Large sharks may be hunting other large sharks, scientists say

(CN) — When a team of scientists attached satellite tags to a pregnant porbeagle shark, they weren’t expecting to solve a shark-on-shark murder mystery, but that is what happened when the satellite tag started to transmit signals from Bermuda just months later.

The research article, published Tuesday in the journal Frontiers in Marine Science, documented the scientists’ foray into unraveling the story behind the shark’s demise.

A team of researchers captured porbeagles, a species of mackerel shark, off Massachusetts’ Cape Cod in 2020 and 2022 to study shark migration. Porbeagles live in the Atlantic and South Pacific Ocean and the Mediterranean.

The sharks were each equipped with two satellite tags, fin-mounted satellite transmitters and a pop-off satellite archival tag.

The pop-off satellite archival tags keep a running log of the depth and temperature and store the information until the tags fall off and float to the surface. The tags are typically set to fall off after a predetermined period.

In the case of a pregnant 7.2-foot-long porbeagle shark, the pop-off satellite archival tag began to unexpectedly transmit signals from the open sea southwest of Bermuda 158 days after being released.

The scientists had hoped to identify important habitats for porbeagle mothers and their offspring, but instead found data showing the pregnant shark had spent five months cruising depths between 100 and 200 meters at night and between 600 and 800 meters during the day until the fateful day of March 14, 2021. For the next four days, the pop-off satellite archival tag remained at a depth between 150 and 600 meters.

The scientists narrowed the explanation down to one possibility: the pregnant porbeagle had been eaten by a larger predator.

“This is the first documented predation event of a porbeagle shark anywhere in the world,” lead author Brooke Anderson, a former graduate student at Arizona State University, said in a statement.

The large and powerful porbeagle sharks live between 30 to 65 years and don’t reproduce until around the age of 13. Females have a gestation period of between eight and nine months and give birth to an average of four pups every one or two years.

“In one event, the population not only lost a reproductive female that could contribute to population growth, but it also lost all her developing babies,” Anderson said. “If predation is more widespread than previously thought, there could be major impacts for the porbeagle shark population that is already suffering due to historic overfishing.”

Recreational fishing, habitat loss and degradation, bycatch and persecution are all existing threats to porbeagles. Now, other sharks may be added to that list of threats.

The scientists narrowed down the list of suspects to two endothermic predators: the white shark and the shortfin mako. Both are large enough to hunt on porbeagles, which can grow up to 12 feet long, and both are found in the same area where the porbeagle was during that time of year.

“The predation of one of our pregnant porbeagles was an unexpected discovery,” Anderson said. “We often think of large sharks as being apex predators.”

Great white sharks have diverse diets, including marine mammals, fish and invertebrates. The scientists suspect a white shark was behind the attack because while shortfin mako sharks also have diverse diets, they typically make rapid dives between deep depths and the sea surface during the day. The pop-off satellite archival tag didn’t capture that type of depth change.

“With technological advancements, we have started to discover that large predator interactions could be even more complex than previously thought,” Anderson said.

Anderson suggested continued study on predator interactions to better understand how frequent shark-on-shark hunting may be.

“This will help us uncover what cascading impacts these interactions could have on the ecosystem,” Anderson said.

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