Scientists piece together clues in a shark ‘murder mystery’

“This is a complete shock,” says Anderson, of the North Carolina Division of Marine Fisheries in Raleigh. “We’ve never seen this before.”

A shark-eat-shark theory

Porbeagles (Lamna nasus) are large-bodied sharks that look like a cross between a great white (Carcharodon carcharias) and a short-fin mako (Isurus oxyrinchus) (SN: 11/10/22). They hunt fish including mackerel, cod and haddock. Some individuals have been known to reach lengths exceeding three meters, which is partly why it’s so surprising that an individual that was more than 2 meters long and a formidable predator in its own right should find itself on the wrong end of the food chain.

To be clear, scientists did not witness a great white shark attack the porbeagle. Rather, Anderson and colleagues pieced together the missing shark’s hypothetical demise using location, temperature and depth data received from tags that had been attached to the shark during a routine catch-and-release survey. These devices also allowed the scientists to track the porbeagle from Cape Cod, Mass., to Bermuda — a migration from which it would never return.

“All of a sudden, on March 24, that temperature increases even at 300 meters depth. And it stays high, even at 600 meters depth,” Anderson says. “That’s a very clear indicator to us that this tag is no longer in the water column attached to the porbeagle. It’s in the stomach of a larger predator.”

To figure out the identity of the killer, Anderson looked at a host of variables that only became available after one of the tags floated back up to the water’s surface, presumably after being excreted by the larger predator.

According to the transmissions, the temperature readings were warmer than expected for the depths the tag had been traveling at, though they weren’t nearly warm enough to indicate being inside the belly of a large marine mammal, such as an orca. And in these waters, that left only two fish large enough and with the right range of internal body temperatures to have gobbled up the porbeagle — a shortfin mako or a great white. Based on diving patterns from previous tagging studies, the scientists propose that the great white seems to be the most likely culprit.

“It really was like a shark murder mystery,” Anderson says.

Not so fast

Shark scientist Megan Winton of the Atlantic White Shark Conservancy in Chatham, Mass., says that there’s plenty of evidence pointing at a large white shark as the suspect. But there are other plausible scenarios.

“There’s no question that something ate the tag,” Winton says. “But it’s hard to tie that necessarily to the mortality of the porbeagle.”

For instance, it’s possible a predator simply ate the tag and not the shark it was attached to, she says. It’s also possible that the predator was another porbeagle.

“We do know that white sharks eat other shark species, including other large shark species, so it’s definitely a potential candidate for what consumed this tag,” Winton says. “But even if it was just the tag being bit off, other than then, you know, another shark eating that shark, it’s still a very interesting interaction between species.”