Lake Victoria fills a shallow depression (no deeper than 80m) between the Western and Eastern Rift valleys, yet it is not part of the Rift system. Until the 1960s, it was home to around five hundred different species of brilliantly coloured tropical fish, known as haplochromines or cichlids, all of them endemic – unique to the lake. Scientists, puzzling over how such a dazzling variety of species came to evolve in this largely uniform environment in the space of no more than a million years, have suggested that, at some stage in its history, the lake must have dried into a series of small lakes in which the fish evolved separately. Lake Victoria’s cichlids are popular aquarium fish, and one of the commonest larger species, the tilapia, is a regional speciality, grilled or fried and served whole.
In the early 1960s, a voracious carnivore, the Nile perch, was introduced to the lake, and proceeded to eat its way through the cichlid population, driving some species close to extinction, though many have held on in parts of the lake which were too shallow for perch, or in smaller lakes around the main one. For local people, the introduction of the perch, which can reach a weight of 250kg, has been a bit of a Trojan horse: while they’re consumed locally and sold for export (good news for the lakeshore economy), traditional fishing and processing have been hit hard by the arrival of modern vessels and factories joining in the feast and taking their profits elsewhere.
The lake has other problems, however. Algae have proliferated, due to industrial and sewage pollution, depriving the lake of oxygen. More than three million litres of human waste drain into the lake every day, and the Swedish development agency, SIDA, estimates that Kenya, with the smallest share of the lake’s shoreline, is its main polluter. As well as suffering a dramatic fall in oxygen levels, the lake is becoming so murky that the remaining cichlids are unable to identify mates, so that hybridization is occurring. Meanwhile, the building of the causeway between Mbita and Rusinga Island has turned the Winam Gulf into even more of a pond, with only one outlet, inhibiting currents and making its water even less healthy.
Another threat comes from the water hyacinth, originally native to Brazil. This floating weed grows quickly around the lakeshore and spreads like a carpet across the surface, blocking out the light, choking the lake to death and snaring up vessels. Since the mid-1990s, Homa Bay, Kendu Bay and Kisumu have all at times been strangled by kilometre-wide cordons of the weed, inhibiting passage to all but the smallest canoes, with disastrous results for the local economy. Solutions have included the promotion of products (furniture, paper, even building materials) made from harvested hyacinth. In 2001, mechanical clearance enabled passenger ferries to resume, only for falling water levels to cause their suspension once more. There’s been a resurgence of the invasive weed since 2006, and the nutrient run-off following occasional rains spreads the deadly canopy even further.