Tropical reservoirs (man-made lakes) have evened out somewhat the distribution of standing waters on a global scale. Natural lakes are well represented in the North Temperate and Arctic regions. In the tropics, the only region with a sizeable lake area is the rift valley region of East Africa. Man's reservoir building activities have created the potential for a rich source of cheap fish protein. This potential is best realized when lacustrine tilapias from Africa are present. Introduction of these fishes to reservoirs has sometimes given spectacular increases in fish yields, as in Sri Lanka. Unlike other freshwater fishes tilapias have the ability to digest and assimilate very efficiently, blue-green algae which predominate in shallow tropical reservoirs. The very high yields of tilapias in tropical reservoirs are due largely to these factors.