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Fish With Whips

By Doug Perrine

 

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Pelagic Thresher Shark
Pelagic Thresher Shark subadult, Alopias pelagicus, Philippines, Pacific Ocean Image #: 003006

It’s festival time in Malapascua, and the number of people crammed onto this 2x4 km dollop of sand just north of Cebu, in the central Philippines, is amazing. The return of natives working off-island, along with extended family, friends, and bon-vivants, has swelled the normal population of 3,000 or so to roughly double that. It seems that nearly everyone is crowded into a single street coursing the length of the small town of Logon, where a stage and bandstand have been set up for the evening’s entertainment. Beer in hand, Paul Foley is giving me an informal tour. We stop to pay respects at the house of a local family he has befriended. A stunning young lady emerges, clad only in a towel. She is getting ready for the beauty contest, she says, and strikes a few poses, clearly for Foley’s benefit. The young British ex-pat is the most eligible bachelor on the island; in fact the only ex-pat in sight without a Filipina beauty glued to his side. Marrying a foreigner has long been the surest way out of the grinding poverty that envelops Malapascua and most other rural communities in the Philippines. Foley, however, has other ideas for the island’s salvation. He believes that destiny has tied the future prosperity and well-being of the islanders to the fate of an obscure and very odd-looking shark that lives offshore – one which only a handful of the islanders have ever seen, and of which the great majority of them are completely ignorant.

In fact, very few people anywhere have ever heard of the pelagic thresher shark. Even fewer have ever seen one. Until very recently there were no photographs or video of this creature alive in its natural habitat. This rarity is exactly what Foley believes will prompt shark enthusiasts to pay a premium to see a thresher alive in the wild. There are at least three species of thresher shark found in the world’s oceans: the common thresher, the bigeye thresher, and the pelagic thresher. A 1995 study, comparing tissue proteins, indicated that there may be an unrecognized fourth species, similar in appearance to the bigeye thresher, but this has not been confirmed. For all but one species of thresher, there is no place on earth where a diver can have a reasonable expectation of seeing one in the water. For the pelagic thresher, however, there is precisely one spot on the planet where such sightings occur regularly – on the edge of an underwater shoal just offshore of Malapascua.

All species of thresher shark exhibit the anatomical marvel that gives them their common name: a scythe-shaped tail that is as long as the entire rest of the body (or nearly so). This unique appendage is said by fishermen to be used like a bullwhip to herd small fish into a tight ball and then knock them unconscious. Supporting evidence comes from the fact that threshers caught on fishing lines sometimes have the hook embedded in the tail rather than the mouth. When swimming, the tail is held at an upward angle and ripples like a ribbon, propelling the shark forward by anguilliform undulations – like an eel or sea snake, but very unlike any other shark. This odd form of propulsion must nonetheless be quite efficient, because the pelagic thresher is one of a very few species of sharks that can, and do, leap entirely clear of the water. The reason for this breaching behavior cannot be known with certainty, but observers report that it occurs entirely independently of the presence of food fish. The additional observation that the shark tends to land on its side with a loud crack (that can be heard for quite some distance underwater) lends credence to the theory that it may serve to loosen or remove parasites. It could, however, also serve as a form of social communication, as is believed with whales and dolphins. The habit of occasionally going airborn is shared with two other members of the order Lamniformes (mackerel sharks): great white sharks and makos. The order also includes porbeagle sharks, salmon sharks, goblin sharks, crocodile sharks, basking sharks, megamouths, and sand tigers.

Apart from the opportunity to see a large (up to 3.5m total length) wild animal that few other humans have ever laid eyes on, and the slim possibility of witnessing a spectacular breach, shark watchers are attracted by the sheer beauty of these graceful creatures. The elongated caudal fin fluttering like a banner in the breeze, the enormous soulful eyes (though not so big as the outrageous saucers on the bigeye thresher, which actually fold over onto the top of the head), and the bronzy sheen produced by thousands of reflective dermal denticles ("skin teeth") embedded in the hide produce a mesmerizing countenance.

