The pregnant porbeagle shark, subject of the study, after her release after tagging (Credit: Jon Dodd)
TEMPE, Ariz. — In the vast, mysterious depths of the Atlantic Ocean, a gruesome crime has been committed. The victim: a pregnant porbeagle shark, once thought to be near the top of the ocean’s food chain. The culprit: unknown, but a new study suggests that this shark’s killer may have been a member of its own species!
This scientific thriller, recently published in the journal Frontiers in Marine Science, marks a groundbreaking discovery in the world of shark research. For the first time ever, scientists have documented evidence of another ocean predator killing an adult porbeagle shark, a finding that sends ripples through our understanding of marine ecosystems and raises alarm bells for the conservation of this vulnerable species.
“This is the first documented predation event of a porbeagle shark anywhere in the world,” says lead author Dr. Brooke Anderson, a former graduate student at Arizona State University, in a media release. “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. 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.”
The porbeagle shark, a cold-water predator known for its endurance and agility, has long been considered one of the ocean’s formidable hunters. Growing up to 3.7 meters long (12 feet) and weighing up to 230kg (507lbs), these sharks are no small fry. As this groundbreaking study reveals, however, even apex predators can fall prey to larger, more voracious hunters.
The story begins off the coast of Cape Cod, Massachusetts, where researchers tagged a pregnant female porbeagle shark with two types of satellite tags: a fin-mounted transmitter and a pop-off satellite archival tag (PSAT). These high-tech devices were the key to unraveling the mystery that was about to unfold.
For five months, the tagged shark behaved typically, diving between 100-200 meters at night and 600-800 meters during the day. On March 24, 2021, something extraordinary happened. The tag’s data showed a sudden and dramatic change in temperature patterns, indicating that it had likely been swallowed by another, much larger predator.
“The predation of one of our pregnant porbeagles was an unexpected discovery,” Anderson explains. “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.”
The identity of the porbeagle’s predator remains a mystery, but the researchers have narrowed it down to two prime suspects: the infamous white shark, known for its massive size and powerful bite, and the sleek and speedy shortfin mako shark. Both species are known to inhabit the waters near Bermuda, where the killing of this porbeagle shark likely occurred.
What makes this discovery particularly intriguing is that it occurred in the mesopelagic zone, often referred to as the ocean’s “twilight zone.” This vast, dimly lit region between 200 and 1000 meters deep is home to a diverse array of marine life and plays a crucial role in the ocean’s food web and carbon cycle. The fact that such large predators are actively hunting in this zone highlights the importance of these deep-water habitats and the need for their conservation.
The implications of this study extend far beyond a single predation event. It raises important questions about the natural mortality rates of porbeagle sharks, which are currently listed as endangered in the Northwest Atlantic and critically endangered in the Northeast Atlantic and Mediterranean on the IUCN Red List of Threatened Species. If predation on adult sharks is more common than previously thought, it could have significant impacts on population dynamics and conservation efforts.
“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 the ecosystem,” Anderson concludes.
Paper Summary
Methodology
To track the porbeagle shark’s movements and gather data, researchers used two types of satellite tags:
- A fin-mounted satellite transmitter: This tag, attached to the shark’s dorsal fin, sent location data to satellites when the shark was near the surface.
- A pop-off satellite archival tag (PSAT): This tag records depth and temperature data every 20 seconds for a year. It was programmed to detach from the shark, float to the surface, and transmit its data via satellite.
Key Results
The tagged porbeagle shark traveled at least 1,120 km from its tagging location off Cape Cod to waters near Bermuda. For about five months, it exhibited normal diving behavior. Then, on March 24, 2021, the tag data showed a sudden increase in temperature (around 22°C) even at depths of 150-600 meters, indicating it had likely been eaten by a larger predator.
Study Limitations
The study only tracked one shark, so it’s unclear how common this predation event might be. The exact location and timing of the predation event are uncertain due to gaps in data transmission. The identity of the predator couldn’t be conclusively determined, though a white shark is suspected.
Discussion & Takeaways
This is the first documented evidence of predation on an adult porbeagle shark. The predation likely occurred in mesopelagic waters near Bermuda, highlighting the importance of this area for large marine predators.
The study suggests that natural predation may be an underestimated source of mortality for porbeagle sharks. This research emphasizes the need for further study of deep-sea ecosystems and predator-prey interactions among large sharks. The findings could have significant implications for porbeagle shark conservation efforts and population estimates, especially given their endangered status and slow reproductive cycle.
Funding & Disclosures
The study was partially funded by the American Elasmobranch Society Donald R. Nelson Award and the Arizona State University Graduate and Professional Student Association Graduate Research and Support Program. The Animal Telemetry Network provided satellite time for the tags. The authors declared no conflicts of interest.
