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From December DIVER
Home | Marine Life Archive |
Appeared in DIVER March 2006
|Secret lives of the
Words by John Boyle, pictures by Chris Goodwin, Gavin Parsons and Tony Baskeyfield
DWARFED BY BOEING 737 PASSENGER JETS, a tiny microlight aircraft taxis and lifts off across the runway of Seychelles international airport. It's only 9am but the temperature is already in the high '80s. Banking sharply, the pilot cuts across the rocky southern tip of Mahe Island and starts flying a search pattern.
The very first European visitors to the Seychelles encountered a whale shark in what is now Victoria Harbour and harpooned it for food. Whale sharks are now protected in Seychelles waters, but this provides only temporary
sanctuary; in many other regions of the Indian Ocean they are the victims of targeted fisheries. Slow-moving and easily spotted in the clear surface waters, their fins and flesh are worth a lot to impoverished Asian fishermen. Many sharks never survive their annual migrations through these heavily fished waters.
But another shark perhaps holds the record for long-distance travelling. Researchers in South Africa are using similar satellite technology to discover the secret lives of perhaps the most fearsome shark in the ocean - the great white.
A female nicknamed Nicole was tagged off South Africa on
7 November, 2003. Soon afterwards, Nicole headed eastwards into the vast Indian Ocean, following an almost direct route towards Australia. Three months and an epic 6897 miles later, she was swimming just a mile offshore at Exmouth, Western Australia, where her tag detached and transmitted all her secrets to the waiting scientists.
During her voyage she had at times swum as deep as 980m,
but most of the time had swum on the surface - like the whale shark, laying herself open to fishing all along the route.
Why did she make the journey? It couldn't be for food, as the region in which she was tagged is one of the richest feeding grounds for shark on the planet. So scientists speculate that it could be for mating - which casts fascinating new light on the possibility that there is just one global group of great whites, rather than isolated breeding groups.
The information gleaned from this single tag could rewrite the books on what we had thought to be the life history of these incredible fish.
Even more astonishing is the fact that on 20 August, 2004, around nine months after first being tagged, Nicole was spotted again off the South African coast! The tag had detached off Australia but there could be no doubt - her distinctively notched dorsal fin was as unique as a fingerprint. Nicole had come home, a return journey of, at the very least, 12,500 miles.
That is not to say that basking sharks aren't also great travellers. They voyage many hundreds of miles along plankton trails to feed in the areas of the greatest plankton density.
They follow the plankton like dogs on a scent trail - or indeed as other sharks would follow a chum slick, or blood from
a wounded prey.
This can result in seasonal anomalies in sightings. 2005 was a sparse year for basking-shark sightings off Cornwall. They were seen throughout the summer, but not in the usual numbers, whereas the Western Isles of Scotland had a boom year for sightings, as the plankton concentrations there were ideal for the grazing sharks. Sometimes they circumnavigate the nation, but basking sharks rarely leave the continental shelf surrounding the British Isles. None of those tagged has carried out trans-oceanic odysseys comparable to those of the whale shark, or indeed of Nicole.
It's also the age and sex of the Seychelles sharks that have intrigued the researchers. Observation has shown that almost 90% of those spotted in the Seychelles are males and none much bigger than 7m in length, so these are young adults that have come inshore to feed, not to mate.
Larger adults and females - up to 14m long - remain far offshore during these annual aggregations. Whale shark are about a metre long when born and grow at around 1m every year for the first four years of their lives, after which the growth rate slows to around 25cm per year.
The biggest whale shark caught was 18m long and probably 100 years old. The males aggregating in Seychelles are probably mere teenagers. Does some primaeval pack instinct drive them back here every year?
David Sims has also discovered that to feed, the basking sharks carry out massive vertical migrations. In terms of the vertical distances covered, he describes them as the champion divers in the shark world, on occasions repeatedly plunging more than 1000m off the edge of the continental shelf to bisect the plankton layers when concentrations are sparse in the upper levels of the ocean.
Whale sharks, in contrast, tend to remain in the warmer surface waters. Tagged sharks spend 37% of their time shallower than 10m, and 98% of their time above 50m. Only rarely have whale sharks been recorded at depths to equal those that basking sharks reach, and then only for the very shortest periods of time.
It is to try to collate sufficient data to support international agreement on protection of these migratory species that the scientists continue their efforts. But time could be running out. Over the past three years, the microlight has logged more than 100 hours each year flying search patterns to locate the whale sharks of the Seychelles.
In 2002, the average number of sightings per flight was 6.16. In 2003 this fell to 3.79 and in 2004 was just 2.94. Similar reductions in numbers have been reported from Australia's whale shark hotspot Ningaloo.
We're gradually discovering the secrets of the planet's giant sharks; but the more we discover, the more we realize how vulnerable they are and how little time we have left to take realistic measures to protect them from extinction.