From villain to saviour - the Nile perch does a flip

Africa's Lake Victoria, a treasure trove of fish species, is under thre at. Martyn Kelly reports on new currents in conservation

Conservationists have described the Great Lakes of Central Africa as the aquatic equivalent of tropical rainforests. It is no mean boast: within these lakes live fish fauna of unparalleled richness. A lake such as Victoria contains 250 or more sp ecies of closely related cichlid fish, nearly all unique to that lake and adapted to exploit particular micro-habitats within it. "It is the most exuberant expression of vertebrate adaption in the world," says Les Kaufmann, of New England Aquarium in Bos ton, Massachusetts. Like the tropical forests, however, these lakes are facing imminent catastrophe.

Until a few years ago, everyone thought they knew the villain of the piece. In the mid-Fifties, well-meaning experts introduced Nile perch into the lake in an attempt to improve the fisheries. This carnivorous fish grows to a massive 150kg and as it spread through the lake, the cichlid fish started to disappear. Their disappearance prompted a letter to Nature from 13 tropical fisheries experts, highlighting not just the loss of biodiversity but also the vulnerability of the entire Lake Victoria fishery and the communities that depend upon it.

New evidence, however, suggests that the lake's ecology started changing well before the introduction of Nile perch. Lakes slowly accumulate sediments at their beds, including plankton, which sinks through the water column as it dies. A vertical core of sediment, therefore, can tell us if the types of algae present have changed over time. From what we know about the habitat preferences of different species, we can get some idea about conditions at the time when the sediment was deposited.

Studies by Bob Hecky, a Winnipeg-based fisheries scientist, showed that the lake started to change in the Twenties - 30 years before the Nile perch was introduced. As the human population around the lake grew, bush was cleared for planting crops. The loss of cover meant that soil was eroded by rainfall and its nutrients ran into the lake. This started the process of eutrophication, which accelerated in the early Sixties.

Rosemary Lowe-McConnell, a freelance consultant formerly of the East African Fisheries Research Organisation, explains that the lake was originally surrounded by swamps. "These acted as filters for nutrients running off the land." When lake levels were particularly high (as between 1961 and 1964), the swamps were drowned and decaying vegetation released nutrients into the lake.

During the next decade changes were slow. Dr Hecky claims: "The Nile perch was held in check by predators." However, extra nutrients in the lake encouraged the growth of small planktonic algae that were, in turn, eaten by small invertebrates - the main food of young Nile perch. "So more Nile perch were able to survive," Dr Hecky explains, "which let populations amass to such an extent that they had extreme effects on the cichlids." Further, as plankton died, it sank to the bottom of the lake and decayed, using up oxygen. As a result, bottom-living cichlids were unable to survive.

"The scientific community is divided by the changes," says Dr Lowe-McConnell. "Those concerned with fisheries are pleased, as yields are much higher. However, people concerned about biodiversity are worried by the vast loss of species." Estimates for thenumber now extinct are as high as 200 out of the 300 cichlid species once present.

The effect of these changes on the local economy is difficult to gauge. Locals did not like Nile perch at first because it damaged their nets and because it is too oily to be sun-dried for export; instead, it had to be smoked, which used scarce fuel wood. Things have changed in recent years, however, and Dr Lowe-McConnell points out that the Nile perch is now regarded as a saviour, precisely because other fish are in short supply. And finding a way of preserving Nile perch by frying it in its own fat has made export easier.

Paradoxically, the next problem might be a decline in Nile perch. "Huge investment in fish processing factories has created great demand for the fish," explains Dr Hecky. "In the past, Nile perch populations were unrealistically high due to a lack of predators. Now it faces pretty tough times."

A fall in Nile perch numbers could bring a resurgence of cichlid and other prey fish; Dr Lowe-McConnell believes there is already evidence that this is happening. This, in turn, might favour small-scale fishermen who cannot afford the stronger nets needed to catch Nile perch.

Drs Hecky and Lowe-McConnell see no reason why Lake Victoria could not be a well-managed system in the future. A first step has been the establishment of the Lake Victoria Fisheries Organisation to implement a unified policy across the three countries - Kenya, Uganda and Tanzania - that border the lake. But research on the lake is in its infancy." Dr Hecky claims: "The options for management remain in the air until a stock assessment has been performed. The question will be how many fish and what kind. There might even be a case for some more introductions."

"This is one of the largest experiments (albeit unwitting) that's ever happened,'' says Dr Lowe-McConnell. "Now we must discover as much as possible about what is going on." With a population of more than 30 million around Lake Victoria, as well as the prospect of fish introductions to other lakes (such as Malawi and Tanganyika), it is an experiment with a great deal at stake.