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When scientists attached a tag to a pregnant porbeagle shark in October 2020 to learn more about the animal’s habitat, they didn’t expect their tag would collect evidence of how large sharks hunt each other.
But when the tracker detected unexpected activity in March 2021, scientists realized a larger shark had eaten their study 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 of a porbeagle shark anywhere in the world,” said lead researcher Dr. Brooke Anderson, a marine fisheries biologist with the North Carolina Department of Environmental Quality, via email.
Porbeagle sharks, found in the Atlantic, South Pacific and Mediterranean, can grow to just over 12 feet (3.7 meters) long and weigh up to 500 pounds (230 kilograms). The elusive, large sharks can also live between 30 and 65 years. However, female porbeagle sharks cannot reproduce until they are 13 years old. Females give birth to four young every one or two years.
Habitat loss, overfishing, and bycatch in fishing gear have threatened porbeagle populations. Northwest Atlantic porbeagle sharks are listed as vulnerable on the International Union for Conservation of Nature’s Red List of Threatened Species.
“In one case, the population not only lost a fertile female that could contribute to population growth, but also lost all of its developing babies,” Anderson said. “If predation is more widespread than previously thought, it could have major implications for the porbeagle population that 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 an eye on sharks
When Anderson and her colleagues tagged porbeagle sharks off Cape Cod, Massachusetts, in 2020 and 2022, their intention was to track where pregnant porbeagle sharks go to identify areas where the sharks and their newborn pups need conservation and protection measures.
She and her team have been studying porbeagle sharks for more than a decade and have become experts in tagging sharks for research.
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’re actually very calming, which makes tagging easy,” Anderson said. “We’ve tagged dozens of porbeagles over the last 10 years and are currently analyzing the data to determine the most important habitats for the population that can be prioritized for conservation and management.”
Each shark was equipped with two satellite tags, a fin-mounted satellite transmitter and a pop-off satellite archival tag. The fin-mounted tags transmit a shark’s current location to satellites whenever its fins are above the ocean’s surface. The pop-off tags measure the ocean’s depth and temperature, storing the data until the tag detaches after a certain amount of time, floats to the surface, and transmits its data to satellites.
The fact that the pregnant porbeagle fell prey to a larger shark was an additional scientific discovery, Anderson said.
The team’s shark, which measured 2.2 meters (7.3 feet) long, remained largely submerged for five months, swimming at depths of 100 to 200 meters (330 to 660 feet) at night and 600 to 800 meters (2,000 to 2,600 feet) during the day. Ocean temperatures ranged from 6.4 to 23.5 degrees Celsius (43.5 to 74.3 degrees Fahrenheit).
But 158 days after the shark was tagged and released, the pop-off tag began transmitting data from the sea southwest of Bermuda, showing that the shark had detached from the shark and was floating on the surface.
For four days in March 2021, the tag recorded a constant temperature of 71.6 degrees Fahrenheit (22 degrees Celsius) at depths ranging from 492 to 1,968 feet (150 to 600 meters). The tag then floated upward.
The team pieced together several factors that indicated the shark had been eaten and that the tag had been excreted by a larger predator that had eaten the shark, Anderson said.
“The first and most important data was the sudden increase in temperature recorded by the tag, even at 600 meters depth,” she said. “This immediately indicated that the tag was now in the stomach of a warm-blooded predator, such as a lamni 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 fell off eight months earlier than expected and the tag attached to the porbeagle’s fin never transmitted data again.
“If the pregnant porbeagle was indeed alive, we expect she would have returned to the ocean surface and the fin-mounted tag would have reported her location,” Anderson said.
Unusual suspects
Porbeagle sharks belong to the lamnid shark family, which also includes the great white shark and the mako shark.
Unlike other sharks, most lamni sharks are endothermic, meaning they can keep their bodies warmer than the water temperature.
“The porbeagle can do this better than almost any of its relatives and prefers the colder waters of Canada and New England year-round,” Anderson said.
To determine what the porbeagle might have been eating while it was swimming near Bermuda, the team narrowed down the list of large predators that swam in the same area and were large enough to prey on porbeagle sharks, including its relatives the great white shark, or Carcharodon carcharias, and the shortfin mako shark, known as Isurus oxyrinchus.
Shortfin mako sharks are known to prey on small sharks, porpoises, sea turtles, seabirds, bony fish, and cephalopods. And great white sharks eat whales, dolphins, seals, and rays.
Anderson’s team suspects the great white shark is the most likely culprit, as shortfin mako sharks make rapid dives between the surface and depths during the day, which the pop-off tag did not register.
“We often think of large sharks as the top predators, but thanks to advances in technology we are beginning to discover that the interactions between large predators may be even more complex than previously thought,” Anderson said.
“It is clear that we need to continue studying predator interactions, for example to estimate how often large sharks prey on each other, and to begin to uncover the cascading effects that these interactions can have on the ecosystem.”
It is not the first time that a large shark has been eaten by another large shark, but such events are rarely documented.
Sharks hunt sharks in the open sea
Some of the largest shark species aren’t afraid to hunt others of their own kind. It’s a fascinating part of the shark world that often goes unnoticed, says shark biologist Dr. Adrian Gutteridge, fisheries assessment manager for the international nonprofit Marine Stewardship Council and a member of the International Union for Conservation of Nature’s Shark Specialist Group.
Gutteridge, who was not involved in the study, agrees that a great white shark is the most likely culprit.
“This particular porbeagle, at 2.2 metres (7.3 feet), may have seemed quite formidable, but white sharks are about 1.5 metres (4.9 feet) long when they’re 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 out-competing other sharks. So the fact that this porbeagle met its end at the hands of a much larger white shark isn’t so much of a surprise, but it’s a reminder that white sharks are at the top of the food chain.”
He said satellite tags help researchers track and discover shark nurseries, seasonal movements and shark behavior patterns, which is especially critical for protecting vulnerable populations.
For decades, Northwestern Atlantic porbeagle sharks were hunted for consumption. Fortunately, that population is stabilizing and growing, but continued protection is vital to sustain this recovery, Anderson said.
Now the team wants to find out how often other sharks hunt porbeagle sharks.
“Unraveling the mysteries of the open ocean has always been a challenge,” Anderson said. “It’s possible that the more large sharks we can tag and track, the more behaviors like this will be revealed.”
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