeagle mothers and their offspring.

However, an unexpected turn of events occurred. After 158 days, the PSAT began transmitting from an area off Bermuda, indicating that the tag had detached and was floating at the surface.

The transmitted data revealed that this female had spent five months cruising at depths between 100 and 200 meters at night and 600 to 800 meters during the day, in water temperatures ranging from 6.4 to 23.5 °C. Throughout this period, the fin-mount tag had only transmitted once, confirming that the shark had mostly remained underwater.

But then, on March 24, 2021, over the span of four days, the temperature recorded by the PSAT remained consistently around 22 °C at depths between 150 and 600 meters. 

This sudden change in data could only mean one thing: the unfortunate porbeagle had been hunted and consumed by a larger predator. The PSAT was likely excreted four days later, which is when it began transmitting again.

Large sharks hunting each other

“Two endothermic predator candidates large enough to predate upon mature porbeagles and located within the vicinity and at the time of year of the predation event include the white shark Carcharodon carcharias and shortfin mako Isurus oxyrhinchus,” the authors wrote.

Shortfin mako sharks typically prey on cephalopods, bony fish, smaller sharks, porpoises, sea turtles, and seabirds, while great white sharks are known to feed on whales, dolphins, seals, and rays. 

Of the two suspects, the great white shark is the more likely culprit, as shortfin mako sharks usually exhibit rapid, oscillatory dives between the surface and deeper waters during the day – behavior that was not detected by the PSAT.

Complex interactions of large predators 

“The predation of one of our pregnant porbeagles was an unexpected discovery. 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,” Anderson noted.

“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,” she concluded.

This case, while tragic, opens new avenues for understanding the complexities of marine predator-prey dynamics, particularly among the ocean’s most formidable inhabitants.

The research is detailed in the journal Frontiers in Marine Science.

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