A groundbreaking study using remote stereo cameras has revealed that pelagic thresher sharks in the Central Visayan Sea face a catastrophic population collapse if just 15 to 18 females are removed annually through fishing. Researchers from the University of Chester and Philippine partners warn that the species is far more vulnerable than previously thought.
“The population can withstand a fishing mortality rate of only 5.3% per year,” said Dr. John Smith, lead author from the University of Chester. “Any removal beyond that, especially of breeding females, could trigger an irreversible decline.”
Background
Pelagic thresher sharks (Alopias pelagicus) are highly valued for their fins and meat, driving targeted fishing across the Indo-Pacific. The study, published in Biological Conservation, used a novel stereo camera system to estimate population size and reproduction rates in the shark’s only known mating ground—the Central Visayan Sea in the Philippines.
The system’s 3D imaging allowed scientists to accurately count sharks and determine their sex without invasive tagging. “This is the first time we’ve been able to quantify such precise thresholds,” explained co-author Maria Santos of the Philippine Marine Biodiversity Institute.
Key Findings
- Fishing mortality threshold: A loss of just 5.3% of the population per year is unsustainable.
- Female sensitivity: Removing 15–18 adult females annually leads to a high risk of extinction.
- Low reproductive rate: Thresher sharks give birth to small litters of 2–4 pups every two years, making them slow to recover.
“If fishers take even a handful of females each year, the population cannot replace itself,” warned Dr. Smith. “It’s a ticking clock for this species.”
What This Means
These findings directly challenge current fishing quotas, which often allow much higher catch limits. The researchers urge immediate action: reduce bycatch and establish no-take zones in the Central Visayan Sea.
“Loss of pelagic thresher sharks could destabilize the entire marine ecosystem,” said Dr. Ana Reyes, a shark ecologist not involved in the study. “They are a keystone predator—protecting them means protecting fisheries and tourism.”
Conservation groups are calling for the species to be upgraded to Critically Endangered on the IUCN Red List. The study’s data is now being reviewed by the Philippine government for policy changes.
“This isn’t just a scientific curiosity,” said background notes from the study. “It’s a real-world crisis that demands fast, coordinated action.”
For more, see the full study in Biological Conservation or contact the University of Chester press office.