The enormous grey whale shark glides effortlessly in the murky waters off Donsol in the eastern Philippines, its distinctive pale yellow spotted back and fins clearly visible as excited tourists prepare to enter the water from nearby outrigger canoes. They swim to within a few metres (feet) of these gentle giants of the deep as their guide makes sure they give the whale shark plenty of room to move.The world's largest fish, some as big as a school bus and weighing up to 30 tonnes, are not aggressive."But it's advisable not to get too close in case they decide to turn over," Angela Quiros, a marine biologist and one of the country's leading experts on the whale shark, told AFP recently.Whale sharks have been a common site in the waters off this coastal town for as long as anyone here can remember, feasting on the rich plankton between January and June.Once hunted for its soft white meat, known throughout Southeast Asia as "tofu shark," the whale shark is now protected and has transformed this sleepy corner of the Philippines 600 kilometres (372 miles) south east of Manila into a major eco-tourism centre."Since 1998 when the government passed a law protecting the whale shark, known locally as 'butanding,' tourism and revenue numbers have shot up dramatically," said local tourism coordinator Salvador Adrao.Last year, almost 11,000 tourists visited Donsol, up from around 900 in 1998. Revenues from eco-tourism have risen to an estimated 12 million pesos (261,723 dollars), from 454,875 pesos (9,920 dollars) in the same period."Swimming with whale sharks has transformed Donsol from a sleepy fishing village into an eco-tourism centre," Adrao said."The only problem we have is the infrastructure. We can't cope with the tourism numbers we now have, let alone any increase."Compared to some of the more developed areas for whale shark watching such as Ningaloo Reef off Western Australia, Donsol is still very cheap."That's why the tourists, especially the foreigners, come to Donsol," he said.Ten years ago environmentalists fought a bitter campaign to end the slaughter of the whale shark, forcing the Philippine government to pass a law protecting the creature."I can't believe how this place has changed," said Korina Escudero, an underwater film maker and one of the original campaigners for the protection of the whale shark."The attitudes of the local people towards conservation rather than killing has been quite extraordinary. They can see the value in protecting these magnificent creatures. People come from all over the world just to see and swim with them."The publicity we gave to the senseless slaughter just hit a chord and people reacted. Villagers even managed to free two whale sharks that had been tied by their tails to coconut trees as they floundered in shallow water off a local beach."You can still see them today off Donsol. They are easy to identify as they still have the rope around their tails. We call them Big Lucky and Little Lucky."One local fisherman said: "There was a time when the butanding was considered a pest. They would plough through our nets driving away smaller fish. "Tourism has changed all that. Now we see the value in protecting them. Not only Filipinos but people from around the world come to swim with these giant fish." Quiros and a small team of marine biologists and volunteers have spent more than a week photographing, measuring and taking tissue samples from the whale sharks for genetic analysis. "It will help us build a better picture of the whale shark," said Deni Ramirez, a marine biologist from Mexico who specialises in whale shark genetics. The programme is the first of its kind in the Philippines and is being partly financed by Mexico-based cement multi-national CEMEX Philippines Foundation and Conservation International. "We hope that through the genetic analysis we will get a better understanding about their migratory patterns, age and breeding habits," Ramirez said. "No one knows how long these fish live for. Some estimates have said up to 180 years. But we just don't know." Quiros said a female can carry up to 300 eggs, each measuring up to 18 centimetres (seven inches). When hatched, the young are 40 to 60 centimetres long. "They hatch inside the mother but very few survive to maturity," said Quiros. "You very rarely see juvenile whale sharks. I haven't seen any here in Donsol but the local fishermen have so we know that the young are still in the area. "Once the juvenile has established itself the mother will leave it and move on." Despite its name and size, the whale shark is not a whale but classified in a family of its own called Rhincodontidae. Its closest relatives are the leopard and nurse sharks. Romeo Trono, executive director for Conservation International in the Philippines said eco-tourism has helped save the whale shark from extinction but it hasn't stopped the killing. "It still goes on in some parts of the country but not to the extent it did a few decades ago. "It takes time to educate poor people that there are alternatives to killing a species off. "Donsol is an example of what can be done if a unique species is protected."