Marine biologist Brooke Anderson couldn’t believe her eyes. Something was wrong with the data from a satellite tag she had placed on a pregnant, seven-foot-long porbeagle shark (Rhinoceros) a few months earlier.
“It was very unusual to see that temperature increase and know that the label came off early on,” Anderson said. Popular science“I kept convincing myself, honestly, that it wasn’t a predatory act.”
[Related: When the ocean got hot, the sharks bulked up.]
She tried to think of other explanations than another large animal attacking and eating the pregnant shark. Maybe there was a rise in body or water temperature when the shark died?
“All the signals were the same,” Anderson said. “The shark must have been eaten.”
Anderson is a co-author of a study published Sept. 3 in the journal Frontiers in Marine Science that describes the first documented instance of a porbeagle being preyed upon. But what could have killed and then partially or completely consumed this large shark?
Slow reproduction, strong babies
Porbeagles are powerfully built sharks that live in the Atlantic Ocean, the South Pacific Ocean, and the Mediterranean Sea. They grow up to 12 feet long and weigh up to about 500 pounds. They are related to the better-known great white shark (Carcharodon carcharias) and shortfin mackerel (Isurus oxyrhinchusPorbeagle sharks are also long-lived, with some documented sharks living up to 65 years.
Like some other large sharks, porbeagles give birth live despite being fish, and they are also oophagous. Pregnant female sharks continue to produce and ovulate unfertilized eggs that their developing young eat in utero.
[Related: Baby sharks stick to the shallows.]
“This makes them really big 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 about 13 years old. They give birth to an average of four young every year or two, after a gestation period of eight to nine months.
Because porbeagle sharks have a relatively slow reproductive rate, the death of a female can harm population growth. Their populations cannot recover quickly from the overfishing, bycatch, and habitat loss and degradation that they currently face. The IUCN Red List of Threatened Species lists the Northwest Atlantic porbeagle shark as Vulnerable, and populations in the Northeast Atlantic and Mediterranean are listed as Critically Endangered. If they are preyed upon by other animals, this could also threaten population levels.
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-mounted satellite transmitters send the shark’s location back to satellites whenever its fins are above the surface. The pop-off satellite archival tag (PSAT) continuously measures and stores depth and temperature until the tag eventually falls off. PSAT data is then stored and sent back to satellites.
Among the sharks the team tagged and released back into the ocean was a pregnant female measuring about two metres in length.
Scientists tag and release a porbeagle shark off the coast of Massachusetts. CREDIT: Jon Dodd.
“We really want to understand what habitats are important to them and where they might be giving birth, and then identify areas that can be used to protect that super important part of the population,” Anderson explains.
Unexpectedly, the pregnant shark’s PSAT tag began transmitting off the coast of Bermuda 158 days after she was tagged and released. This indicated that the PSAT had become detached and was now floating on the surface of the water. The data showed that the shark had been roaming between 328 and 658 feet at night and between 1,968 and 2,624 feet during the day for five months. The water temperature remained between 43 and 74 degrees Fahrenheit during the same period. The fin-mounted tag had only transmitted once, confirming that she was submerged most of the time.
[Related: With new tags, researchers can track sharks into the inky depths of the ocean’s Twilight Zone.]
Starting on March 24, 2021, and for a period of four days, the temperature measured by PSAT remained steady at 71°F, at depths between 492 and 1,968 feet. With 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 excreted the PSAT about four days later, when the tag began transmitting.
“I would expect at least part of the shark was eaten, since we also had a thin satellite tag on the shark,” Anderson said. “That tag is designed to transmit location data in real time. We would expect that if that animal was still alive, it would eventually come back to the surface, but we never heard from that tag again.”
Family and suspects
The team believes that a great white shark or shortfin mako shark would be large enough to have eaten an adult porbeagle, as both species are in the area at that time of year. Although killer whales are apex predators known to prey on great white sharks, they are not often found in Bermuda’s waters. Shortfin mako sharks also feed on cephalopods, bony fish, small sharks, porpoises, sea turtles and sea birds. Great white sharks have also been known to eat whales, dolphins, seals and rays, among other things.
[Related: Great whites don’t hunt humans—they just have blind spots.]
Of the two species, the team believes a great white shark is the most likely candidate. Shortfin mako sharks typically make rapid, oscillating dives between the surface and deeper depths during the day in the open ocean. This behavior was not recorded by the PSAT.
“The predation of one of our pregnant porbeagles was an unexpected discovery. We often think of large sharks as apex predators,” Anderson said in a press release. “But with advances in technology, we’ve learned that interactions between large predators can be even more complex than previously thought.”
