Environment : Lake Victoria’s Demise Is a Fish-Eat-Fish Story : Scientists say 300 species of indigenous fish have disappeared during the last 10 years. The villain in the story is man; his weapon: the Nile Perch.

It was from a point not far from this strip of grass and beach that the British explorer John Hanning Speke gazed over the blue waters of Lake Victoria in 1858 and made a judgment--seemingly audacious at the time, but ultimately proved correct--that it was the source of the Nile.

Ringed by graceful hills and dotted by lush islands that Africans had been cultivating and pasturing for centuries, Lake Victoria invited such boldness. No one who viewed its shores could doubt that the lake harbored wealth as majestic as the monarch after whom Speke named it.

Victoria would prove to be, after Lake Superior, the largest freshwater lake in the world. The neighboring land was the most productive in the region, sustaining some of Africa’s proudest tribes. The water nurtured a bounty of fish.

As if to testify to Victoria’s potential, on one recent day the grass running up from the beach at Kasenyi glimmered with silver. The sparkles were fish scales, stripped by an army of fishermen’s wives from the mounds of perch dotting the field under swarms of flies, awaiting transport to city markets. Yet the story the scales tell is really not about abundance. It is about a coming famine.

More and more, the talk from the fishermen who ply the inshore bays in shallow dhows and from the buyers who throng the piers is of scarcity. The most cherished fish, the tastiest and most valuable, have disappeared.

“Labeo, for instance,” said George Latigo, a fisheries official observing the catch at Kasenyi. “I haven’t seen one of those caught in five years. If one lands, everyone rushes to buy it, because it’s very delicious. That’s a species that is becoming extinct.”

It is one of many. Lake Victoria, under its glittering surface, is dying.

Scientists say that as many as 300 species of indigenous fish have disappeared from this lake in the last 10 years, an unparalleled environmental calamity.

The lake’s ecosystem has changed so dramatically that any environmental survey of its waters dated before 1975 is worthless. Plans calling for new fish-processing plants need to be rewritten, proposals for trawler fleets scrapped.

The villain of this story is man, of course, but his weapon was another fish. In 1962 fisheries officials of Uganda and Kenya, hoping to increase the lake’s output of food, stocked it with an exotic fish known as the Nile perch.

The voracious predator has unexpectedly ravaged the schools of indigenous fish. Now that most of the perch’s own food is gone, scientists say, another of its remarkable habits is becoming apparent: It is a cannibal.

The extinction of the native schools is more than a scientific issue here; it bears ominous implications for the health and livelihoods of thousands of lake-shore dwellers and millions of Africans dependent on fish for protein. Malnutrition has already begun to rise on the Kenyan side of the lake, where fish has long been a cheap staple protein.

The fate of Victoria underscores the difficulty of husbanding natural resources in the Third World.

Africa is not a region of environmental impact statements and involved public debates over conservation; this is a place where development opportunities are so scarce, the few that exist are often hastily exploited to exhaustion.

Lake Victoria, observes A. W. Kudhongania, director of the Uganda Freshwater Fisheries Research Organization, has long been a laboratory of ecological destruction.

The protection of the lake is complicated by a 2,100-mile shoreline that falls within three countries not always on the best of terms. In the 1950s, Kenya, Uganda and Tanzania (then Tanganyika) all tried to impose fishing restrictions to protect the resource; by 1962 each had conceded the impossibility of regulating an international lake and thrown their waters open to indiscriminate exploitation.

The perch is only the most destructive of the many changes men have wrought here. Textile and paper mills have gone up on shore, their effluent fouling the water. Kenya built a causeway to one of its islands from the mainland, closing a strait through which lake currents circulated.

Overfishing on Victoria was observed by Westerners as long ago as 1928, more than 30 years before the perch appeared, said Kudhongania. More recently, the unsophisticated fishermen have jeopardized their own futures by depositing on shore a lot that should be left in the water.

This is the result of beach seining, in which weighted nets are strung out from the shore and left overnight to gather fish. When they are dragged in at daybreak, their bottom edges scrape the lake clean, stripping it of fish eggs and the plants serving as their nursery.

As fish become more scarce, the fishermen set out nets with progressively smaller mesh, taking younger fish that should be left to reproduce.

“The disappearance of most of these species has been caused by the action of man, not the Nile perch,” Kudhongania said.

There have been periodic surges of fishing activity so intense that some portion of the lake is fairly vacuumed clean. In 1976, Tanzania erected a modern fish meal plant at the southernmost point of the lake and sent a fleet of four trawlers out into its Mwanza Bay for abundant sardine-like species called haplochromines.

