Where to dive with thresher sharks

What they are

The pelagic thresher (Alopias pelagicus) averages about 300 cm (10 feet) and can reach 365 cm total length. Its long upper tail lobe makes it easy to recognise.^1,4

Researchers, including members of the Thresher Shark Research and Conservation Project in the Philippines, filmed threshers using that tail to hunt: the sharks lift their rear body and slap the tail into schools of small fish, creating shockwaves that stun or kill the prey.⁵

Where and when divers see them

The pelagic thresher lives in warm and temperate offshore waters of the Pacific and Indian oceans. It is found from the surface to at least 150 metres, and some sources give depths to 300 metres.^1,4

At Monad Shoal in the Philippines, a seamount whose top lies at 15 to 25 metres, reef fish pick parasites and loose skin off the sharks at cleaning stations around 30 to 35 metres deep. A study there recorded threshers being cleaned at all times of day, but less often from morning towards evening; a WWF, Manta Trust and Project AWARE factsheet says dives there mostly take place between dawn and 9 am, when sightings are most regular.^2,3

Conservation status

The pelagic thresher is listed as Endangered globally on the IUCN Red List, with a decreasing population, according to a document submitted to the 15th Conference of the Parties of the Convention on Migratory Species. The assessment year is not stated there, only that the global thresher assessments are more than seven years old. A 2011 study still described it as Vulnerable, an older listing.^3,4

How to behave around them

Threshers at cleaning stations are vulnerable to disturbance, and researchers at Monad Shoal suggested they may interact less with cleaner fish when many divers are present. A thresher code of conduct developed with dive operators and local authorities in Fuvahmulah, Maldives, sets out these rules for dives there.^3,6

• Stay at least 5 metres from the sharks.⁶
• No touching, chasing or harassing.⁶
• No flash, strobes or video lights.⁶
• Do not block the sharks' path or approach from above, and avoid sudden movements.⁶
• Stay close to the reef, follow your guide and keep off the cleaning stations.⁶