Tofo's Underwater Giants
by Elizabeth Willoughby
Where giant mantas and whale sharks go
LUNGING AND LURCHING past the shallow reef just beyond Mozambique's Tofo beach, our boat is transporting us out to Manta Reef. We are six divers all anxious to witness the enormous, elegant manta rays that glide around underwater like eagles soar across the sky, and our chances are good at this popular "cleaning station" with one of the largest populations of mantas in all the oceans. The cleaning station attracts all sorts of creatures that require parasites and dead skin to be picked off. There are over seven species of cleaner fish there, including butterfly fish that specialize in the removal of dead tissue from wounds, preventing infection, sergeant major damsel fish that work on mouths, and cleaner wrasse that go to the back of the throat to pick gills clean.
We're hoping against reason for the sighting of a whale shark along the way, which would be another thrilling gift due to the sheer size of them. They have been uncharacteristically absent from this southeastern coast of Africa for weeks, causing growing anxiety for locally-stationed researchers. Tiger sharks, bull sharks, orcas and great whites are found here; hump backs and southern rites are regularly seen, but Tofo's researchers say this area in the Indian Ocean normally has the highest visitation of whale sharks in the world.
We sit in wetsuits on the soft sides of the zodiac, gripping the cord strung around it as the inflatable boat whomps the waves and flops our torsos forward and back, our feet slipped under floor straps to keep us from bouncing out. Our skipper stands shirtless at the bow controls, flip flops, light cotton breeches, big sunglasses and sun hat, maneuvering through the swells and on the lookout for the dark shadows cast by the huge sharks. Underwater has had poor visibility for days – a condition less than ideal for diving, but it's fuelling our hope for a sighting because plankton is what's causing the cloudiness, which is what whale sharks feed on, several kilograms per day.
Skipper cuts the engine, though we are nowhere near the dive site – he has spotted a large shadow. I scramble to get my mask, snorkel and fins on, plop off the boat backwards and roll, then lay in the water, spread eagle, bobbing with the waves. Many feet beneath the boat the shark emerges, gliding swiftly in my direction. The head, the deep blue body with horizontal white dotted lines, then a long tailfin swaying side to side, it's so large that I cannot view it all at once. I am stunned by its enormity and proximity. It passes gracefully below me as I hover at the surface, overwhelmed, and then it disappears from sight.
During our hour of meandering at Manta Reef, we see seagrass pipefish, yellow trumpet fish, anemone fish and cube boxfish, peacock groupers, parrot fish, variegated lizard fish and lion fish, moray eels, a school of five-lined snappers, a giant lobster and more. What we don't see at Manta Reef are mantas, and I can't help being disappointed except for the sweet glimpse of the whale shark en route. So, the whale sharks are back. But where have they been?
THE ANSWER is elusive. Despite their immensity, manta rays (manta alfredi), giant mantas (manta birostris) and whale sharks (rhincodon typus) are not easy to study. They are ocean wanderers appearing when feeding is plentiful, such as at Australia's Christmas Island when crabs come down from the land to release eggs and sperm into the water, in Belize when twice a year there is a large fish spawning event, and in the Maldives monsoon when the spring tide currents bring nutriment up from the depths.
Mozambique's coastline, on the other hand, is normally home to these animals year round due to a continual undercurrent that brings nutrients up towards the surface. This makes it a goldmine for marine megafauna researchers. We know that whale sharks are sharks (not whales), are pelagic and found in large groups, which makes them vulnerable to fishery markets, and despite their size they eat the smallest things in the ocean, such as plankton, krill, small fish species and fish eggs. From strandings of whale sharks stuck in bays when the tide goes out, we know that they have a skeleton composed of cartilage rather than bone, and that the age can be determined by examining the vertebrae, much like a tree's age can be determined by counting the rings in a cross section of the trunk. Such specimens have provided evidence that a 20-foot whale shark is about 22 years old and a 23-foot shark is about 27 years old. Researchers estimate that maturity is probably reached at around 30 feet, 30-33 years old. The oldest recorded whale shark is thought to be 100 years old.
Whale shark populations are dwindling at an alarming rate, exacerbated by the fact that they are slow to mature. Although the record for the longest whale shark is 65 feet, today the individuals being caught by fisheries are only around 16 feet long, whereas just 20 years ago they averaged 40-50 feet.
Manta rays may share a similar diet with whale sharks, but not biology. There are differences even between mantas and other rays, the most obvious being size and shape. A manta has a diamond shaped body with massive pectoral fins that propel it through water with agility and speed, but it can also sustain long periods of swimming during migrations, travelling over 125 miles per week. Lateral eye placement (instead of on top) allows for greater range of vision in an open sea environment. Cephalic fins on either side of the mouth appear to allow for more efficient feeding by angling the fins in different ways to funnel water into the mouth whether swimming flat out in a water column, barrel rolling in an area thick with zooplankton, or vacuuming along the sea floor. When not eating, mantas can roll the fins up to be more hydrodynamic.
