Sinister Find in Cape Cod: A Female Shark Possibly Consumed by a Fearsome Predator

Scientific studies don't always go as planned. Scientists made a grim discovery while studying porbeagle sharks (Lamna nasus) off Cape Cod, a peninsula in the U.S. state of Massachusetts.

Their goal was to monitor the movements of several porbeagle sharks, also known as porbeagles or porpoise sharks. This study was all the more important because this species is considered to be in danger of extinction.

It all started in 2020 and 2022 when scientists captured several porbeagle sharks. They fitted them with fin-mounted transmitters and PSAT tags that communicate via satellite.

The female shark was eaten just 158 ​​days after being released by scientists

Transmitters on the fins can transmit data only when the shark is at the surface of the water. PSATs (Pop-up satellite archival tags) can record everything that happens continuously. These tags work until they detach and float back to the surface of the water.

A pregnant female was part of the group of porbeagle sharks that participated in this study. She measured 2.2 meters long, while the maximum size of these sharks is about 3.2 meters. This made her an excellent specimen to study to help conserve the species.

This female unfortunately met a tragic fate just 158 ​​days after being released into the ocean at the end of 2020. She did not even have time to give birth to her offspring.

The beacon recorded a constant temperature of 22°C at several hundred meters depth.

Scientists quickly figured out what had happened. By checking the data from the transmitter and the tag, they were able to confirm that the porbeagle had been cruising steadily for 5 months.

In March 2021, researchers noticed a drastic change in the data received. The temperature became constant at 22 °C while the tag was suddenly at a greater depth.

The researchers immediately realized that the female had been eaten by a larger predator. As Dr. Brooke Anderson, the study's lead author, explains, “When a marked animal is eaten, we often see patterns in the data that indicate predation has occurred.”

Predator swallowed female shark's tag

So the question was not what happened to the porbeagle shark, but rather who was the predator responsible? To determine this, there is no choice but to rely on the data transmitted by the tag.

In this case, the depth at which the tag was located clearly indicated that it was no longer the porbeagle shark swimming in the ocean. Additionally, “you can also see high and stable temperatures if the predator is endothermic, because the tag is now recording the temperature of the predator's stomach rather than the temperature of the water.”

There are also tags that transmit the level of light. This obviously drops to zero when the tag lands in the belly of a predator. This leaves little doubt about the fate of the animal being studied.

However, the tags on the sharks in this study did not record light levels. “So we had to use all the other data to solve this mystery,” adds Dr Brooke Anderson.

The pregnant female was a “great value for money meal”

The researchers immediately analyzed the data from the tag as soon as it surfaced. It was excreted by the mysterious predator about four days after the attack.

The data revealed that the hunt took place at a depth of 300 meters in the open sea. This is not necessarily the best place to find abundant prey. It would therefore be an “opportunistic” predation, according to the researchers.

This means that the predator did not hesitate to attack the pregnant female, even though she was quite large. The porbeagle shark represented “excellent value for money in terms of a meal”, according to Dr Brooke Anderson.

Nor should we rule out the possibility that these hunts are more common than we think. We simply do not have the means to monitor them, at least not yet.

The Great White Shark and the Mako Shark are the Prime Suspects

So how do you know what animal ate the porbeagle shark? The constant temperature of 22°C indicated that the culprit was a warm-blooded shark. Then, you just have to eliminate potential predators based on the size of the female and the area where she was eaten.

So the researchers identified two potential culprits: the great white shark (Carcharodon carcharias) and the shortfin mako shark (Isurus oxyrinchus). They are leaning more toward the great white shark, which quickly dove into the ocean and then remained at a constant depth.

The mako shark is known for its rapid dives, but it tends to swim at shallower depths than the great white shark. As Dr. Brooke Anderson concludes, “in an instant, the population lost not only a breeding female, but all of her developing pups.”

Source: IFLScience