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The Thresher Sharks of Malapascua

The Thresher Sharks of Malapascua

 
     
Nov 2014

Words Theodora Sutcliffe, Pics Mark Pacey & Theodora Sutcliffe

As our outrigger dive boat bounces over light chop, the rising sun tints the sky blush pink and a cry goes up from the crew. Silhouetted against the fluffy clouds and dawn sky, a dark shape leaps from the Visayan Sea and arches into the air, pectoral fins drooping like a spaniel’s ears, its long tail limp behind it. Down it splashes. And another! We’re in luck. Monad Shoal, off the coast of Malapascua Island, in Cebu, Philippines, is the only dive site in the world where you’re virtually guaranteed of a chance to see thresher sharks, particularly if you visit around dawn. But it’s rare for a diver to see them breaching. 

Sleek, smooth creatures, from the same family as the Great White and the basking shark, thresher sharks are, pretty much all tail. The elegant, whip-like caudal fin can make up as much as 50% of their length, and powers them through the water with incredible grace. Malapascua’s thresher sharks are pelagic threshers, the smallest of the three known thresher shark species – although they can grow to as much as 3.65m in length, both common and bigeye threshers are bigger. But what are these creatures, that had lived in the ocean many millions of years before humans appeared on land, doing in the air?

Zac and I drop gently down the line that leads us to the seamount, Monad Shoal, a hotspot of diversity that rises from the coastal seabed below, invisible in the pristine waters that surround us. Even in the open ocean, seamounts like this are home to incredibly sophisticated ecosystems – and a magnet to larger, ocean-going creatures. 

We are barely 14m below the surface, still metres above the reef, when a thresher emerges from the blue, appearing, almost ghostly, like a Polaroid snap slowly developing out of photographic paper. 

There’s none of that rush of primal adrenaline, an ancient whimper from the reptilian brain that sharks sometimes produce. Her tiny mouth, her invisible teeth, her blunted snout and her outsize eyes deny her sleek power and size.  She looks... friendly, almost gormless, like a kindly, cartoon shark. 

We hang on the line and watch. She comes a little closer, as if to inspect us, then, finding nothing of interest, she turns, flicks that powerful tail like a rhythmic gymnast’s ribbon, and disappears, cruising effortlessly on her powerful pectoral fins. All of a sudden she doesn’t seem gormless. Just beautiful. A beautiful, strong, curious animal in an environment that’s all hers, a place we’re privileged to visit for the short time our tanks allow. 

And, as always when a shark disappears, the eternal question rises. Where did she go? What will she do? What is it LIKE down there, in the blue, those dark depths where the pressure racks up at tens of kilos per square centimetre? The mountain whose gently sloping summit forms Monad Shoal rises “only” 250m from the coastal sea floor. But it’s likely she spends much of her time far deeper than that, heading into shore and rising up to this ugly rubbled reef for cleaning.

“Thresher sharks are physiologically adapted for depth,” says Simon Oliver, of the Thresher Shark Research and Conservation Project. “Their sense organs, their ability to regulate their body temperature and cope with salinity – all this suggests that they go well below the thermocline, to 500, maybe 700 metres.”And how did she find this place? Nobody knows. Perhaps, as hammerheads seem to, she uses the earth’s geomagnetic field to navigate the deep and trackless ocean, sensing changes with a system we don’t yet understand.

Like most oceanic sharks, pelagic thresher sharks are more unknown than known; our sole encounters with them are in waters shallow enough to dive. It’s part of their magic: they emerge from the blue like spirits and evanesce again, untraceable. 

How far does she travel? Nobody knows. Simon Oliver says, “We suspect their life strategies take them long distances. They’re oceanic sharks, which by definition means far moving. But some reports suggest they’re tied to quite specific habitats and places. It’s an open question.”

It was, in fact, a Dutch-Filipina couple, Dik and Cora de Boer, who brought pelagic threshers to the diving world’s attention, when they set up Exotic, the first dive resort on Malapascua, in 1998. This was despite the fact that Cora, like many Cebuanas, couldn’t swim until well into adulthood – she learned to swim at the same time that she learned to dive.  

At the time, this white sand, palm-fringed atoll was purely local – a very traditional island fishing community. “We bought the land and started to look for the nice dive spots,” says Dik. “We’d heard from the fishermen that they were catching sharks around that shoal. Luckily, the fins were too small to be of much value in the shark fin soup trade.” 

