Don’t think that sharks are the top predators; they can also be prey.
On a chilly early morning off the coast of Cape Cod, Massachusetts, marine biologist Brooke Anderson and her team stood on the deck of a research vessel, preparing for a challenging mission.
Today, they would be shark fishing.
As the boat quietly approached an area known to be home to the porbeagle shark, one of the large shark species inhabiting the Atlantic Ocean, Anderson focused intently. She scanned the horizon with binoculars until she spotted an animal several hundred meters away.
The vessel then “held its breath” as it neared the creature, while Anderson and her team dropped nets and scattered bait. Nearly an hour later, they successfully pulled the animal from the water. The moment she saw the over 2-meter-long shark lying still on the deck, Anderson knew she had hit the jackpot.
She realized it was a pregnant porbeagle shark.
Illustrative image.
The marine biology research team at the University of Arizona, led by Anderson, had never found a pregnant female shark before. This would be their first opportunity to study one.
“We really want to understand which habitats pregnant sharks choose and the precise locations where they may give birth. This information will help identify areas that could be used to protect this threatened shark population,” Anderson explained.
To do this, she quickly retrieved a small drill from her toolkit and made a small hole through the shark’s dorsal fin. Then, Anderson attached a satellite tag (PSAT) to its fin, which would collect data on temperature, depth, and location, storing it on a memory card.
The PSAT satellite tag can be programmed to automatically detach after a certain period. Since the gestation period for porbeagle sharks can last up to 9 months, Anderson set it for that duration.
She anticipated that after the tag detached and floated to the surface, it would transmit data to a satellite and automatically download to Anderson’s computer. This information would allow her to track the porbeagle shark’s movements over the past year, including its activities and where it chose to give birth.
These satellite tags (PSAT)…
…were attached to the shark’s fin.
However, things did not go as planned. 158 days after the porbeagle shark was tagged and released back into the Atlantic, Anderson suddenly discovered that the PSAT tag had surfaced earlier than expected by several months.
Notably, it was drifting somewhere in the Bermuda Triangle, over 1,000 km from Cape Cod. After decoding the data sent back from the satellite, “everything led me to a single conclusion,” Anderson stated:
“The shark was eaten.”
The porbeagle is a large and robust shark species. They inhabit the Atlantic Ocean, South Pacific, and the Mediterranean Sea. A porbeagle shark can grow up to a maximum size of 3.7 meters and weigh 230 kg. They are related to the more famous great white shark (Carcharodon carcharias) and the shortfin mako shark (Isurus oxyrinchus).
Porbeagle sharks have a very long lifespan. Some individuals have been recorded living up to 65 years. Like some other large shark species, porbeagle sharks give birth to live young rather than laying eggs, even though they are a type of fish.
The female shark will shed its eggs inside the womb, where the fertilized eggs will hatch. The shark pups will continue to grow in the womb before being “born,” which is actually a release process. “This allows them to be larger at birth. This gives them a bit of an advantage in terms of survival and higher chances of survival,” Anderson explained.
The IUCN lists porbeagle sharks in the Northwest Atlantic as a vulnerable species, and populations in the Northeast Atlantic and Mediterranean are considered critically endangered.
Female porbeagle sharks typically do not reproduce until they are about 13 years old. They usually give birth to an average of 4 pups every 1 to 2 years. The gestation period for porbeagle sharks lasts from 8 to 9 months.
Because porbeagle sharks have a relatively slow reproduction rate, the death of a female shark can significantly impact the growth of the entire population. Currently, porbeagle sharks are being overfished, and habitat loss and degradation are also affecting them.
The Red List of the International Union for Conservation of Nature (IUCN) currently lists porbeagle sharks in the Northwest Atlantic as vulnerable, with populations in the Northeast Atlantic and Mediterranean considered critically endangered.
If another animal is hunting and eating them, this could also put this shark species at risk of extinction, especially if it’s a pregnant female.
A Mysterious “Murder” in the Depths of the Sea
In a study recently published in the journal Frontiers in Marine Science, Anderson and her colleagues played detective to detail the arguments suggesting that the porbeagle shark her team was tracking had been eaten.
After analyzing data from the PSAT satellite tag that had surfaced, Anderson noted that during the first 5 months, the shark swam steadily at depths of 1-200 meters at night and dove to about 6-800 meters during the day.
The average water temperature surrounding it ranged from 6-23 degrees Celsius.
