Thresher Shark

Ian dodges dynamite blasts to check out an unusual gathering of Pelagic Thresher sharks in waters off the island of Cebu in the Philippines. Normally ocean roamers, the sharks are found year round here circling the reef of an undersea mountain. Ian thinks they are using it as a cleaning station, and sets out to test his theory.

Produced by NHNZ

There is no mistaking the Pelgic Thresher shark with its incredibly long tail and ability to leap from the water.

Shark Gordon presenter Ian Gordon takes us to the waters of the Philippines to investigate a rare haven for Pelagic Thresher sharks, recently discovered 45 miles (72 km) from the island of Cebu. Thought to be a pelagic species of ocean roamers, the Threshers are found all year round at an undersea mountain there. It’s a riddle that doesn’t make sense and Ian intends to solve it.

Ian explains the threat to wildlife from the rubbish and plastics blown into the water by a typhoon; weather which delays his departure and destroys visibility.

Heading for the island Ian’s guide, marine consultant Charles Frew, warns about the dangers from dynamite blasts in the area. A small percentage of local fishermen use dynamite to bomb the reef up to 80 ft (24.6 metres) below so they can gather the dead fish from the surface.

Ian kits up with a heavy re-breather rig to lessen his air bubbles underwater so as not to spook this shy species further.

The noise and percussion of the blast he experiences is enough to knock you senseless even from miles away. Ian reveals the devastation to an area of reef already blasted by this destructive form of fishing.

But he finds a patch of untouched reef where Threshers circle each dawn before the light gets too bright. After noting identifying marks on each shark he discovers that after one shark has circled here for a while, another appears to take its place. It’s behaviour no-one else has documented.

Above the surface the Threshers are performing spectacular leaps out of the water and Ian believes this intriguing behaviour could provide some clue as to why they hang around the seamount.

One by one a series of 8 to 10ft (2.4 to 3 metre) Thresher sharks queue up for this circling behaviour and Ian thinks he’s discovered why.

Tiny cleaner wrasse are eating parasites off fish as they come past and Ian believes the area is used by large predators like Thresher sharks as a cleaning station.

A pre-dawn dive reinforces this theory. If the Threshers are here to be cleaned then they’re not likely to be circling during the hours of darkness while the cleaner wrasse are asleep and they’re not.

The Thresher’s sensitive big eyes are probably what keep them in the darkest depths, possibly 500ft (153.8 metres) or deeper. The necessary calculations are made for Ian to dive to more dangerous depths of over 100ft (30.7 metres), but he doesn’t see any Threshers there either.

The crew has recorded almost seventy Thresher jumps. Being cleaned has to be a little ticklish and Ian concludes that the jumping behaviour is probably to dislodge the last of the parasites.

But sadly the blasting continues and Ian fears it’s probably only a matter of time before this cleaning station receives a direct hit and is gone forever.

Although much of the local population relies on less destructive means of earning a living Ian hopes the dynamite fishermen never find this tiny oasis. Threshers are worth much more to the Philippine economy alive than dead.

Episodes From This Series

White Tips of Osprey

30 mins / 2001

A Whale of a Shark

30 mins / 2001

Mako—Friend or Foe?

30 mins / 2001

Great White—The Ultimate Predator

30 mins / 2001

Thresher Shark

30 mins / 2001

Hammerheads

30 mins / 2001

Galapagos Sharks

30 mins / 2001

Grey Nurse — A Visit to the Nurse

30 mins / 2001

Prickly Sharks

30 mins / 2001

Hawaiian Tigers

1 hour / 2001

Port Jackson – A Family Secret

30 mins / 2001

The Sailor’s Nightmare

1 hour / 2001

Shark Attack

30 mins / 2001

Great White Bite

2 Minutes / 2007

Great White – The Ultimate Predator

2 Minutes / 2007

Feeding Frenzy

3 Minutes / 2007

Bull Sharks

3 Minutes / 2007

A Whale of a Shark

2 Minutes / 2007

Mako

3 Minutes / 2007