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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 tracking devices to capture evidence of how the big sharks hunt each other.
But when the tracker recorded unexpected activity in March 2021, scientists realized a larger shark was eating their research subjects.
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,” lead study author Dr. Brooke Anderson, a marine fisheries biologist with the North Carolina Department of Environmental Quality, said in a statement via email.
Found in the Atlantic and South Pacific oceans and the Mediterranean Sea, Porbeagle sharks can grow to be a little over 12 feet (3.7 meters) long and weigh up to 507 pounds (230 kilograms). These elusive, large sharks can live to be 30 to 65 years old. However, female Porbeagles cannot reproduce until they are 13 years old. Females give birth to four cubs 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’s Red List of Threatened Species.
“In one event, the population not only lost a reproductive female that could have contributed to population growth, but also lost all of its developing offspring,” Anderson said. “If predation is more widespread than previously thought, there could be major implications for the porbeagle shark population, which is already suffering due to historic overfishing.”
Researchers now say they may have identified two suspects in this scientific murder mystery—the great white shark and the shortfin mako shark—and it could change the way researchers think about how large sharks interact.
Chasing sharks
When Anderson and her colleagues tagged porbeagle sharks off Cape Cod, Massachusetts, in 2020 and 2022, their goal was to track where pregnant porbeagles traveled to identify areas where the sharks and their newborns needed conservation and protection efforts.
He and his team have been studying sharks for more than a decade and have become experts at tagging sharks for research.
The researchers used fishing rods and reels to catch the sharks and bring them back to their boat, where they were fitted with saltwater pumps to help them breathe.
“They actually calm down very well to make tagging easier,” Anderson said. “We’ve tagged dozens of porbeagle sharks over the past 10 years, and we’re currently working to analyze the data to identify 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 archive tag. The fin-mounted tags transmit the shark’s current location to satellites when its fins are above the ocean surface. The pop-up tags measure depth and ocean temperatures and store the data until the tag pops up after a certain period of time, floats to the surface and transmits its data to the satellites.
Anderson said the discovery of a pregnant porpoise falling prey to a larger shark was a bonus scientific discovery.
The team’s 7.2-foot-long (2.2-meter) shark remained largely submerged for five months, navigating 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. Ocean temperatures fluctuated between 43.5 and 74.3 degrees Fahrenheit (6.4 to 23.5 degrees Celsius).
But 158 days after the shark was tagged and released, the deployed tag began sending data from the sea southwest of Bermuda, suggesting it had broken off from the shark and was floating on the ocean surface.
For four days in March 2021, the tag recorded a steady temperature of 71.6 degrees Fahrenheit (22 degrees Celsius) at depths of 492 to 1,968 feet (150 to 600 meters). The tag then floated upward.
Anderson said the team pieced together several factors that suggest the shark was eaten and the tag was discarded by a larger predator that swallowed the shark.
“The first and most important piece of data was the sudden temperature increase recorded by the tag, even at a depth of 600 metres,” he said. “This immediately indicated that the tag was now in the stomach of a warm-bodied predator such as a lamnid shark. There was also a slight change in the dive pattern recorded by the tag, indicating that the tag was now tracking another animal (predator).”
The tag fell off eight months earlier than expected, and the porpoise’s fin-mounted tag never transmitted data again.
“If the pregnant porbeagle shark is indeed still alive, we would expect it to have returned to the sea surface and the fin pocket tag would have transmitted its 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 stampede can do this better than almost any of its relatives, and it loves the cold waters of Canada and New England year-round,” Anderson said.
To determine what might be eating porbeagle sharks swimming near Bermuda, the team narrowed down the list of large predators swimming in the vicinity that were large enough to prey on porbeagles, including the porbeagle’s relatives the great white shark (Carcharodon carcharias) and the shortfin mako shark (Isurus oxyrinchus).
Short-finned makos are known to prey on small sharks, dolphins, sea turtles, seabirds, bony fish, and cephalopods, while great whites eat whales, dolphins, seals, and rays.
Anderson’s team suspects the great white shark is the most likely culprit, considering that shortfin mako sharks make rapid dives between the ocean surface and the depths during the day, which were not recorded on the pop-off tag.
“We often think of large sharks as top predators, but with technological advances we are beginning to discover that interactions with large predators may be 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 prey on each other and to begin to uncover what cascading effects these interactions might have on the ecosystem.”
This isn’t the first time a large shark has been eaten by another large shark, but documented incidents of this nature are rare.
Sharks hunting sharks in the open seas
Some of the largest shark species are not shy about preying on their own kind, and it’s a fascinating part of the shark world that often goes unnoticed, said shark biologist Dr. Adrian Gutteridge, director of fisheries assessments at 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, said the great white shark was the most likely culprit.
“This particular 2.2m porbeagle may have looked quite intimidating, but white sharks are around 1.5m (4.9ft) at birth,” Gutteridge said. “Once they reach their full size of 4 to 5m (13 to 16.4ft), they are capable of preying on other sharks. So it’s not surprising that this porbeagle was doomed by a much larger white shark, and it’s a reminder that white sharks are at the top of the food chain.”
He said satellite tagging helps researchers track and explore shark breeding grounds, seasonal movements and behavioural patterns in sharks, which is vital for protecting particularly vulnerable populations.
For decades, northwest Atlantic porbeagles were hunted for consumption. Fortunately, that population is stabilizing and increasing, but Anderson said continued protection is vital to ensure such a recovery continues.
Now the team wants to uncover how often other sharks prey on porbeagle sharks.
“Uncovering the mysteries of the open ocean is always challenging,” Anderson said. “The more large sharks we can tag and track, the more likely we are to uncover behaviors like this.”
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