Lake Victoria covers about 68,800 square kilometres, making it Africa's largest lake and the largest tropical lake in the world. It is shared by Kenya, Uganda and Tanzania and is the main source of the White Nile. Its fishery feeds millions of people and earns large exports, but its fish life was changed in the 20th century by one of the most famous species introductions in history.

The cichlid flock

Before the 1950s, the lake held several hundred species of haplochromine cichlids, small fish found nowhere else, which had evolved in the roughly 15,000 years since the lake last dried out. They ate algae, insects, snails, other fish and even the eggs and scales of other cichlids. Scientists saw them as a natural laboratory of evolution.

Nile perch and the new fishery

The Nile perch, a large predator, was introduced in the 1950s to give fishers a bigger fish. In the 1980s its numbers exploded, and many native cichlids vanished; perhaps 200 species are thought to have gone extinct. Nile perch fillets became a major export to Europe, and the small silver cyprinid, dagaa or omena, became the largest catch by weight. Pollution, water hyacinth and overfishing remain problems, and some cichlids have recovered as perch numbers fell.