Science / Medicine : The Fish That’s Killing LAKE VICTORIA
On the waters of Africa’s Lake Victoria, a Kenyan fisherman strains to retrieve his catch from the long lines he set hours before. Balancing in his handmade wooden boat, he uses a pair of 2-foot steel hooks to impale the gills. He heaves aboard his handmade wooden boat a 6-foot fish weighing 400 pounds. It is a Nile perch, also known as the “elephant of the water.”
Hauling it in is barely worth the effort. The big fish, which resembles a monstrous snook, is nearly worthless. But it is just about the only fish one can catch off the Kenyan shores of the lake these days--an area that teemed with valuable fish only a generation ago.
The voracious perch is eating to extinction hundreds of species of fish and freshwater crustaceans--traditional food species, economically valuable aquarium fish, and 250 scientifically precious species found nowhere else in the world.
Man’s introduction of the fish has so upset the natural balance of the lake that even its life-giving oxygen is threatened. Today, as a result of the introduction of the Nile perch, Africa’s largest lake is dying, and no one knows how to save it.
“We consider this one of the greatest ecological tragedies of this century,” said Perez Olindo, former chief ecologist for the Kenyan Lake Victoria Basin Development Authority. “We do not see a practical solution. We accept the fact that Lake Victoria’s fisheries are doomed, leaving behind the single species,” said Olindo, who directed a 1985 study of the lake.
The rapidly diminishing numbers of native fish species in vast portions of Lake Victoria is also a disastrous blow to scientists. With its remarkable array of 400 freshwater species (by comparison, all of Western Europe hosts only 60) Lake Victoria offers “a laboratory of evolution found nowhere else in the world,” said Dr. Melanie Stiassny, assistant curator of ichthyology at the Museum of Natural History in New York.
“These species make Darwin’s finches in the Galapagos pale in significance.”
Evolution Study
Aside from the sheer number of species, Lake Victoria’s native fish are also scientifically important for another reason: They appear to have evolved in a startlingly short amount of time, explained Richard Vari, curator of fishes at the Smithsonian Institution’s Museum of Natural History in Washington.
The lake was created when gradual warping of the plateau between the two arms of the Rift Valley caused a back-ponding of four rivers about 750,000 years ago. The 250 species unique to the lake evolved, therefore, since the evolution of Homo sapiens-- within a wink’s worth of time, geologically speaking.
Lake Victoria presents “a unique opportunity to evolutionary biologists,” Vari said, “but one that will be soon lost. These native fishes are not going to last very long.”
The consequences of the Nile perch introduction, however, don’t stop here. The carnivorous fish compounds the pollution problems brought on by Africa’s Green Revolution: nutrient-rich runoff from agricultural chemicals is causing algae to proliferate in parts of the lake. With so few plant-eating fish left to eat the algae (Nile perch do not eat plants), the algae simply blooms and decomposes--depleting the life-giving oxygen dissolved in the water. Some parts of the lake are already unable to support life for this reason. “We may be looking to a barren Lake Victoria in which even the Nile perch may not be able to survive,” Olindo said.
The predatory Nile perch was first introduced to Lake Victoria as a sport fish by Ugandan fisheries officials in 1962.
Native to the Nile, its numbers there are kept in check by other predatory fish that eat its young and by fish diseases native to that area. But in Lake Victoria, none of these natural controls are present, and the population of Nile perch is exploding unchecked. Now it is destroying a lake upon whose resources 200 million people in Uganda, Kenya and Tanzania rely.
“This is probably the most stark example of destruction of vertebrate species anywhere in the world,” Stiassny said.
“If all the fishes in the Mississippi disappeared, it wouldn’t even begin to compare (with the perch’s impact upon Lake Victoria),” Vari said.
The introduction of the Nile perch painfully illustrates the disaster that can result from introducing an alien species to a new environment. America is familiar with the ecological havoc wrecked by alien species: the forest-destroying Gypsy moth and the bark beetle carrying Dutch elm disease were both introduced from other countries.
Ruining Fishing Economies
Slowly eating its way through the 26,828-square-mile Lake Victoria from north to south, the Nile perch is destroying the fishing economies of small lakeside villages in Uganda and Kenya. In some parts of the lake, it has eaten so many of the native fish that it is now eating their food source, the crustaceans.
In other northern areas, the Nile perch has nearly exhausted even this food supply and has turned to eating its own young. And now the giant fish is showing up in Tanzanian waters, the third African country sharing the third-largest lake on earth.
