Massive pregnant shark tracked off Bermuda — then her tag pings inside bigger creature

When you’re a 7-foot-long predator with rows of razor sharp teeth and a powerful body, there isn’t much to be afraid of in the open ocean.

That’s the life of porbeagle sharks, a species that travels from the Atlantic to the Mediterranean and South Pacific Ocean.

“They are large, active, powerfully built — up to (12 feet) long and weighing up to (507 pounds) — and long-lived, living up to 30 or even 65 years,” researchers said in a Sep. 3 news release from the peer-reviewed journal Frontiers in Marine Science.

Their bodies would suggest the species is successful in the oceans, but due to an extremely slow reproductive cycle, some porbeagle shark populations are threatened and others are critically endangered.

“Females don’t reproduce until they are about 13 years old, and then give birth to an average of four pups every one or two years, born live after a gestation period between eight and nine months,” researchers said.

Hoping to learn more about these animals, a research team led by Brooke Anderson, a former graduate student at Arizona State University, caught porbeagle sharks off the coast of Cape Cod from 2020 to 2022, according to a study published Sep. 3 in Frontiers in Marine Science.

Pregnant shark caught

The sharks were fitted with “pop-off satellite archival tags (PSATs)” and “fin-mount satellite transmitters,” according to the study. The fin-mount tags, attached to the fin as the name suggests, send a satellite signal with the shark’s location each time they break the surface of the water. The PSATs measure environmental conditions like water temperature and depth of the shark based on the amount of light the tag detects, and the tag is designed to “pop-off” and float to the surface after a predetermined length of time, researchers said.

One of the porbeagles caught in Massachusetts was a 7-foot-long female, and she was pregnant, according to the study. Researchers were hoping they could use her migration data to identify where the sharks were giving birth and where newborn sharks may congregate.

She was fitted with tags and released back into the water. The tracking data is stored in the tag and not transmitted to the researchers until the tag has popped-off from the shark, according to the study, so it was a waiting game.

“But fate intervened,” researchers said. “Unexpectedly, this female’s PSAT started to transmit off Bermuda 158 days after its release. This implied that the PSAT had popped off and was now floating at the surface.”

The tag was collected, and researchers began to sift through its data.

For five months, the shark stayed at a depth between about 500 and 2,000 feet at night and at higher depths during the day, according to the study. Her fin-mount tag only pinged one time meaning she only came to the surface once during that period, researchers said. The shark was also swimming in waters of varying temperatures throughout the day as she changed depths, in a consistent pattern, according to the study.

“But suddenly, from 24 March 2021 onwards over the period of four days, the temperature as measured by the PSAT remained at a constant (71.6 Fahrenheit), at a depth between (500 and 2,000 feet),” researchers said. “Only one explanation was possible: that day, the unfortunate porbeagle had been hunted and eaten by a larger predator. The PSAT must then have been excreted about four days later, starting to transmit.”

The constant temperature wasn’t the shark staying perfectly in one spot. Instead, it was the tag moving through the digestive system of the attacking shark.

‘Scientific whodunnit’

Now, researchers are calling it a “scientific whodunnit” as they make accusations against possible perpetrators.

“Two endothermic predator candidates large enough to predate upon mature porbeagles are located within the vicinity and at the time of year of the predation event include the (great white shark) and shortfin mako,” according to the study.

Mako, while large enough, typically feed on squid, fish, small sharks, sea turtles, porpoises and seabirds, researchers said. They also typically dive from the surface to the depths multiple times in a day while out in the open ocean.

Great whites, on the other hand, have been known to eat whales, dolphins, seals and rays, and to stay at consistent depths, researchers said. They predict an attack from a great white is more likely, but the data is unable to provide a concrete answer.

“The predation of one of our pregnant porbeagles was an unexpected discovery. We often think of large sharks as being apex predators. But with technological advancements, we have started to discover that large predator interactions could be even more complex than previously thought,” Anderson said. “We need to continue studying predator interactions, to estimate how often large sharks hunt each other. This will help us uncover what cascading impacts these interactions could have on an ecosystem.”

The porbeagle was attacked in the Sargasso Sea off the coast of Bermuda, an archipelago in the western Atlantic.

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