Throughout those 5 months, the satellite tag transmitted data back to Anderson’s computer only once, indicating that the pregnant shark surfaced only once. The shark had been swimming continuously beneath the surface in the waters off Cape Cod, Massachusetts.
Depth of the shark (left column) and water temperature surrounding it (color of the right column). While the depth varied continuously, the temperature around the shark remained stable at about 22 degrees Celsius for the last 4 days.
However, starting from day 154, scientists noticed that the average temperature of the water surrounding the PSAT tag remained at 22 degrees Celsius for 4 consecutive days. Despite this thermal stability, the depth of the shark continuously varied, ranging from 150-600 meters.
After those 4 days, the PSAT tag suddenly surfaced and transmitted a signal continuously. It surfaced in the Bermuda Triangle, over 1,000 km away from Cape Cod.
“That was clearly an unusual event,” Anderson stated. She searched for explanations for the depth variations in contrast to the thermal stability of the water. But no reason was as convincing as the possibility that the pregnant porbeagle shark had been eaten.
“When a tagged animal is eaten, you often see patterns in the data indicating that a predation event has occurred,” Anderson said. “You can see temperature spikes if the predator is a warm-blooded species, as the tag records the predator’s stomach temperature instead of the water temperature.”
Indeed, the 22 degrees Celsius reading was the temperature in the stomach of the creature that had eaten the over 2-meter-long pregnant porbeagle shark. The predator completely digested the shark and its pup. During this process, it swam from Cape Cod to Bermuda, then excreted the satellite tag attached to the unfortunate shark’s fin.
Position where the shark was released (square), where it surfaced once (triangle), and where the PSAT tag on its fin permanently surfaced.
“Some types of satellite tags have the capability to record and send light level data to researchers, which is one of the easiest ways to identify a predation event, as light intensity would drop to zero inside the stomach of an animal,” Anderson said.
“However, the tag we deployed on the pregnant porbeagle shark did not send us light level data, so we had to use all the remaining data to solve this mystery.”
The Portrait of the Culprit Unveiled
“I believe at least part of the shark was eaten because we also attached a satellite tag on the fin of the shark,” Anderson said. “That tag was designed to send location data in real-time. We had hoped that if the mother and pup were still alive, they would eventually return to the surface, but we never received a signal from that tag again.”
Anderson noted that this study documents the first recorded case of a porbeagle shark being eaten by another creature. But which creature has the capability to kill and consume this massive shark?
“I think this could be an opportunistic event,” Anderson said. “Remember that the predation occurred at a depth of about 300 meters in the ocean, where you don’t always find a rich prey like this.”
“In this case, a large pregnant porbeagle shark would be a great meal. So, if you think you can take it down and eat it, then the effort is worth it.”
Suspect #1: Orca.
Suspect #2: Shortfin Mako Shark.
Suspect #3: Great White Shark.
After screening other giant animals in the ocean that could hunt and consume porbeagle sharks, Anderson identified three suspects:
First is the orca, a creature known for its ability to hunt and eat both whales and sharks. Second is the shortfin mako shark, which is large enough to prey on an adult porbeagle shark. And third is the great white shark, a creature that sits at the top of the ocean’s food chain.
The first suspect, the orca, was ruled out, as this species is not commonly recorded in the waters off Bermuda. Shortfin mako sharks were also excluded, as they tend to feed more on squid, sea turtles, and seabirds.
Whenever they finish a meal, shortfin mako sharks typically perform rapid dives, oscillating between the ocean surface and greater depths during the day. Meanwhile, data from PSAT satellite tags did not record these dives.
Ultimately, Anderson believes that the great white shark is the most likely suspect. This creature is warm-blooded, meaning it has a stable stomach temperature. They often dive to stable depths and can consume various large marine animals from dolphins and whales to other shark species.
“The predation of our pregnant porbeagle shark is a surprising discovery. We often think of sharks as top predators,” Anderson said. “But with technological advancements, we are beginning to explore interactions among top predators that may be more complex than we previously thought.”
Sharks are top predators, but they also eat each other.
This discovery marks a surprising and noteworthy predation event for the porbeagle shark, while also raising concerns about extinction since this species has a relatively slow reproductive rate.
“The most concerning aspect of this discovery is that the porbeagle shark being preyed upon was pregnant,” Anderson said. “In a moment, the population not only loses a reproductive female but also all the developing young in her womb.”
This represents a significant loss for the threatened porbeagle shark population in the Atlantic.
