The Biodiversity Crisis: Lake Victoria
Part of the Biodiversity Crisis Curriculum Collection.
Melanie L.J. Stiassny
Freshwater ecosystems around the world exist in a complex balance, and most face severe threats. Prominent among these threats are overharvesting, pollutants (usually washed in from the land), and the introduction of foreign or exotic species. Today, the combined effect of these three types of threats has put one of the world’s great ecosystems—Lake Victoria—close to death.
Lake Victoria—called the freshwater heart of Africa—is the world’s largest tropical lake; it covers an area about the size of Scotland. It was once home to an astonishing diversity—more than 350 species—of cichlid (pronounced “sick-lid”) species found nowhere else. Although cichlids are small fish, they were a major food resource for millions of people in the three countries surrounding the lake: Kenya, Uganda, and Tanzania.
Popular for home aquariums because of their typically vivid colors, cichlids are almost unique among fish in the way they protect their newborns from predators. They carry their newly hatched young in their mouths until the fry are large enough to have a good chance of defending themselves. The drawback of this behavioral adaptation is that it severely limits the number of offspring; in contrast, typical fish lay thousands of eggs and let them fend for themselves. This limit to the cichlid’s reproduction rate makes them very vulnerable to overfishing; however, for thousands of years they thrived in Lake Victoria while playing a key role in keeping the lake’s ecosystem in balance.
Around 1900, the colonial British government established a large fishing industry on the lake. They introduced gill nets, which allowed larger numbers of cichlids to be caught. For decades the lake was heavily overfished. Simultaneously, more and more local people began to settle by the lake. As land was cleared for agriculture, soil and fertilizer began washing into the lake in the runoff from the region’s frequent rains. The fertilizers caused an increase in the population of surface algae. When these died, they sank, and their decomposition absorbed oxygen, reducing the oxygen available for fish living in deeper layers of the lake.
After undergoing these stresses, the lake’s ecosystem suffered its severest blow of all in 1954—with the introduction of the Nile perch, a huge, voracious predator. Weighing up to 136 kilograms, the perch were introduced to give the local fishers a bigger fish to catch. Unfortunately, bigger was definitely not better. The local people had preserved the much smaller cichlids by drying them in the sun. But the only way they could preserve the much bigger Nile perch was to fry them or smoke them. Ever more forest land was cleared for the needed firewood, and this additional deforestation increased the harmful runoff into the lake while devastating the surrounding forest ecosystems.
For reasons not fully understood, the perch population remained small until the late 1970s; then it exploded—largely at the expense of the cichlids, its prey. In 1978, cichlids were about eighty percent of the biomass—the total mass of living organisms—in Lake Victoria; the Nile perch then were only about two percent of the biomass. Fewer than ten years later, the Nile perch were more than eighty percent of the lake’s biomass, while cichlids were a tiny portion of the remaining twenty percent. Today less than one percent of the mass of fish caught in Lake Victoria comes from cichlids. More than half of the cichlid species may now be extinct or close to extinction.
The devastation of cichlids has led to drastic changes in Lake Victoria; even the perch’s future is in doubt. The disappearance of most algae-eating cichlids allowed algae to flourish unchecked; this further depleted the lakes oxygen levels through the process described above. Today, the Nile perch and most other fish can survive only in a thin region near enough to the surface to receive sufficient oxygen from the atmosphere. The perch has virtually eaten its way through all its potential prey species except one: a species of small shrimp that appears to thrive in oxygen-depleted water. How long will the shrimp hold out? No one knows. If the shrimp goes, the whole ecosystem will collapse; Lake Victoria will die. This would be a catastrophe of enormous proportions. It could spell famine, or even starvation, for millions of people across East Africa who depend on the lake for their survival.
Although there is no way to exterminate the Nile perch, researchers are looking for ways to stabilize and possibly save the surviving cichlid populations. In 1993, conservationists introduced a Species Survival Plan for the lake’s cichlids. This plan called for a widespread educational effort for people who use the lake, cichlid captive breeding programs in the United States and Europe, further assessments of the status of the remaining wild cichlid populations, and attempts to reintroduce cichlids to the lake in the future.
Will these attempts be enough to save Lake Victoria? It is too soon to know for sure, but scientists will keep trying.