When scientists attached a tag to a pregnant porbeagle shark in October 2020 to learn more about the creature’s habitat, they didn’t expect their tracker to capture evidence of how large sharks hunt one another.
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 Tuesday in the journal Frontiers in Marine Science.
“This is the first documented predation event of a porbeagle shark anywhere in the world,” said lead study author Dr. Brooke Anderson, a marine fisheries biologist in the North Carolina Department of Environmental Quality, via email.
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, overfishingand 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.
“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,” Anderson said. “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.”
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.
Keeping tabs on sharks
When Anderson and her colleagues were tagging porbeagle sharks off Cape Cod in Massachusetts in 2020 and 2022, their intention was to track where pregnant porbeagles go to identify areas where the sharks and their newborn pups require conservation and protection efforts.
She and her team have been studying porbeagles for more than a decade, and they have become experts at tagging the sharks for study.
The researchers used rods and reels to catch the sharks and bring them aboard their boat. Saltwater pumps were placed in the sharks’ mouths to allow them to breathe.
“They actually calm down really well to make tagging easy,” Anderson said. “We’ve tagged dozens of porbeagle sharks in the past 10 years and are currently working on analyzing the data to determine the most important habitats for the population that can be prioritized for conservation and management directions.”
Each shark was outfitted with two satellite tags, a satellite transmitter mounted on the fin, and a pop-off satellite archival tag. The fin-mount tags transmit a shark’s current location to satellites when its fins are above the ocean’s surface. The pop-off tags measure depth and ocean temperatures and store the data until the tag pops off after a certain amount of time, floats to the surface and transmits its data to satellites.
The fact that the pregnant porbeagle was prey for a bigger shark became a bonus scientific discovery, Anderson said.
The team’s shark, measuring 7.2 feet (2.2 meters) long, 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. The ocean temperature fluctuated between 43.5 and 74.3 degrees Fahrenheit (6.4 and 23.5 degrees Celsius).
But 158 days after the shark was tagged and released, the pop-off tag began to transmit data from the sea southwest of Bermuda, suggesting it had come off the shark and was floating on the ocean’s surface.
For four days in March 2021, the tag registered a constant temperature of 71.6 degrees Fahrenheit (22 degrees Celsius) 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, Anderson said.
“The first and most important data was the sudden spike in temperature recorded by the tag, even at 600 meters depth,” she said. “This indicated right away that the tag was now inside the stomach of a warm-bodied predator such as a lamnid shark. There was also a slight change in the diving pattern recorded by the tag, which also indicated the tag was now tracking another animal (the predator).”
The tag popped off eight months earlier than expected, and the porbeagle’s fin-mount tag never transmitted any data again.
“If the pregnant porbeagle shark was indeed still alive, we expect that she would have returned to the sea surface again and the fin-mount tag (would) transmit her location,” Anderson said.
Unusual suspects
Porbeagles belong to a family called lamnid sharks, which also includes great white sharks and mako sharks.
Unlike other sharks, most lamnid sharks are endothermic, meaning they can keep their bodies warmer than the water temperature.
“The porbeagle can do this better than almost all of its relatives and loves the colder waters of Canada and New England year-round,” Anderson said.
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, or Carcharodon carcharias, and the shortfin mako shark, known as Isurus oxyrinchus.
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.
“We often think of large sharks as being apex or top predators, but with technological advancements, we have started to discover that large predator interactions could be even more complex than previously thought,” Anderson said.
“It’s clear that we need to continue studying predator interactions, for example to estimate how often large sharks are hunting each other, and start to uncover what cascading impacts these interactions could have on the ecosystem.”
It’s not the first time one large shark has been eaten by another large one, but documentation of such events is rare.
Sharks hunting sharks in the open seas
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 shark biologist Dr. Adrian Gutteridge, fisheries assessment manager for international nonprofit Marine Stewardship Council and member of the International Union for Conservation of Nature’s Shark Specialist Group.
Gutteridge, who was not involved in the study, agrees that a white shark is the most likely culprit.
“This particular porbeagle, at 2.2 metres, might have seemed pretty formidable, but white sharks are around 1.5 metres (4.9 feet) the moment it’s born,” Gutteridge said. “By the time they reach their full size at 4 to 5 metres (13 to 16.4 feet), they’re fully capable of taking down other sharks. So, this porbeagle meeting its end from a much larger white shark isn’t so much surprising as it is a reminder of white sharks being at the top of the food chain.”
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.
For decades, northwest Atlantic porbeagles were hunted for consumption. Fortunately, that population is stabilizing and increasing, but continued protection is vital to allow such recovery to continue, Anderson said.
Now, the team wants to uncover how often other sharks hunt porbeagle sharks.
“Uncovering the mysteries of the open ocean has always been challenging,” Anderson said. “It’s possible that the more large sharks we can tag and track, the more behaviors like this are going to be revealed.”