“By 1979 there were no fish to process in Mwanza Bay,” said John Okedi, chairman of the zoology department at Uganda’s respected Makarere University, who spent the 1970s in Mwanza in exile from Idi Amin’s dictatorship. The trawlers were beached and the plant shuttered.

Yet few actions seem to have packed as much destructive wallop, and been implemented with such little advance study, as the introduction of perch.

“The Nile perch was introduced basically on sentiment,” said Okedi, who was a young marine researcher at the time. “No work had been done on its biology. We started to pay attention to those aspects after the damage was done.”

The program was controversial from the start. Twenty-five years ago the dominant fish were 300 species of haplochromines, all too small to be efficiently caught by the lake’s “artisanal”--that is, unmechanized--fishermen. It was as if all this living matter was going to waste by eluding the fishermen.

Recalls Okedi: “The argument was to introduce a big predator fish to convert these tiny bony fishes into big fillets,” as if they were recycling factories with fins.

Scientists warned that the impact of an alien fish on a delicately balanced ecosystem was unpredictable. But they lost the battle. One day in 1962 Ugandan officials dumped about 35 perch fry off the pier at Entebbe, about five miles south of Kasenyi. A year later, 335 more joined them. Soon Kenya followed suit.

For several years nothing happened. A lakewide survey in 1970 found that tilapia and bagrus (a catfish popular as food) were maintaining their traditional share of about a quarter of the catch. As both a portion of the catch and of the lake’s “biomass,” or its total living matter, Nile perch did not register as statistically significant.

In the next decade, however, the perch overwhelmed everything. By 1980 it was half of the catch, by 1985 as much as 95%.

Meanwhile the stockers’ arguments appeared to have been borne out, for total landings had boomed as well. Almost all of the tremendous increase in the catch from 1977 to 1985 could be traced to the growth in the perch fishery. It was a phenomenon that even today does not go unremarked around the lake.

“For these people this fish has been the biggest blessing, in terms of income,” said the Rev. Marinus Tielen, a Dutch missionary who has preached for nearly 30 years on the Kenyan lake shore from a gaily painted church built to resemble a fully rigged dhow.

Yet it did not take long for the Lake Victoria fishermen to come to regard the perch as a mixed blessing. They were the first to notice that bagrus and labeo were no longer filling their nets. The marketplace noticed next: The prices of those increasingly rare fish soared, as did that of the tilapia , which for many lifetimes had been the most popular food fish. Prices slumped for the odd-tasting but abundant perch.

Today perch fetch less than 50 cents a pound at the market; tilapia , so prized that many restaurants in Uganda and Kenya keep it listed on the menu while secretly serving perch, brings five times that much for each 2-pound specimen.

On the fish piers around the lake the dynamics of the food chain have not been much of a mystery.

“These big ones are eating all the other fish,” remarks fisherman Agustine Sari, indicating a yellow bucket of perch with the needle he is using to repair his net.

So far the fishermen’s incomes have stayed afloat only because of the huge abundance of perch. But that is bound to be short-lived.

“It was true in the short term that the Nile perch was a boon,” said David Wiley, director of the African Studies Center of Michigan State University and a student of the Lake Victoria fishing ground. “Over the last 10 to 15 years the fishermen reaped the harvest as the Nile perch ate up the bonanza of 13 million years of evolution. So there were these huge, wonderful fish to harvest.”

But in recent years the perch have been getting smaller.

“Now they’re catching any size Nile perch in some areas,” said Kudhongania. He holds his thumb and index finger an inch apart. “They’ve been catching perch this small!”

In some villages on the Kenya side of the lake, said Wiley, almost all of the perch catch is of immature fish, meaning the fishermen are mortgaging their future by cutting into the perch’s reproductive cycle.

Meanwhile, the governments around the lake have yet to catch up with the limitations of Lake Victoria. Kenya and Uganda are both proceeding with plans to construct fish-processing plants like the ghost structure haunting the Tanzanian shoreline; 11 trawlers are scheduled to be launched to collect the raw material.

“It’s a wrong assumption that the lake can be fished so heavily,” remarked Makarere University’s Okedi. “The authorities have not listened to our advice, but we say that within five years, you’ll have no resources to fill these factories.”

He continued: “I think we’ve reached the climax of Nile perch, and from now on the population will decline very rapidly. That will spell doom for the fishermen, and malnutrition for the people.”

A Dying Source of Life

Lake Victoria, source of the Nile, is a textbook case of ecological destruction, with 300 species of fish having disappeared over the last decade. Among the reasons: industrial dumping, overfishing and the introduction of the ravenous Nile Perch, which began gobbling up smaller fish and is now eating its own kind.