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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.
Many thousands of miles further north, in Cornwall, it's three hours earlier and a wee bit chillier, but on an early June morning it's already daylight as a fishing boat slices through the glass-calm ocean, leaving Plymouth's breakwater behind and heading out towards the Eddystone Lighthouse.
On opposite sides of the planet, two small, dedicated teams of scientists are starting their daily search for the ocean's two greatest fish - the whale shark and the basking shark. Both are using the latest technology to unravel the secrets of these ocean giants. But intriguingly, although these huge fish that divers love to see have much in common, the scientists are discovering massive differences in the behaviour patterns of these creatures - facts that will be essential in securing their long-term survival.
Both sharks survive on diets of tiny plankton, filtering their food from hundreds of litres of sea water every hour. And both seem to appear regularly each year in certain locations. Peak season for whale sharks around Seychelles is between August and November, and the basking shark appears off Cornwall and elsewhere in the UK in early summer.
Initially, tracking them was a matter of luck - could researchers recognise individual fish by unusual markings or fin shapes with enough certainty to establish that these were the same sharks returning every year?
Then various types of tags were tried, attached to the shark by speargun or harpoon, all with limited degrees of success. But these were still mere aids to visual recognition of the shark, and helped only if the shark happened to be seen again.
All they could tell science was that these were indeed the same sharks that had been spotted and tagged in previous years and that they were following a regular pattern of returning each year to the same waters at the same times. The lives of the sharks when out of sight remained a mystery.
Then technology stepped in to help. The new generation of satellite tags can record data for as long as they remain attached to the shark.
And when, after a pre-set period, they are released from it - an electrical discharge melting the copper wire holding them on - they float to the surface and transmit their data to satellites passing overhead, and so to the scientists who can piece together the fish's movements for the whole period.
Researchers can learn where it has been, how deep it has dived, and a host of other factors that build into the jigsaw puzzle of the shark's life. So without ever seeing that shark again, a vast amount of information can be collated.
But to attach the tag, first find your shark. In the Seychelles,
the microlight can cover huge areas in its search patterns, and when the sharks are located the pilot talks in the boat with David Rowat from the Seychelles Underwater Centre, joint
co-ordinator of the project, and his tagging crew on board.
Slipping overboard, a snorkeller swims alongside the shark and, using a speargun, attaches the tag to the shark with a dart that must penetrate skin up to 11cm thick. The team also records the size and sex of the fish, its behaviour, and any obvious markings.
Without the luxury of a microlight, Dr David Sims from Plymouth's Marine Biological Association has to rely on a web of contacts around the Cornish peninsula, and sightings of the huge fins of basking sharks slicing through the water.
So he is generally restricted to flat calm days - sometimes a rarity in a Cornish summer!
When he has managed to find his target sharks and sneak up on them, David attaches his tags by firing a harpoon from the bow of the boat.
The findings being uncovered about these huge sharks by the research projects carried out by the two Davids have excited scientists around the world, not only for the insights into their secret lives but for how different the lifestyles of these two apparently similar ocean giants are turning out to be.
Perhaps the most stunning discovery David Sims has made is that populations of basking shark are in fact relatively localised. The sharks we see in early summer off the Cornish coast are actually around our shores all through the year - they are truly British sharks.
These findings contrast starkly with the massive and apparently random journeys carried out by whale sharks when they eventually leave Seychelles waters.
Of the first three tagged:
Intriguingly, up to 25% of the tagged sharks may return to Seychelles the following year. But it is their lives as long-distance travellers that bring them into danger.
- The first headed west to Kenya, then north along the African coast to Somalia, where it circled for about three weeks.
- The second headed south-west, ending up on the east African coast near Zanzibar.
- The third, tagged only 30 minutes after the second and fewer than 500m away, headed north, surfacing three weeks later 1850 miles away off Sri Lanka, then, seven weeks later, surfacing again off Thailand.
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.
• Whale sharks swim by moving their body from side to side, not just the tail, as some other sharks do.
• They often eat while in a vertical position.
• Males tend to migrate long distances, while females migrate short distances and always return to their place of birth.
• The biggest individual caught was 18m long, weighed 34 tons and was reckoned to be 100 years old.
• Whale sharks have more than 4000 teeth, each only 3mm long.
• Basking sharks have been seen swimming in single file so that their dorsal fins and the upper lobes of their caudal fins penetrate the surface - giving the impression of one huge animal.
• Liver oil makes up about 30% of a basking shark's body weight. A single adult can yield up to 900 litres.
• Basking sharks filter about 2000cu m of water an hour while feeding.
• The Isle of Man was the first place in Britain to protect basking sharks.
• The average basking shark weighs 14,500kg.
GREAT WHITE SHARK
• Great white wharks can go three months without eating.
• As well as an awesome sense of smell, white sharks can also sense the electrical fields radiating from living creatures.
• By the number of reported incidents, dogs kill more people each year than great whites have killed in the past 100 years.
• Unlike most fish, white sharks' bodies are warmer than their surroundings.
• Great whites are the only sharks that can hold their heads up out of the water. This allows them to seek potential prey at the surface.