The native fish that the Nile perch is destroying represent the major source of protein for the human settlements that ring the lake. The perch is wiping out colorful cichlid species for which aquarium hobbyists pay up to $300 a pair. It is eradicating a species of native fish that destroys the snail that causes schistosomiasis, a deadly liver disease.
But because the lake is so vast--about the size of Ireland--eradicating the Nile perch seems impossible.
Olindo and other African leaders have considered the possibilities, and discarded them all: To fish out the Nile perch would be impossible. They couldn’t poison the Nile perch without killing everything else in the lake. Neither could they risk introducing a new virus or disease to attack the fish--it could affect other species as well. And they don’t dare introduce yet another new species to eat the young of the Nile perch.
In the meantime, fish catches in the region have dropped dramatically.
As recently as the 1960s, the East African High Commission estimated that the three nations sharing the lake could each safely harvest 200,000 tons of haplochromis, which resemble sunfish, each year. But the fish has become so rare today that a Kenyan fisherman trawling Lake Victoria would be lucky to haul in 6 pounds for his days’ work.
Kenya’s annual catch of all species now totals only 20,000 tons. About 80% of that catch is Nile perch--a species of very little value to the 25,000 Kenyan fishermen who ply the lake.
Nile perch has no place in the traditional African diet, Olindo said. Although tasty by Western standards (its flavor is similar to sea bass), Africans don’t like it.
And because the fish is so oily, it cannot be preserved by sun-drying. To preserve it would require either extensive processing to extract the oil, which is too expensive, or smoking, a method that is already seriously threatening the fuel resources of an increasingly treeless land.
Although distributors will buy the Nile perch, Olindo said, they pay little for the unwanted species. At the lakeside of the Nyanza Gulf, a kilo of Nile perch sells for a shilling; in areas where it is still available, a kilo of the native species oreochromis fetches 30 shillings. “The fishermen are forced to sell their (Nile perch) at giveaway prices,” Olindo said.
Because of the Nile perch, “fisheries have not been merely damaged, but destroyed,” researcher C. D. N. Barel and his colleagues in Leiden in The Netherlands wrote in a recent issue of the British journal Nature.
All three countries sharing the lake have been struggling toward a way to cope with the Nile perch problem. But as developing Third World nations vie for foreign aid, Olindo said, such proposed environmental projects often are passed over in favor of more immediate efforts to feed starving millions.
A 1984 symposium of the United Nations Food and Agriculture Organization concluded that the future of the lake is so dim that it would be “extremely unwise” to invest further in industrial fisheries there.
For his part, Olindo has proposed cordoning off certain shallow areas of the lake with plastic nets in order to create “exclusion zones” to keep the perch out of selected fisheries. But there are no formal plans, lacking funding.
To learn how to arrest the pollution problems, Olindo hopes to send two of his staff scientists for university training in the United States, but he has not been able to secure the $50,000 needed for the scholarships.
To try to exploit the protein potential of the Nile perch, all three countries would like to set up processing plants that would extract the oil from the big fish. The lean flesh then could be eaten, the bones used for fertilizer, and the oil exported to vitamin or cosmetic companies. But again, finding investors for this expensive--although potentially lucrative--project remains the obstacle.
“Because of what a few men did back in 1962,” Olindo said, “we are dealing with a situation we are not even sure we can begin to deal with.”
LAKE VICTORIA: THE GALAPAGOS OF LAKES Lake Victoria, the world’s third-largest freshwater lake, boasts a remarkable array of 400 fish species. Aside from the sheer dazzling number of species, Lake Victoria’s native fish are also scientifically important for another reason: They appear to have evolved in a startlingly short amount of time, creating a great opportunity for evolutionary research.
The lake was created when gradual warping of the plateau between the two arms of the Rift Valley caused a back-ponding of four rivers about 750,000 years ago. The 250 species unique to the lake evolved, therefore, since the evolution of Homo sapiens--within a wink’s worth of time, geologically speaking.
Some species endangered are the:
Oreochromis nilotica, which derives its name from the fact that females carries developing eggs inside her mouth. Even when the young are hatched, they will return to her mouth if danger threatens. Despite this, the mouthbrooder has a small mouth, tiny teeth and is a sturdy fish with a long-based dorsal fin. Although it mainly feeds on plankton, insects and crustaceans are also a part of its diet.
The many Lake Victoria Haplochromines, only 3 of which are pictured here, show remarkably little diversity in body shape. They are generally recognized by their rather elongate form and large mouth. Contrasting with the relative uniformity of body form, it is in the head, the
jaw teeth and dental patterns that the real diversity of these species becomes apparent. This particular diversity, more than any other factor, has contributed to the success of these fishes in the lake.
