Swallowed Tracking Tag Confirms Large Sharks Are Hunting Each Other

During a recent effort to track porbeagle sharks, researchers made a grisly discovery. One of their tags transmitted strange water depth and temperature readings, leading them to conclude that their subject had been eaten by a larger shark.

This is the first recorded instance of sharks preying on porbeagles, a development that could have dire consequences for a species already facing severe population decline.

The marine biologists, which included former Arizona State University graduate student Brooke Anderson, had gone fishing for porbeagle sharks in the waters southeast of Cape Cod in October 2020, and again during the same month, two years later. After they were caught, the researchers stuck satellite-linked transmitters onto the sharks’ fins. The tags were designed to stay on the sharks for one year, after which they would rise to the surface. Once there, they could transmit the data they collected to Anderson and her colleagues.

In April 2021, while tracking the sharks, they noticed that one—a seven-foot-long (2.2 meters) pregnant female—was sending back some odd data. Her transmitter had detached near Bermuda after only five months, which was unusual enough. Even stranger were the readings from the week before the tag surfaced. The temperatures measured during that time ranged between 61.5 degrees Fahrenheit (16.4 degrees C) and 76 degrees Fahrenheit (24.7 degrees C), far warmer than the waters she was supposed to be swimming in. There could only be one reason: During that week, the tracker, and presumably pieces of the shark it had been attached to, was in the digestive system of a predator.

Porbeagles are big, growing up to 12 feet (3.7 meters) in length and weighing up to 500 pounds (230 kilograms), and they can be found lurking in the waters of the north and south Atlantic, Pacific, and Indian oceans. In the study, published in the journal Frontiers in Marine Science, Anderson and her team concluded that there are only two predators who would have been present in the part of the Atlantic where the porbeagle was eaten at that time of year—great whites and shortfin makos, both large sharks.

The discovery is the “first documented predation event of a porbeagle shark anywhere in the world,” said Anderson in a press release.

The incident is alarming because of what it could mean for porbeagle shark populations. The sharks are classified as endangered, with a 2016 National Oceanic and Atmospheric Administration report estimating that stocks have declined by as much as 90% due to fishing and habitat loss. Porbeagles generally live to around 30 years old (though some as old as 65 have been recorded), but females only reach sexual maturity at about 13. The females give birth to an average of four pups every year or two. That’s a relatively slow reproductive cycle, which means the population has a tough time bouncing back once they start declining. Adding other sharks to the dangers, particularly to pregnant females, just makes the situation more dire.

“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,” said Anderson. “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.”

Discovering that a rare, endangered shark was eaten isn’t exactly welcome news for marine biology, but it could prove significant. Now that researchers like Anderson know it’s happening, they can study the phenomenon of large sharks eating each other further. That could lead to new strategies to save these fearsome, awesome creatures while there’s still time.