evidence: Lake Nyasa, lake, southernmost and third largest of the Eastern Rift Valley lakes of East Africa, which lies in a deep trough mainly within Malawi. The existence of the lake was reported by a Portuguese explorer, Caspar Boccaro, in 1616. David Livingstone, the British explorer-missionary, reached it from the south in 1859. [...] Nyasa (meaning "lake") is fed by 14 perennial rivers, the largest being the Ruhuhu; the sole outlet is the Shire River, a tributary of the Zambezi. Hundreds of species of fish have been recorded in the lake, many of which are endemic, being isolated from the Zambezi fauna by the Murchison Falls. Commercial fisheries exist at the southern end of the lake, based chiefly on the freshwater fish Tilapia. Fly hatches on the lake occur in clouds large enough to obscure the horizon. Increased environmental degradation has had adverse effects on wildlife in the lake, however; excessive silting disrupts fish feeding and breeding grounds, reducing their numbers. In addition, overfishing, the use of nets with a mesh size smaller than those recommended by fisheries experts, and the disregard of the ban on fishing in the breeding season have also had a detrimental effect on fish populations there.