When threshers were first spotted on Monad Shoal several years ago, nobody knew what they were doing there, and they were only seen occasionally. "People used to tell me they were coming up onto the shoal to feed," says dive instructor Martin Kirkman, "but I never saw any fish." Pelagic threshers are considered to be, as the name implies, sharks of the open ocean, so their presence on top of a shoal that ranges from 12-24m depth was perplexing. Furthermore, the sharks were always in close association with the bottom whereas pelagic creatures, by definition, are normally found away from any substrate. For years the sharks were seen on rare occasions, but not frequently enough to make detailed observations of their behavior.

Then, during a chance dive at dawn, several of the "foxtail sharks" were sighted coming up onto the shoal from deep water. The dive operators learned that by putting their divers in the water just as the sun was coming over the horizon, they could virtually guarantee sightings. At first, the divers would swim towards the sharks to get a better look, and the sharks would vanish over the edge of the drop-off. Then Kirkman and the other instructors learned that by getting all their divers to drop to the bottom and hold still when a shark was sighted, it was possible to observe the sharks’ behavior for up to 20 minutes. That’s when he realized what the sharks were doing there.

"Once we quit chasing them," says Kirkman, "it was easy to see they were coming to get cleaned." Many reef-dwelling fish utilize special sites called "cleaning stations" where they rendezvous with smaller fish that remove their parasites in return for an easy meal. Only recently has it been learned that larger pelagic animals also regularly come to reefs from deeper water in order to seek out the services of parasite removal specialists such as the cleaner wrasse, Labroides dimidiatus. Kirkman points out that the depth at the edge of Monad Shoal is just about the maximum depth where this wrasse species is normally found in the region. The cleaner wrasse is active only by day, while thresher sharks are suspected to be primarily nocturnal, and appear to have very sensitive eyes. A dawn visit to the cleaning station may be a compromise between the shark’s desire for deep, dark water, and its need to be freed from its parasites. The leaps may be a way of loosening them up for easier removal by the wrasses and other cleaner fish. At least three species of smaller fish (two wrasses and a damselfish) have been observed attending threshers on Monad Shoal. It may be that each species specializes in removal of certain types of parasites or damaged tissue. The sharks typically swim about one meter off the bottom in a figure eight pattern. Without swim bladders to float them, sharks must swim constantly to keep from sinking, but this tight formation enables the threshers to stay close to the territories of their cleaners.

Will divers willingly pay a premium for a glimpse of the daily hygiene rituals of a medium sized predator that is exceedingly unlikely to appear in the slide shows of their friends’ and relatives’ vacations? Paul Foley believes that they will. He terms the situation a "unique selling proposition." Thresher sharks are not available elsewhere. Therefore shark watchers that come to Malapascua will gladly pay a bonus to dive where threshers are likely to be seen. Foley also has very definite ideas about how the Monad bonus should be collected and spent. He proposes a fee to be tacked onto the price of every dive undertaken at Monad Shoal. The fees will be collected by the dive operators and passed on to a community committee that will disburse funds for two purposes: community development; and the creation, administration and enforcement of a marine protected area on Monad Shoal.

The first category would include such projects as the funding of a clinic on Malapascua. Currently residents must undertake a half-hour boat ride to the larger island of Cebu in order to seek medical attention. It might also include funding an expansion of the local school so that island children would not have to go to Cebu to complete high school. Such projects would not only fulfill vital local needs, but also generate local support for the second part of the project, the creation of a reserve or sanctuary on the shoal.

Without local support, a sanctuary would have little to no chance of success. Up to 80% of the workforce on Malapascua is employed in fishing, although tourism and sport diving are growing rapidly. Fishing has been the mainstay of life on Malapascua ever since it was first settled, but the once-abundant catches have dwindled as a result of overharvesting and destructive practices, such as blast fishing. The large snappers and groupers once pulled from the reefs fringing the island are a memory of the past, and islanders now scour the flats for sea cucumbers, sea urchins, small crabs, or any other living thing that could even remotely be considered edible. Fishermen have receive some respite in their downward spiral of income, however, from the recent boom in prices of shark fins for export to China. The fins of a singe shark can net a sum equivalent to several months’ earnings. As a result, sharks are being harvested by any means possible, with no regard for their low rates of reproduction. In the case of threshers, make that an extremely low rate of reproduction.

