From Fishing Planet Unofficial Wiki

Perch is a common name for fish of the genus Perca, freshwater gamefish belonging to the family Percidae. The perch, of which there are three species in different geographical areas, lend their name to a large order of vertebrates: the Perciformes, from the Greek perke, simply meaning perch, and the Latin forma meaning shape. Many species of freshwater gamefish more or less resemble perch, but belong to different genera. In fact, the exclusively saltwater dwelling red drum is often referred to as a red perch, though by definition perch are freshwater fish. Though many fish are referred to as perch as a common name, to be considered a true perch, the fish must be of the family Percidae.

(Source: Wikipedia)