Whalesharks of Mafia Island

Whalesharks of Mafia Island

 

The average human (data culled from various signs in lifts) weighs 70 kilos and (empirical evidence) is less than 2m tall. Also, we’re terrestrial. So it’s quite something to be floating in 25m of warm blue water being approached by an animal which, as an adult, weighs an average of 9 tonnes (that’s 9 000 kilos in case there was any doubt) and is close to 10m in length. Plus, it’s leading with its cavernous 1.5m wide mouth. And the animal which owns the mouth is actively feeding. At 2m away and closing, there’s a certain sense of drama.

That’s a really big animal getting really close. And that’s presumably why they’re not called hamstersharks.

Luckily for me, the whaleshark, for that’s what it was, ignored my feeble attempts to evade it, effortlessly flicked its enormous tail and submerged. And circled back and repeated the feeding lunge, this time not quite so close to me. The plankton must have moved. Whalesharks, of course, are not interested in feeding on humans, though an unintended whack from a tail could certainly do some damage; worth considering when in the water and trying to photograph one.

This was offshore of Mafia Island, off the Tanzanian coast. We’d gone there specifically for just this experience. Here, from September to March, congregations of whalesharks move inshore to feed on the plankton blooms which are generated by river outflow.

As many as fifty whalesharks can be seen gaping their rectangular mouths open, sucking vast gulps of plankton into their maws and flaring their gills. It’s got to take a lot of tiny shrimps to keep 9 tonnes on the go. Whalesharks, indeed, will also eat small schooling fishes and sometimes small tuna or squid. This must make life exciting for the juvenile golden kingfishes we frequently saw swimming just in front of the whalesharks, mouths opening in time to the whaleshark’s.

They’re presumably there because the whaleshark is better at finding plankton blooms than they are. Or maybe they like living dangerously.

The whalesharks of Mafia are mostly sub-adults, so they’re usually only around 8m in total length (i.e. not that big, for a whaleshark), and mysteriously, are predominantly male. Getting to see them and snorkel with them as they feed is awe-inspiring. Waking in the morning, hastily eating breakfast and splashing through the jellyfish-dotted shallows to the dinghy is just the beginning. Then there’s the outward journey searching for the leviathans, in a hand-carved dhow, powered by a 9.9 horsepower engine, driven with dashing flair by the boat captain’s foot. It’s a bit like being back in the days when whalers stood in the bows of their tiny ships and shouted out, ‘Thar she blows!’, except here the sign is the enormous dorsal fin of the whaleshark and its smaller tail fin which breaks the surface in the distance.

Or it might be the surface wave that trails their great heads as they rise to the surface to feed. Or their enormous muscular backs glinting in the tropical light. The captain’s foot does its work and the snorkellers bail overboard searching for signs of the beast. A lot of the time it’s empty ocean but then suddenly and with a weird inevitability, there’s a spotty reef in the water, as though it’s always been there. Sometimes they don’t come near, but often the feeding urge is upon them and it’s close encounter with huge animal time.

Whalesharks have beautiful blue dorsal skin crisscrossed with a chequerboard of pale stripes and spots. They may look like this because they have evolved from bottom-dwelling sharks, though no-one is really sure. They have the aforementioned huge mouths which contain 300-350 tiny teeth: not used for feeding and probably also an evolutionary legacy. To feed they open their mouths, suck in a mouthful of prey and water, then close their mouths and open their gills. The slight delay between closing their mouths and opening their gills results in any particles bigger than 2 or 3mm being trapped either against the filter pads in their mouths or pushed directly back to their throats. This form of suction feeding is so efficient that only water is emitted from their gills. They are sometimes seen ‘coughing’, which is probably to rid their gills of any unwanted food buildup.

They are open ocean animals, known from the tropics and subtropics, which makes it interesting that the first whaleshark seen by Western scientists was found in Cape Town’s Table Bay back in 1829. It had probably come inshore following an unusual meander of an Agulhas gyre. They like warm surface water, but tagging data has shown that they dive to 700m and spend time down there in 10 degree water. No-one is really sure what they’re doing there.

And when it comes to mysteries, it’s astonishing just how many secrets swirl around those huge slow-moving animals. No-one has ever seen them mating, and it was only in 1995 that a pregnant female was caught with 300 embryos inside her. Today it is known that whalesharks are ovoviviparous, which is to say, the females carry eggs within their two uteri and the embryos grow inside the eggs inside the mother, finally being birthed at about 60cm long. It is thought that females carry many embryos at varying stages of development, probably mate only a few times and may store sperm, fertilising their eggs at different times.

But back to being in the water with whalesharks. They’re huge, a fact which can hardly be overstated, and though they look like they’re moving slowly, they far outpace even the fastest human swimmer. Best to aim ahead of their apparent feeding direction and hope for the open mouth shot.

Finally, around lunch-time, the plankton swarm has dissipated (or been swallowed by the whalesharks) and the snorkellers return to the dhow. If the wind is right, it’s a creaking smooth sail homewards to Mafia to eat, process images and scheme for the next days’ possible encounters.

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