Current studies by Dr. Jose Castro, working in Mexico’s Sea of Cortez, confirm the conclusions of earlier studies in Japan and Taiwan. Pelagic threshers produce a litter, roughly annually, of only one or two pups. Only one fertilized egg is released into each of the shark’s two oviducts. If both develop, then two pups will be born. Throughout the pregnancy, the mother continues to produce eggs, but these are not fertilized. They are released in capsules of about ten eggs each, which serve as food for the developing embryos. Unlike their relatives, sand tiger sharks, embryonic threshers do not eat their siblings within the womb, but eat only eggs produced specifically for this purpose. Embryonic teeth appear to be specialized for the purpose of opening egg capsules. These teeth are lost as the embryo grows large enough to swallow eggs whole, then adult teeth develop just before birth.

Not a lot is known about what the adults eat with those relatively small adult teeth, but it is assumed that the diet consists mostly of small fish and squid. In the Sea of Cortez, Dr. Castro has found jumbo squid, a.k.a. Humboldt squid, up to half a meter long in the stomachs of some of the sharks. Jumbo squid are themselves aggressive and well armed (with parrot-like beaks and rings of sharp claws around the tentacle suckers) predators that have been known to attack humans (large specimens can exceed 3m in length). Castro’s studies, as well as all the previous studies, have been based on specimens purchased from fishermen. There has never been a scientific study of the natural behavior of any species of thresher shark. Until the discovery at Monad Shoal, the opportunity did not exist.

Pelagic threshers not only give live birth, like mammals, but may also be warm-bodied. Studies of the circulatory system suggest that they are able to maintain at least their eyes and brains well above the ambient temperature of the water (unlike most fish), and possibly their entire bodies. Like large mammals, they are slow growing, long-lived, and slow to mature. They are estimated to reach sexual maturity at an age of 6-10 years, and live for 20-30 years. These characteristics place them closer to mammals than to most fish in terms of their population dynamics. The authors of the Taiwan study (Liu, Chen, Liao, and Joung) conclude that "… a single female produces only about 40 embryos per generation if it gives birth once every year. This evidence suggests that this species is extremely vulnerable to overexploitation and in need of close monitoring."

Information on catches of pelagic threshers is hard to come by. The species appears to be widely distributed in tropical and subtropical regions, and is probably frequently taken as bycatch on longlines set for tuna, swordfish, and other species, as well as in directed fisheries. However, few good records are kept, and it is easily confused with common and bigeye threshers. Pelagic threshers are known to be regularly taken in Taiwan, the Sea of Cortez, around the Hawaiian Islands, and off the Pacific coast of Guatemala. At one time, at least, many were taken in the Russian longline fishery in the northwestern Indian Ocean. Management of fisheries for pelagic thresher sharks is rare to nonexistent. Shark fishermen around Cebu catch threshers from time to time, but have not yet, apparently, developed techniques to target them.

Nothing is known of the migratory patterns of pelagic threshers, but repeated sightings of recognizable individuals at the Monad cleaning stations suggest they may be less nomadic than was previously believed. If the sharks utilizing these stations constitute a resident population, then both the sharks and, Foley believes, the children of Malapascua, may be in jeopardy. Fishing, he asserts, can no longer feed the ever-expanding families on the island. Already large numbers of local children are leaving the island after schooling for employment in Cebu, Manila, or even farther afield. The rapidly increasing numbers of "beach resorts," mostly catering to backpackers, provide some employment opportunities, but also add demands to infrastructure. Tourism taxes will not necessarily come back to the island to help deal with local crises, for example if increased pumping to supply the new resorts should cause the water table to drop below the reach of the wells that supply local families. In Foley’s view, the sharks are the only drawing card the island has, and he hopes to help the locals play it wisely.