There wasn’t much by way of maps to go by. “In the Philippines, even today, they’re still finding entire islands that aren’t on the maps,” says Dik. “The nautical maps were no good at all.” 

With local boatmen entirely unused to divers and their needs, exploring was a challenge. “One time, when we were looking for the Doña Marilyn wreck, the boatmen were playing cards, or something, and they lost their anchor and drifted, without noticing. When we came up, the boat was a speck on the horizon – the size of a cigarette,” Dik recalls. “It’s terrifying. Because no one was going to come and get us, out there, no one was going to look for us.”

Undaunted, they dived different spots at different times of day, dropping into pristine, uncharted waters to see what there was for others to enjoy. And, finally, with help from a friend, Mikael Persson, who was happy to spend months diving daily in exchange for desert island bliss, they found the various cleaning stations pelagic threshers visit, most often in the period around sunrise. Other discoveries? Mandarin fish mating and a myriad seahorses hanging out at Lighthouse around sunset (I saw eight seahorses in a single dive, which is not unusual); Gato Island is home to wonderful macro life, a dramatic tunnel and a cave where reef sharks sleep; Pantau Pantau and North Point are stunning muck dives, with frogfishes, nudibranches and far better vis than you’d expect; hammerheads visit Kemod Shoal in season; while Kalanggaman is the ultimate desert island dream.

This is not to say that the waters around Malapascua were entirely uncharted. Wreck divers and underwater explorers the world over know to talk to fishermen first – wrecks, hidden reefs and sea mounts make fertile fishing grounds. In the Visayas, many local fishermen practice compressor diving, sending untrained divers, often children, down to depths that can exceed recreational diving norms, equipped only with swimming goggles and a plastic tube down which a compressor pumps “air”. The tube is wrapped around the diver’s waist, and he keeps one end in his mouth – fins aren’t used, wetsuits are uncommon, and of course there is no buddy but the guy manning the compressor up top. “They adapt the compressors from the brake pumps of a truck,” says Cora. “They use Strawberry Fanta or candies to take away the taste of diesel fumes.” 

Though, obviously, Fanta, whatever flavour, can’t take away the carbon monoxide risk – or the danger that the jerry-rigged apparatus will just stop working, leaving a diver who could have spent hours deep underwater with no other option but to ascend as fast as possible. Some compressor divers work herding tuna into nets. Others work salvaging metal from wrecks or harvesting shellfish. A few work as miners. It is, to put it mildly, a career with a short life expectancy. By way of safety precautions? One former compressor diver, who retired aged 12 after a neighbour died, and is now an eagle-eyed dive guide for Exotic, told me: “You have to pee before you come up, and you have to come up slowly. If there is a problem, you tug the line three times, and they will bring you up. If you get sick, you need to go back down, wait three hours and try again.”Compressor divers directed Dik and Cora to the site of a deep, World War II Japanese wreck, and also the Doña Marilyn. An inter-island ferry, she went down in 1988, killing almost 400 people – only parts of the wreck can be explored, out of respect to the folk whose remains are likely still on board.

On Monad Shoal, we descend and swim close to the cleaning station, an unappealing table coral, where we join a line of other divers, flat on our stomachs behind a rope, for all the world as if we’re shooting - or watching - deer. Diving on Monad Shoal is carefully regulated, as is fishing: locals recognise the value of dive tourism, and line fishing for sharks or dynamite fishing of smaller fish is banned. That said, in the first few years, divers killed great patches of coral, and disturbed the threshers by arriving in midwater. Today there is a timetable for the resort dive boats, while divers are restricted to very specific, already damaged patches, at a distance that does not bother the sharks. 

What’s brilliant about this experience? There is no feeding. No interruption of the animals’ behaviour. The aim is entirely to observe, to let the threshers do what comes naturally with minimal interference. We don’t need to wait long. A thresher shark emerges from the blue, nudging over the edge of the drop-off, and begins to slowly circle, tail low. I see a wrasse latched on. The shark disappears into the distance, and then reemerges from our right. Like great whites, pelagic threshers need to swim to breathe. They have no spiracles through which to “inhale” water: they must swim fast enough for the oxygen-bearing water to flow over their gills through their open mouths. So they can’t stop to let the cleaners latch on.