Also unlike other rays, mantas are curious, can communicate with sound, and spend their time in the water column rather than on the sea floor. Giant mantas, with wing spans up to 25 feet (6 feet wider than regular reef mantas), are more difficult to study since they only stay in Tofo for a few days per year, suggesting that they are migratory, contrary to the resident reef mantas.
Nevertheless, this coastal area has proved an exceptional manta observation post at inshore cleaning stations and feeding along the coast during daylight hours. Identified as a reef manta breeding site in 2003, Tofo has also provided much in the way of mating behavioral study. When a female wants to mate, she leads a string of males on a rollicking high speed chase that appears highly choreographed with bumper-to-bumper banks, barrels and rolls. Once the winner is decided, he holds the female's pectoral fins with a Velcro-like band of 4,000 short (less than .1 inches) teeth to help maintain his grip on her while they mate. After a 12-month pregnancy, she disappears for a couple of weeks to give birth to usually one pup, and then most often takes one to two years off before the next high speed chase.
IMPRESSIVELY LARGE, megafaunae still face threats, human and otherwise. While dolphin-safe tuna measures are meant to ensure that dolphins are not brought up in tuna catches, whale sharks do not have any such protection, and although whale shark by-catches are often released overboard, in gravity the soft cartilaginous skeleton cannot hold its own weight; it crushes the organs, causing death even after they are returned to the water.
Sharks with teeth are another threat. Three quarters of Tofo's mantas have significant injuries from shark attacks. Whale sharks' skin, six inches thick on their backs, is their heaviest armor, but they do display the occasional shark bite and youngster whale sharks have been found in the stomachs of marlin, blue sharks and great whites.
Whale sharks' and mantas' worst predators, however, are human. They suffer boat propeller strikes. They get caught in direct fishing and in nets where fishermen will cut up the animal rather than cut the net to free it. Chinese syndicates pay big bucks for shark fins for the Asian shark fin soup market (in Thailand there has been a 97% decline in whale shark populations), and mantas are highly prized in traditional Chinese medicine. A dead manta can yield $250-$500 for the skin, cartilage and gill rakers used in spurious Chinese medicinal products that claim to treat everything from diabetes to colds to libido.
THE IUCN (International Union for Conservation of Nature and Natural Resources) lists manta rays, giant mantas and whale sharks as vulnerable to extinction. Mozambique is the only aggregation site in the world where there is no legal protection for whale sharks as a species or for their habitat. Recording the populations and studying their habits and biological and ecological data is key in determining the conservation requirements of these threatened species in order to help develop policy that is absent in southern Africa. Over the long term, the data gathered here should enable the design of a protected area in the region for manta rays and whale sharks, and a plan for effective management both locally and internationally.
To this end, Dr Simon Pearce and Dr Andrea Marshall, Tofo's resident researchers of whale sharks and manta rays respectively, set up office in a hut-like room off a beach lodge, give weekly presentations of their efforts to tourists in a make-shift room off the bar, and do the underwater photography and tagging. Their photo identification system has catalogued hundreds of individual whale sharks, mantas and giant mantas so far. This is possible because each animal's spot pattern is distinct. These spot patterns are compared with previously sighted individuals using a program that was originally developed by NASA to compare star constellations. Dr Pearce has incorporated laser technology into his picture data gathering to determine whale shark length, and therefore approximate age, finding no individuals around Tofo that are less than 13 feet (i.e. no juveniles) or greater than 30 feet (i.e. no adults).
Pictures also help compare the numbers of mature male and female mantas. Although only about half of the females in the area are mature, the vast majority of the male population is, even though Tofo's female mantas outnumber the males three to one. Dr Marshall suggests that the high female to male ratio may be due to females staying year round in order to mate and give birth. Satellite tags, underwater transmitter tags and tissue samples from identified individuals provide valuable tracking information as well.
There is another kind of data, though, that policy makers might hear louder: how megafauna is linked to the tourism industry. By surveying tourists in Belize who visit specifically for a chance to see a whale shark, it is estimated that one whale shark brings about $34,000 into the country per year, over a million in an average whale shark's lifetime. This prompted Drs Marshall and Pearce to conduct such surveys in Tofo, and plan to use the economy as part of the argument to gain protection for the marine megafauna off Mozambique's shores.
But for all the study and discovery at Tofo, data is slow to accumulate and the more that is discovered, the more questions seem to arise. Where do mantas go at night and where do they give birth? Where do giant mantas come from and go to? Why are there only teenage whale sharks and where are all the females?
While standing on Tofo beach feeling lucky to have encountered the fleeting whale shark, I watch two men carry a cumbersome catch ashore from their little boat and start hacking it up on the beach with machetes. Finally, I've seen a manta ray. When they've finished, all that's left is the tail.
© Elizabeth Willoughby 2012
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