Ironically, most of the divers currently flocking to see the sharks at Monad are not experienced shark watchers hoping to add a rare jewel to their life lists, but rather divers of moderate experience who, in many cases, have never seen a shark before. They are often not even aware of what kind of shark they will be seeing, or have any idea how rare are encounters with this particular species. In many parts of the tropical Indo-Pacific region, divers are regularly entertained with controlled feedings of blacktip reef sharks (the common lagoon species), gray reef sharks (the common shallow reef species), and silvertip sharks (the common deep reef species). However, in the Philippines, all three once-common species are now rare. Any attempt to set up a shark-feeding entertainment would immediately draw fishermen to the area, who would relieve the performers of their swimming appendages and put an abrupt end to the show. After appreciating the small tropical reef fishes and invertebrates on the over-fished reefs throughout the Philippines, traveling divers sometimes ask where they can see BIG fish. They are often told, "Go to Malapascua. They have sharks." Most of the divers arriving in Malapascua did not come to the Philippines in order to see thresher sharks.

Paul Foley expects that situation to change dramatically. Just as word of the new discovery has spread like wildfire through the Philippines in the first two years, the buzz is now going global, with reports appearing in dive magazines throughout the world. At least two television documentaries have been filmed on the threshers of Malapascua. Soon, Foley fears, a massive influx of shark enthusiasts could pose a more immediate threat to the long-term viability of the shark-watching business than do fisherman. The shy, light-sensitive sharks have only a brief period of less than two hours each morning after the wrasses wake up to get their parasites removed before they are compelled by increasing light intensity to descend to deeper water. If repeatedly driven from the cleaning stations by hyperactive divers during this period, they might abandon Monad Shoal, and, if no suitable alternative is available, begin to suffer the effects of increased parasite loads.

So far, the various dive operations taking visitors to view the sharks at Monad have done a commendable job of briefing their customers on "shark etiquette" and controlling their approaches so as not to frighten the sharks away before they can get serviced by the cleaner fish. However, increasing diver traffic may produce increasing competition for each dive outfit to get their customers to the sharks first or closest. The sharks may also be simply overwhelmed by the sheer numbers of divers surrounding them and filling the water with noisy exhaust bubbles. Foley’s proposal for a marine reserve also includes a provision to limit the number of dive operations with licenses to visit Monad, and the number of divers there at any one time.

Foley’s proposal has met with a cautiously receptive response from local fishermen and community leaders, and tentative approval from the local government. However, the wheels of action grind slowly in the municipal bureaucracy, and no official edicts have been proclaimed. Each day that passes without protection increases the danger that, as word of the sharks spreads from mouth to mouth, fishermen will descend on Monad and strip it of its sharks just as happened at the nearby resort of Cabilao. Cabilao was famous for its schooling hammerhead sharks until several years ago when a fishing boat anchored right on the dive site and fished until there were almost no sharks left at all. Even proclamation of a reserve may provide little protection unless there is adequate enforcement. The marine reserve on nearby Gato Island offers little encouragement. A sign next to a guardhouse threatens stiff fines for anyone violating the reserve, and an armed guard stands by, but dynamite fishers conduct their devastating trade on a near daily basis with apparent impunity.

Although rarely seen, pelagic thresher sharks are distributed widely through the world’s oceans, and are not in imminent danger of being wiped out. What is in dire danger of disappearing is mankind’s only known opportunity to observe and study these beautiful, elusive, and remarkably adapted creatures. With it may disappear a small island’s hope for a sustainable future. The small children collecting sea urchins from the shallow grass beds along Malapascua’s beaches have never seen a thresher shark, and have no idea that their future health, education, and livelihood may be intimately tied to this odd creature swimming just out of sight. With luck those children have not yet even had to think about what they will eat when the urchins have gone the way of the snappers and groupers that once inhabited the reefs surrounding the island.