With several exceptions, the fishes included in this checklist are all obligatory freshwater fishes (i.e., fishes that need fresh water to complete at least part of their life cycle). Select marine and brackish water fishes are included if they meet one or more of the following criteria: they are naturally capable (or presumed to be capable) of spawning in fresh water, they commonly penetrate far inland, and/or they are common or abundant in freshwater habitats on a regular basis. This excludes many species that are rare in North American fresh water habitats (e.g., Bull Shark, Carcharhinus leucas), or that are generally limited to estuaries or the extreme downstream portions of rivers where salt and fresh water mix. Exotic fishes are included if they have (or are suspected to have) established populations in the wild.
North America is herein defined not as the entire continent, but as the Nearctic zoogeographic realm. The Nearctic Realm includes the entire continental landmass, including Greenland, Alaska, Canada, the lower 48 states, and Mexico south to the where the Mexican plateau breaks down into the lowlands of Central America.