Marine biologist Brooke Anderson simply could not believe her eyes. Something was off about the data from a satellite tag that she had placed on a pregnant, seven-foot-long porbeagle shark (Lamna nasus) a few months earlier.
“It was very unusual, seeing that increase in the temperature and knowing that the tag popped off early,” Anderson tells Popular Science. “I honestly kept trying to talk myself out of it being a predation.”
[Related: When the ocean got hot, the sharks bulked up.]
She tried to come up with other explanations than another large animal attacking and eating the pregnant shark. Maybe there was a body or water temperature increase when the shark died?
“All signs just came back to the same thing,” says Anderson. “The shark must have been eaten.”
Anderson is a co-author of a study published September 3 in the journal Frontiers in Marine Science that details the first documented example of a porbeagle shark being preyed upon. But what could have killed and then eaten all of or part of this large shark?
Slow reproduction, strong babies
Porbeagles are powerfully built sharks that live in the Atlantic Ocean, South Pacific, and Mediterranean Sea. They grow up to 12 feet long and weigh up to about 500 pounds. They are related to the more famous great white shark (Carcharodon carcharias) and shortfin mako (Isurus oxyrhinchus). Porbeagles are also long-lived, with some documented sharks living up to 65 years.
Like some other large sharks, porbeagles give live birth despite being fish and they are also oophagous. Pregnant female sharks will continue to produce and ovulate unfertilized eggs throughout that their developing pups eat in utero.
[Related: Baby sharks stick to the shallows.]
“This helps them be really large and heavy at birth, which hopefully gives them a little bit of an edge in terms of survival,” Anderson says.
Female porbeagle sharks generally do not reproduce until they are roughly 13 years old. They give birth to an average of four pups every one or two years, after an eight to nine month gestation rate.
Since porbeagle sharks have a fairly slow reproduction rate, the death of a female can hurt population growth. Their populations cannot recover quickly from the overfishing, bycatch, and habitat loss and degradation that they are currently facing. The IUCN Red List of Threatened Species lists Northwest Atlantic porbeagles as vulnerable, and the Northeast Atlantic and Mediterranean populations are considered critically endangered. If other animals are preying on them, this could also put population levels in jeopardy.
Playing shark tag
To better understand their reproductive patterns, Anderson and her colleagues tagged several porbeagle sharks off Cape Cod, Massachusetts in 2020 and 2022. Each shark was equipped with two different satellite tags. The fin-mount satellite transmitters send back the shark’s location to satellites whenever their fins rise above the surface. The pop-off satellite archival tag (PSAT) continuously measures depth and temperature and stores it until the tag eventually falls off. PSAT data is then stored and transferred back up to satellites.
Among the sharks that the team tagged and released back into the ocean was a pregnant female that was about seven feet long.
“We really want to understand what habitats are important to them and where they might be going to give birth, in order to help identify areas that could be used to protect that super important part of the population,” explains Anderson.
Unexpectedly, the pregnant shark’s PSAT tag began to transmit off Bermuda 158 days after it was tagged and released. This indicated that the PSAT had popped off and was now floating at the surface of the water. The data showed that the shark had been cruising for five months at a depth between 328 and 658 feet at night and between 1,968 and 2,624 feet during the day. The water temperature stayed between 43 and 74 degrees Fahrenheit over that same time period. The fin-mount tag had only transmitted only once, confirming that she was underwater most of the time.
[Related: With new tags, researchers can track sharks into the inky depths of the ocean’s Twilight Zone.]
Beginning on March 24, 2021 and over a period of four days, the PSAT-measured temperature remained the same at 71°F, at a depth between 492 and 1,968 feet. From such consistent data Anderson and the team could only come up with one possible explanation. The porbeagle had been hunted and eaten by a larger predator. The predator likely then excreted the PSAT about four days later, when the tag began to transmit.
“I expect at least a portion of the shark was eaten off since we also had a thin mount satellite tag on the shark,” says Anderson. “That tag is designed to send location data in real time. We would expect, if that animal was still alive, that eventually it would come back to the surface, but we never heard from that tag again.”
Kin and suspects
The team believes that a great white shark or shortfin mako are large enough to have eaten a mature porbeagle shark, as both species are known to be in the vicinity at that time of year. While orcas whales are apex predators that are known to go after great whites, they are not commonly found in the waters off Bermuda. Shortfin mako sharks also feed on cephalopods, bony fish, small sharks, porpoises, sea turtles, and seabirds. Great whites are also known to eat whales, dolphins, seals, and rays among other organisms.
[Related: Great whites don’t hunt humans—they just have blind spots.]
Of the two species, the team believes that a great white shark is the most likely candidate. Shortfin makos typically make rapid, oscillating dives between the sea surface and deeper depths during the day while in the open ocean. This behavior was not registered by the PSAT.
“The predation of one of our pregnant porbeagles was an unexpected discovery. We often think of large sharks as being apex predators,” Anderson said in a press release. “But with technological advancements, we have started to discover that large predator interactions could be even more complex than previously thought.”
