N I G E L M A R S H P H O T O G R A P H Y
U N D E R W A T E R I M A G E S A N D A R T I C L E S
ONE HELL OF A TAIL
By Nigel Marsh & Helen Rose
Pelagic thresher sharks are a deep water species, but they do visit shallow water at a very special place in the Philippines, Malapascua Island.
Little is really known about the behaviour and biology of most deep water sharks, including the pelagic thresher shark (Alopias pelagicus). The smallest of the three thresher shark species, the pelagic thresher is an oceanic wanderer found in tropical and subtropical waters of the Indo-Pacific region.
The pelagic thresher is a very distinctive species that reaches a length of 3.3m and has very large eyes (adapted for its deep water habitat), a small mouth, a short snout, large pectoral fins and an extremely elongated tail caudal fin, which makes up half the shark’s total length.
Occasionally seen by divers along deep drop-offs, most encounters with this species are rare and fleeting, except at Malapascua Island in the Philippines. Located off the top end of Cebu, Malapascua is a diver’s paradise surrounded by lovely reefs loaded with a diverse range of marine life. However, as wonderful as these dive sites are every diver that heads to Malapascua is there to see one thing – a pelagic thresher.
To see a pelagic thresher requires a bit of commitment, as you have to get out of bed at 4.30am to be on the dive boat by 5am for the trip to a nearby sea mount called Monad Shoal. The reason for the very early start is because the thresher sharks visit the top of this sea mount at dawn to get cleaned.
All the dive operators at Malapascua visit Monad Shoal at dawn each day, and while they do stagger the visiting times it can be crowded with a dozen dive boats at times. Fortunately we booked with Thresher Shark Divers that have recently found another cleaning station, so we were pleased to avoid the crowds.
Descending into dark water, with the sun still hidden behind the hills of nearby Leyte, we settled on top of Monad Shoal at 14m. This sea mount is surrounded by waters 250m deep, dark waters where the thresher sharks spend their day. We didn’t know how long we would have to wait or if we would even see a pelagic thresher at all, so were thrilled to see a large shape rising from the deep after only a minute. As it got closer we could tell it was a shark, and then we saw the very distinctive tail, it was a thresher and boy was it impressive!
The 3m long shark then paraded before us, quickly joined by two others, swimming up and down the sloping bottom. We expected to see the sharks getting cleaned, but they appeared to be more interested in the divers, coming in very close to see the bubble-blowing-aliens. Photographing the thresher sharks in the dark dawn waters proved to be a real challenge, as no lights or strobes are allowed.
On other mornings we saw the sharks being cleaned. A shark would swim slow circuits around the cleaning station to allow both cleaner wrasse (Labroides dimidiatus) and moon wrasse (Thalassoma lunare) to rise up and pick its skin free of parasites. It was like watching an exotic dance.
Each morning we dived Monad Shoal we saw between two and six pelagic threshers, and according to the dive operators there are more sharks seen now than ever before thanks to nightly patrols to keep fisherman away.
While little research has been done on pelagic threshers, most of what we do know comes from the sharks of Monad Shoal. The Thresher Shark Research and Conservation Project has spent the last eight years studying these sharks and have discovered some remarkable things.
Headed by Dr Simon Oliver, they discovered that the sharks visit the cleaning station at any time of the day, but dawn is the busiest time. They also did a detailed study on the cleaning behaviour, the first of its kind on any shark species, and also documented the impact of divers on the sharks and the reef. But the most amazing thing they discovered was how the sharks feed.
It had long been theorised that thresher sharks developed their long tails to assist in feeding, but it wasn’t until Dr Oliver and his team visited another site off Cebu that this was confirmed. Moalboal is located off the south west coast of Cebu and is a very popular dive destination; it is also regularly visited by immense schools of sardines. The research team were able to record incredible footage of pelagic thresher sharks feeding on these sardines. The sharks would speed around the sardines then suddenly stop and whip their tail over their head to stun the fish. They recorded this behaviour 25 times and calculated that the sharks lash their tails at 80kmh.
The research team also helped to get thresher sharks protected in parts of the Philippines in 2008, but unfortunately this species is listed as vulnerable worldwide and targeted in many places for their meat and especially their fins. With an estimated 80% decline in thresher shark numbers in the last decade more has to be done to ensure the future of this unique shark with a hell of a long tail!
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