The Enigmatic Thresher Shark and Why It Needs Protection
Beneath the turquoise waters of Indonesia's coral reefs lurks a marine phantom—the pelagic thresher shark (Alopias pelagicus).
With its extraordinarily long, scythe-like tail that can be as long as its body, this species is among the ocean's most evolutionarily distinct and graceful predators. Yet despite its captivating appearance, the pelagic thresher shark is silently vanishing from our oceans, victim to both targeted fishing and accidental bycatch.
The pelagic thresher shark was reclassified as Endangered on the IUCN Red List in 2019, with populations estimated to have declined by 50-79% over the past three generations.
In central Indonesia, where these sharks regularly gather, scientists are racing against time to uncover their hidden lives using cutting-edge tracking technology. What they're discovering is transforming our understanding of shark conservation and revealing surprising behaviors that challenge conventional wisdom.
Meet the Pelagic Thresher: Evolution's Masterpiece in Trouble
The smallest of the three thresher species with a remarkable hunting adaptation.
Size & Characteristics
Typically reaches about 3 meters in length with the upper lobe of its caudal fin making up nearly half its total length.
Hunting Technique
Uses its tail like a whip to stun and herd small fish, making it one of the ocean's most specialized predators.
Adaptations
Large eyes suggest adaptation to low-light environments, hinting at its deep-diving capabilities.
Why Indonesia Matters: Global Epicenter for Shark Diversity and Fishing Pressure
Biodiversity Hotspot
As the heart of the Coral Triangle—the global epicenter of marine biodiversity—Indonesian waters support an incredible array of marine life, including more than 200 shark and ray species.
The Sunda Banda seascape in eastern Indonesia has been recently identified as a critical hotspot and migration corridor for pelagic thresher sharks 7 .
Fishing Pressure
Indonesia's rich marine resources have made it a fishing hotspot as well. Shark fishing has a long tradition in Indonesian coastal communities.
Today, pelagic threshers are caught both by nearshore small-scale fisheries and as bycatch in offshore large-scale tuna and swordfish operations using longlines, gill nets, and purse seines 4 .
Critical Situation in Alor
The situation is particularly dire in the Alor archipelago of East Nusa Tenggara, where a traditional fishery has targeted thresher sharks for over 50 years 4 .
100,000+
tons of sharks landed over the past two decades
Decoding Shark Movements: The Technology of Satellite and Acoustic Telemetry
Satellite Tags
Archive or transmit data about an animal's location and depth to researchers via satellite networks. These tags provide broad-scale movement data over ocean-basin scales but have limited battery life and are more expensive.
Acoustic Tags
Emit coded sound signals that are detected by underwater receivers moored at specific locations. These provide fine-scale data about an animal's presence in a particular area but only when the animal comes within detection range of a receiver.
Pioneering Study: Tracking Thresher Sharks in Indonesia
Research Team
In 2024, an international team of researchers from the USA, New Zealand, and Indonesia conducted groundbreaking research on pelagic thresher sharks in the Selat Pantar Marine Protected Area (MPA) in East Nusa Tenggara, Indonesia 1 4 .
Local Collaboration
The research team collaborated with local shark fishers whose traditional knowledge proved invaluable for locating and safely capturing the sharks.
Tagging Process
Once captured, the sharks were carefully brought to the research vessel and immobilized upside down to induce tonic immobility—a natural tranquil state that occurs in sharks when they're inverted.
| Tag Type | Number Deployed | Successful Deployments | Attachment Period |
|---|---|---|---|
| Satellite tags | 14 | 12 | 81-181 days |
| Acoustic tags | 15 | 7 | 44-175 days |
| Both tags | 5 | - | - |
Revealing the Secrets: Surprising Discoveries About Thresher Shark Behavior
Horizontal Movements
The study revealed two distinct patterns of horizontal movement:
- Strong residency within the Selat Pantar MPA for acoustically tagged sharks
- Broad movements (approximately 90 km from tagging locations) for satellite-tagged sharks
These regions are known for unregulated and unreported longline and gillnet fisheries, putting the sharks at significant risk when they venture beyond protected waters 1 .
Vertical Movements: Deep-Diving Champions
Perhaps most astonishing were the vertical movement patterns. The sharks displayed clear diel vertical migration, spending significantly more time in deeper water during the day and moving to shallower waters at night 4 .
| Time of Day | Depth Preference | Behavioral Pattern |
|---|---|---|
| Daytime | Deeper waters | Maximum recorded depth: 1,889.5 m |
| Nighttime | Shallower waters | Presumed feeding in shallow waters |
| Diel pattern | Significant (p < 0.001) | Consistent across tracked individuals |
From Data to Action: How Science Is Driving Conservation Policy
The research findings had immediate practical applications for conservation. The team communicated their results to relevant governmental bodies, resulting in increased political will and new legislation to protect pelagic thresher sharks within the Selat Pantar MPA and the broader waters of East Nusa Tenggara 1 4 .
In 2022, the East Nusa Tenggara province enacted a Governor's decree prohibiting capture and trade of the species within the province 7 .
| Conservation Measure | Implementation Level | Key Features |
|---|---|---|
| Selat Pantar MPA | Local | Protects critical coastal habitats |
| East Nusa Tenggara Governor's Decree | Provincial | Prohibits capture and trade of the species |
| IOTC Resolution 12/09 | International | Prohibits retention of pelagic threshers in tuna fisheries |
| Community-based alternatives | Local | Provides alternative livelihoods for fishers |
The Scientist's Toolkit
- Archival satellite tags
- Acoustic transmitters and receivers
- Traditional fishing gear collaboration
- Tonic immobility techniques
- Data analysis software
Future Directions: The Path Forward for Thresher Shark Conservation
While the telemetry study provided groundbreaking insights into thresher shark ecology, much remains unknown about these enigmatic predators. Future research priorities include:
Genetic studies
Fine-scale habitat use
Fisheries interactions
Climate vulnerability
"Area-based protection could be an effective nearshore conservation tool as individuals appear to remain in relatively restricted areas within Selat Pantar MPA for extended periods. However, fisheries regulation aimed at restricting illegal longline and gillnet fishing is also necessary to protect sharks when they move offshore" 4 .