Identify a fish based on the type of water it lives in, the geographic location, its body shape and color, the placement of its fins, and the absence or presence of scales. The placement of other body structures, such as eyes and nostrils, are also important when identifying a fish.
A good field guide is helpful when identifying fish. Chose a field guide based on the type of water in which the fish lives and its geographic location. With very few exceptions, an individual species of fish lives in either salt water or fresh water, and some only occur in small geographic ranges.
With the help of the field guide, determine the class of the fish by observing its body type. Ray-finned fishes are the most common with bony skeletons, a distinct head, and its tail and fins supported by rays. Sharks and rays pronounced gill openings and rough skin instead of scales. Lampreys, lobe-finned fish and hagfish all have elongated body types, though lampreys and hagfish lack jaws. Search the field guide to locate fish of similar size and color to the specimen being identified.
If a field guide is not available, eNature.com provides a comprehensive illustrated fish identification glossary online.