A pregnant shark was tagged and monitored for 5 months, then disappeared. Scientists now know its fate
But when the tracker registered some unexpected activity in March 2021, the scientists realized a larger shark had eaten their research subject.
The team shared these unexpected findings in a new study published in a scientific journal.
Porbeagle sharks, which are found across the Atlantic and South Pacific oceans and Mediterranean Sea, can reach a little more than 12 feet (3.7 meters) long and weigh as much as 507 pounds (230 kilograms). The elusive, large sharks can also live between 30 and 65 years old. But female porbeagles can’t reproduce until they reach 13 years old. The females give birth to four pups every one or two years.
Habitat loss, overfishing, and their fate as bycatch in fishing nets have threatened porbeagle shark populations. Northwest Atlantic porbeagle sharks are listed as vulnerable on the International Union for Conservation of Nature Red List of Threatened Species.
Now, the researchers said they may have identified two suspects — a great white shark and shortfin mako shark — in this scientific murder mystery, changing the way researchers think about how large sharks interact.
When the researchers were tagging porbeagle sharks, their intention was to track where pregnant porbeagles go to identify areas where the sharks and their newborn pups require conservation and protection efforts.
Each shark was outfitted with two satellite tags, a satellite transmitter mounted on the fin, and a pop-off satellite archival tag.
The fact that the pregnant porbeagle was prey for a bigger shark became a bonus scientific discovery.
The team’s shark largely remained underwater for five months, cruising along at depths of 328 to 656 feet (100 to 200 meters) at night and 1,969 to 2,625 feet (600 to 800 meters) during the day.
For four days in March 2021, the tag registered a constant temperature at a depth ranging from 492 to 1,968 feet (150 to 600 meters). Then the tag floated up.
The team pieced together several factors that indicated the shark had been eaten and the tag excreted by a larger predator that gobbled up the shark.
Porbeagles belong to a family called lamnid sharks, which also includes great white sharks and mako sharks.
To determine what could have eaten the porbeagle shark as it swam near Bermuda, the team narrowed down the list of large predators swimming in the same vicinity big enough to hunt porbeagles — including its relatives, the great white shark, and the shortfin mako shark.
Shortfin makos have been known to hunt small sharks, porpoises, sea turtles, seabirds, bony fish, and cephalopods. And great whites chow down on whales, dolphins, seals, and rays.
Anderson’s team suspects the great white shark is the most likely culprit, given that shortfin makos make rapid dives between the ocean surface and its depths during the day, which the pop-off tag didn’t register.
Some of the biggest shark species aren’t shy about hunting their own kind, which is a fascinating part of the shark world that often goes unnoticed, said a shark biologist.
Gutteridge agrees that a white shark is the most likely culprit.
Satellite tagging is helping researchers track and discover shark nurseries, seasonal movement, and behavioral patterns in sharks, which is especially crucial for protecting vulnerable populations, he said.
Now, the team wants to uncover how often other sharks hunt porbeagle sharks.