One of the most biodiverse countries on earth is losing its marine life at record speed, but Alasdair Harris, a scuba-diving scientist, has an eight-pronged plan to stem the tide. Deborah Dunn goes in search of his eco-holistic octo-cure

For the past two decades, Condé Nast Traveler has been honoring environmental visionaries around the world who have found solutions to seemingly intractable problems. Every year, we receive dozens of nominations from leading nonprofits (Ashoka nominated this year's winner) and, with the help of our expert judges, select one winner for a $20,000 prize. Each of the three runners-up receives $2,000.

Standing on the beach in Andavadoaka, a remote fishing village on Madagascar's southwest coast, you'd be hard-pressed to find anything wrong. Candy-colored bungalows overlook a turquoise lagoon. Young British and American tourists just back from their morning scuba dive gush over their sightings (manta rays as big as cars, humphead wrasses the color of stained glass). The Vezo, the master fishermen of Madagascar, paddle along the shore in brightly painted pirogues.

But the idyllic setting masks a looming crisis. Rampant deforestation, climate change, and overfishing have led to catastrophic loss of sea life, jeopardizing not only the area's fragile ecosystem and vast tourism potential but the livelihood and food supply for tens of thousands of locals.

The most promising solution has come from an unlikely source: an amiable, highly caffeinated 30-year-old Scotsman. Alasdair Harris first arrived in Madagascar in 2001 on a university-funded expedition to study the Grand Recif, an extensive barrier reef lying outside Toliara, the biggest city in the southwest, and found the reef choked by four decades of sedimentation and pollution. "Madagascar supports some of the largest reefs in the Indian Ocean, but while no one's been watching, they've been slowly dying," Harris tells me at Toliara's Fisheries and Marine Sciences Institute, which overlooks the muddy bay. When the university funding ran out in 2003, Harris, who was by then earning a master's in environmental management from Oxford University, launched the conservation nonprofit Blue Ventures ("My mother tried to talk me out of the name because she thought it sounded like a skin flick," jokes Harris). At first the idea was simple: Enlist tourists to fund research and help survey Madagascar's little-known reefs. But over the past six years, Blue Ventures has morphed into the leading marine conservation nonprofit in the country, launching marine reserves, aquaculture businesses, education programs, and even family-planning clinics. The secret to all this success? The humble octopus.

An Indian Ocean island the size of Texas, Madagascar is home to such an extraordinary number of endemic plant and animal species—about 150,000—that it's referred to as a mini continent. But while naturalists have been cataloging every leaf and insect on the island for the better part of a century, its vast marine resources have been largely ignored. Decades of slash-and-burn farming and deforestation have left much of the country's singular habitat and wildlife critically endangered, and though far less reported, the soil running off into the sea is killing Madagascar's reefs and the sea life along with them.