Introduction

Fishes constitute more than 50% of the living vertebrate species and are characterized by a level of sexual plasticity that is unrivalled among other vertebrates. They inhabit every conceivable aquatic habitat and are found in the world’s highest lake, Titicaca (3,812 m above mean sea level) and the lowest lake, Baikal (7,000 m below sea level). Some are reported from almost distilled waters (0.01 g/kg salinity), while others live in very salty lakes (100 g/kg s). The African tilapias inhabit hot soda lakes (44˚C), while Trematomus are found under the Antarctic ice sheet at –2˚C. Some of them are capable of breathing atmospheric air and survive in stagnant tropical swamps (Pandian, 1987).