Native fishes of the Central Plateau area-Tasmania

In comparison with the northern hemisphere, the major continents of the southern hemisphere have a relatively small freshwater fish fauna. This is probably due to the fact that the predominant portions of the main southern hemisphere land-masses lie in tropical, subtropical or temperate zones and except for the southern-most extremities are not overendowed with rivers and lakes. Consequently the potential for invasion of fish into freshwater areas where they can become established is limited. Most freshwater fishes living today are considered to be derived from marine ancestors which have subsequently invaded freshwater.
Evidence for this can be found in the fact that many groups of fishes which are considered to be predominately freshwater species periodically descend to the sea or to the estuaries to spawn or carry out some other phase of the life cycle.