Yet, since cleaner-predator interactions are fraught with danger -- it’s far from unknown for predatory species to dine on their cleaners – sharks also need to cue their cleaners in. Thresher sharks drop their tails and swim slowly in circles as a sign that they’re ready to be cleaned. The wrasses need to latch on, hitch a ride, fill their guts and bail before the creature heads away from the reef and into hostile depths. The thresher disappears, its grooming finished, and we change our angle. Another thresher! And then a third! 

They’re beautiful. It’s hard to believe the power and elegance of their tails. 

Like all sharks, threshers use their caudal fins to swim – the tail is the motor that drives them through the water, with the pectoral fins providing mainly lift, a little like the wings of a plane. Thresher shark tails, however, are not just for locomotion, but a lethal hunting weapon. With their small mouths and tiny teeth, pelagic threshers favour smaller fish, like sardines, and easy prey, like squid. Like some killer whales, they use their tails to hunt small schooling fish. 

And it’s an incredible phenomenon. Simon Oliver documented pelagic threshers hunting sardines off Pescador Island, near Moalboal, also off Cebu – though sadly both the sardines and their hunters have moved elsewhere for now. 

Video footage shows the sharks whirling their tails up almost over their heads in an explosion of power too fast for a camera to capture fully. Big, strong pelagic threshers can accelerate their tails at rates of up to 350 metres a SECOND – as fast as the speed of sound in air – then seamlessly turn 180 degrees to collect their helpless victims from the water. Some of the slaps seem to be violent enough to precipitate gas bubbles from the ocean. It’s a sensible hunting strategy. Killer whales can stun or kill 30-odd herring with a single tail slap; while threshers may not knock out that many sardines, it’s an easy way to separate individual fishes from their defensive school, leaving the dead or stunned ones floating in the water column – easy meat. And the process is much less labour-intensive for a big shark that likes small fish than chasing after sardines one by one. How does the tail knock out the sardines? No one’s quite sure. It’s probably a combination of turbulence, pressure changes, impact, maybe even sound.

But what are the threshers doing at the seamount? Pretty much every part of a shark’s anatomy, from the liver to the denticles to the gills, can be host to an itch-inducing welter of parasites: flatworms, tapeworms, leeches, lampreys and copepods, the shrimp-like family to which sea lice belong, are just a few of the species that live off different sharks.

Parasites in sharks are a serious business. They are known to cause anaemia, respiratory difficulties, reproductive problems and intense, chronic skin diseases. Gill parasites, in particular, can be so debilitating that some sharks have been seen rubbing sand into their gills – it’s hard to imagine that this doesn’t involve something similar to our experience of pain. And cleaner fish, it appears, feed on parasites, among other things. Simon Oliver studied where the cleaner fish choose to feed on thresher sharks’ bodies, and found a disproportionate fondness for the pelvis. Each and every one of the threshers he inspected in the Philippines’ fish markets had large flat worms around their cloacal opening, the slit through which their intestinal tracts empty, and through which female sharks receive the male’s claspers and produce their pups. So is Monad Shoal an STD clinic for sharks? Oliver is cautious. “We don’t know anything about their reproductive habits, their parturition, their juveniles, or how they interact with each other,” he says. “We know they’re oophagous, that their young grow in the uterus by eating eggs supplied by the mother. Publications say that they have one male, one female pup, one for each uterus. But that seems unusual.”

And the ones who breached? That too, most likely, was to treat its parasites, this time in the gills. Ant-sized copepods live in and feed off thresher sharks’ delicate gill filaments, causing sores, and swelling, blanching and –naturally – respiratory problems.

So the breaching sharks we saw were, paradoxically, leaping from their native element, the ocean, into the inhospitable air – in an attempt to help them breathe. The violent splash with which they broke the surface was, most likely, to dislodge the parasites from their gills. Parasites may be one of the grosser parts of the marine ecosystem. But, from a diver’s perspective at least, they’re a bonus. Without them, after all, we’d barely ever see a thresher shark – and they’re truly worth the seeing.

THE FACTS:

Theodora and Zac stayed and dived with Exotic and their excellent dive guides – www.malapascua.net. Four night, five day, ten dive packages for two people sharing a room and using their own gear start from 31,300 Philippine pesos for two, or around 3700 ZAR per head. The nearest airport is Cebu: buses to the port of Maya, where the public boat to Malapascua runs until around 4pm, depart from the North Bus Terminal. The Thresher Shark Research and Conservation Project is at www.threshersharkproject.org.


Source: The Divesite

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