Among freshwater fishes, there is a wide range in tolerance to salinity. Species that are intolerant of salty water are unable to stray into estuaries and are termed primary freshwater fishes by ecologists. Their dependence on fresh water restricts their ability to invade and colonize new areas, limiting their travels to freshwater routes.
Common species of the Everglades are largemouth bass, warmouth, bluegill, redear sunfish, golden shiner, and yellow bullhead.
The Florida largemouth bass, found only in peninsular Florida, is the most famous of the primary freshwater fishes of the region. It is a subspecies of the largemouth bass that occurs naturally in North America, from the Great Lakes southward to the Gulf and lower Atlantic coasts, and is now widely stocked around the world. The largemouth bass is a spectacular game fish, and the Florida subspecies is especially renowned. Frequently growing to 10 pounds, with a maximum nearer 20, it averages substantially larger than the northern subspecies. Because it is more wary, the Florida subspecies is also more difficult to catch.
Freshwater fishes, with more tolerance to salt water and the ability to live there temporarily, are termed secondary freshwater fishes. They can colonize new areas by swimming from the mouth of one river system into the next through interconnecting estuaries, salty coastal wetlands, or even temporarily in seawater. This ability allows them to colonize faster than primary species. Most of the eight species of secondary freshwater fishes in southern Florida are the killifishes and their relatives. Generally small and colorful, many of these fishes make good aquarium specimens.