Lake Malawi is an inland sea 580 kilometres long in the Great Rift Valley between Malawi, Mozambique and Tanzania, and it holds more species of fish than any lake on Earth. At least 700 of them, almost all found nowhere else. Only the upper third of the lake holds oxygen. Below about 250 metres the water is still and lifeless, so every fish is pressed into a bright band along the shore. That crowding is the reason for the number: hundreds of species evolved side by side, each on its own patch of rock or sand. The ones you meet are the mbuna, rock-dwellers in electric blue, tangerine and gold. At Cape Maclear a mask and a step off the rocks puts you in a wall of them an arm's length away. Most are mouthbrooders: after spawning the female scoops the eggs into her mouth and carries the young there for weeks, and the fry dart back in at any sign of danger.
At first light a fisherman pushes a dugout off the beach, hollowed from a single tree trunk and still the everyday craft here. It rides so low that you sit almost level with the water, and every knock of the paddle carries the length of the hull. After dark the far shore pricks into light, one lantern at a time, as the fishermen paddle out over the deep. Now and then a grey column rises off the lake like smoke. It is not smoke but billions of lake flies hatching at once, which villagers sweep out of the air with wet baskets and press into a dark, protein-rich cake. Far out lies Likoma: eighteen square kilometres of Malawi inside Mozambican water, a border drawn in the 1880s around a mission station. Ten thousand people live on it, and the church those missionaries built is one of the largest in Africa. The fish stay with the rock, so when you swim out over the sand between two headlands the colour stops dead, then starts again on the next outcrop.