There is some disagreement over the classification of living fishes, but they are generally divided into three major groups: the jawless fishes (hagfishes and lampreys), the cartilaginous fishes (sharks, skates, rays, and related fishes), and the ray finned fishes (also called bony fishes). There are about 25,000 described species worldwide, 97 percent of which are ray-finned fishes. More than 2,500 species are found in North American waters.
Biologists divide living organisms into major groups called phyla (singular: phylum). Fishes belong to the large phylum Chordata (animals having a spinal cord), and to the subphylum Vertebrata, along with amphibians, reptiles, birds, and land dwelling mammals. Phyla are divided into classes, classes into orders, orders into families, families into genera (singular: genus), and genera into species. The species is the basic unit of classification and is generally what we have in mind when we talk about a "kind" of fish.
The formal definition of a species is rather difficult to understand, but in practical terms species are groups of organisms that breed successfully and produce fertile offspring. Subspecies are populations of a species that differ from one another in some way and, most important, are completely or partly isolated geographically.
The written descriptions of fishes are arranged phylogenetically, according to scientific classification, which reflects the general evolutionary history of fishes from the jawless hagfishes through the cartilaginous sharks and rays to the most advanced ray-finned fishes. Each grouping of fishes (superclass, class, order, family) is introduced by a discussion of its distinctive features; when reading about a specific fish, readers are urged to review these introductions. Within each group, the orders and families are arranged in the sequence adopted by the American Fisheries Society in 2001. The arrangement of genera and species within each family is alphabetical by scientific name. The following details are given in the species accounts:
Common names can be unreliable because the same species may be known by different common names in different areas; in addition, the same common name may be applied to more than one species. Scientific names are more precise because each species can have only one valid scientific name. However, scientific names continue to be modified as our knowledge and understanding of the biological relationships among fishes expand. The common and scientific named used in this guide are those given in A List of Common and Scientific Names of Fishes from the United States and Canada, published by the American Fisheries Society in 2002. Each scientific name consists of two words, usually derived from Latin or Greek. The first, always capitalized, is the name of the genus (plural: genera); the second, in lower case, is the specific epithet. For example, the scientific name of Rainbow Trout is Oncorhynchus mykiss. It is the only member of the genus Oncorhynchus that bears the specific name mykiss. Scientific names for genera and species are always italicized; those for the larger groupings - such as classes, orders and families- are not.
