LAKE Malawi sparkles under the African sun as the boat, its outboard engine coughing and wheezing, chugs out toward the island. Heaving a sigh, a hippo rises from among the reeds, flicking its ears, and a lone fish eagle surges in a slow, graceful flight across the bay.
Approaching Lamareng Island, an outcrop of rock covered by shrub and guano, one can see the mountainous shoreline of Mozambique some 30 miles to the northeast. The craft anchors a few yards off the shore and the three divers, all wearing orange T-shirts with the words ``Tropical Fish -- Salima Fish Team,'' ready their equipment.
Jagwa, the team leader, starts the air compressor. ``We fish here,'' he explains. His two companions don goggles and fins. Then, armed with a finely meshed nylon net, they drop into the clear aqua waters, clutching their breathing hoses.
For the next two hours, the divers roam the rocky verges of the island, trailing the net behind them. The variety of darting fish is spectacular. It is hard to believe that one is swimming in fresh water and not off an ocean coral reef. Every so often, a diver rises to the surface to bring in a colorful catch: peacock blue or dark red Malawian cichlids, mouthbreeders -- destined for tropical fish enthusiasts in the United States and Europe.
Lake Malawi, also known as Lake Nyasa, stretches 350 miles from north to south, and is Africa's third-largest freshwater body after Lakes Victoria and Tanganyika. In places, it drops to more than 2,200 feet below sea level, where the water is dead and turbid for lack of oxygen. From a biological point of view, specialists rate this vast inland sea on Malawi's eastern border as the most unusual of the three.
According to scientists, Lake Malawi boasts the earth's greatest variety of freshwater fish, as many as 1,000 species, half of which have yet to be identified. Many, too, are found nowhere else and are even endemic to specific areas of the lake.
``Lake Malawi is an isolated basin, so they have evolved fairly rapidly on their own,'' observes American biologist Peter Reinthal of Duke University.
In 1982, Malawi became the first country in the world to establish a national park with the sole aim of preserving freshwater fish populations. The park was also recently designated a ``world heritage'' by UNESCO -- a distinction shared by, among other natural wonders, the Grand Canyon and Yellowstone National Park, and Tanzania's Serengeti.
Cichlids are the main variety of lake fish.
``Malawian fish are known for their color,'' says Stuart Grant, a British tropical fish exporter. ``Enthusiasts go for their visual appeal. If you've got a tank of really switched-on Malawian fish, it's a blaze, a riot of color.''
From his camp at Senga Bay near Salima, Mr. Grant runs a collecting operation with the help of 28 Malawian divers and assistants. Concentrating on some 120 species with exotic names such as Blue Orchio aulonacara, Red Cyrtochara boadzulu or ``marmalade cats'' (catfish), Grant ships his catches out once a week by air to London and beyond.
Almost every day, his men dive off various islands and shore areas, mainly in the southern areas of the lake where the water is shallower and richer in aquatic life than in the deepwater northern areas.
Grant maintains that his operations do not harm the environment. ``Our catching methods are notoriously inefficient. We're not using drugs, electricity, or other systems like cyanide in the Philippines. We change varieties and are careful to restrict our catches to certain species.''
The British exporter is now planning to breed his own fish in ponds regularly replenished by wild stock. For years, American enthusiasts have relied on Malawian fish bred in Florida or New Jersey. But domestic breeding has resulted in genetic dilution, he says, and there is now a trend toward more vigorous, wild-caught species to strengthen bloodlines.
Lake Malawi is not just of environmental or tourist interest. It is also essential to Malawi's economy. According to development sources, 75 percent of the country's protein intake comes from the lake. Some 30,000 tons of fish are caught every year. Much of the catch is smoked, dried, and sent inland for sale in local markets.
With more than 16,000 fishermen, Lake Malawi is the main lakeside industry. At each village, one sees smoke rising from the bamboo racks on which fish are cured, and later dried in the sun. Commercial trawlers account for 7,000 tons a year, but most of the catching is done from dugout canoes and plank boats.
The Malawian government is trying hard to develop the industry further. Robert Wilson, head of fisheries at Salima, explained that extension services selling supplies -- nets, hooks, twine, and even more efficient fishing craft -- have been established up and down the coast. ``Our main effort is to improve marketing and preserve the catch,'' he said.
Fishery experts believe that Lake Malawi's production capacity can be significantly increased, particularly that of its edible species. One suggestion by the UN Food and Agriculture Organization is to introduce kapenta. A type of freshwater ``sardine'' originally from Lake Tanganyika on Tanzania's western border, this fish has already been successfully added to the species in Zimbabwe's Lake Kariba.
Kapenta, it is maintained, could produce 100,000 tons of protein annually. It would be particularly suited to the deep-water regions of northern Lake Malawi, where it could feed on ``lake fly,'' which congregate in thick clouds often rising 3,000 feet over the lake.
Nevertheless, introducing alien species is a touchy issue among conservationists. ``It is always dangerous to introduce exotic species,'' Mr. Wilson says. ``But we should remember that maize, now the staple African diet, was also introduced.''