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When scientists tagged a pregnant porbeagle shark in October 2020 to learn more about the creature’s habitat, they didn’t expect their tracker to find evidence of how large sharks hunt each other .
But when the tracker registered unexpected activity in March 2021, the scientists realized that their research subject was being eaten by a bigger shark.
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 porbeagle sharks anywhere in the world,” lead study author Dr. Brooke Anderson, a marine fisheries biologist in the North Carolina Department of Environmental Quality, said via email.
Pearl sharks, found across the Atlantic and South Pacific and Mediterranean oceans, can reach just over 12 feet (3.7 meters) in length and weigh as much as 507 pounds (230 kilograms). The elusive big sharks can also live between 30 and 65 years of age. But female porbeagles cannot reproduce until they reach 13 years of age. Females give birth to four cubs every year or two.
Due to habitat loss, overfishing and their fate as bycatch in fishing nets, thresher shark populations are at risk. The Northwest China sharks are listed as vulnerable sharks on the International Union for Conservation of Nature’s Red List of Threatened Species.
“In one case, not only did the population lose a reproductive female that could have contributed to population growth, but it also lost all of its developing babies,” Anderson said. “If predation is more widespread than previously thought, it could have a major impact on the porbeagle shark population which is already suffering from historical overfishing.”
Now, researchers say they may have identified two suspects – a great white shark and a 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 thresher sharks off Cape Cod in Massachusetts in 2020 and 2022, they intended to track where the pregnant porpoises go to identify areas where the sharks need conservation and protection efforts and their newborn cubs.
She and her team have been studying porbeagles for over a decade, and are now experts in tagging 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 really set up really well to make tagging easy,” Anderson said. “We have tagged dozens of exhausted sharks over 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 guidelines.”
Each shark was equipped with two satellite tags, a fin-mounted satellite transmitter, and a pop-up satellite archival tag. The fin tags transmit a shark’s current location to satellites when its fins are above the ocean surface. The tags measure the depth and temperature of the ocean and store the data until the tag comes off after a certain period of time, floats to the surface and transmits its data to satellites.
The fact that the pregnant porbeagle was prey for a larger shark was a bonus scientific discovery, Anderson said.
The team’s shark, which measured 7.2 feet (2.2 meters) long, remained mostly underwater for five months, cruising 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 varied 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-up tag began transmitting data from the sea southwest of Bermuda, suggesting that 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 depths between 492 and 1,968 feet (150 to 600 meters). Then the tag floated up.
The team put together several factors that indicated that the shark and the tag were eaten excrement by a larger predator that raised the shark, Anderson said.
“The first and most important data was the sudden temperature spike recorded by the tag, even at a depth of 600 meters,” she said. “This immediately indicated that the tag was now inside the stomach of a warm predator such as a thresher shark. There was also a slight change in the diving pattern recorded by the tag, which also indicated that the tag was now tracking a different animal (the predator).”
The tag exploded eight months earlier than expected, and the bun’s fin tag never transmitted any data again.
“If the pregnant porbeagle shark was indeed still alive, we expect that she would have returned to the surface of the sea and the fin-mounted tag would have transmitted her location,” Anderson said.
Unusual suspected
It belongs to a family called lamnid sharks, which includes great white sharks and mako sharks in the porcine family.
Unlike other sharks, most lamnid sharks are endothermic, meaning they can keep their bodies warmer than the temperature of the water.
“The porbeagle can do this better than almost all of its relatives and it loves the cooler waters of Canada and New England year-round,” Anderson said.
To find out what the pig shark might have eaten while swimming near Bermuda, the team narrowed down the list of large predators swimming in the same vicinity that are 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 are known to prey on small sharks, porpoises, sea turtles, seabirds, bony fish and cephalopods. And great whites cry over whales, dolphins, seals and rays.
Anderson’s team suspects that the great white shark is the most likely culprit, since shortfin makos make rapid dives between the ocean’s surface and its depths during the day, which the tag didn’t register.
“We often think of large sharks as apex or apex predators, but with technological advances, we’re beginning to discover that large predator interactions may be even more complex than previously thought,” Anderson said.
“It’s clear that we need to continue to study predator interactions, for example to estimate how often large sharks are hunting each other, and to begin to find out what cascading effects the these interactions on the ecosystem.”
This is not the first time that one large shark has eaten another large shark, but such events are rarely recorded.
Sharks hunt sharks in the open seas
Some of the larger shark species aren’t shy about hunting their own kind, which is a fascinating and often overlooked part of the shark world, said shark biologist Dr. Adrian Gutteridge, fisheries assessment manager for the international non-profit Marine Stewardship Council and member of the Council. the Shark Specialist Group of the International Union for Conservation of Nature.
Gutteridge, who was not involved in the study, agrees that a white shark is the most likely culprit.
“This particular porbeagle may have looked very large, at 2.2 metres, but great white sharks are around 1.5 meters (4.9 feet) at birth,” said Gutteridge. “By the time they reach their full size at 4 to 5 meters (13 to 16.4 feet), they are fully capable of taking down other sharks. So it’s no surprise that this porbeagle ends up with a much larger white shark because it’s a reminder of white sharks being at the top of the food chain.”
Satellite tagging helps researchers track and discover shark nurseries, seasonal movement and behavioral patterns in sharks, which is critical to protecting vulnerable populations, he said.
For years, Northwestern Atlantic porpoises were hunted for consumption. Fortunately, that population is stabilizing and increasing, but continued protection is critical to allow that recovery to continue, Anderson said.
Now, the team is trying to find out how often other sharks prey on thresher sharks.
“Unraveling the mysteries of the open ocean has always been challenging,” Anderson said. “The more large sharks we can tag and track, the more behaviors like this will be revealed.”
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