Fish (specifically, the ray-finned bony fish) are the most successful vertebrates in terms of numbers of species (more than 22,000) and individuals. About 4 percent of these fishes, almost 800 species are known from the fresh water environments of the US and Canada. Some, such as minnows and shiners, are small and may only reach a few inches as adults while others, such as bass and trout, can reach considerable sizes and are greatly desired sporting fish. Their varied diets often include algae, microscopic animals (especially as hatchlings), insect larvae, crayfish, and other fish.

Because so many will be eaten prior to adulthood, females typically lay thousands (even hundreds of thousands) of eggs per year. Males often make nests into which females lay the eggs with fertilization of the eggs occurring outside the female's body. Males guard both the nest and hatchlings in many species although there is no parental care in other species. Many specialize in certain types of habitats: small and large rivers, small and large lakes, swamps, etc. Some spend part of their lives in the ocean and swim up rivers such as the Delaware and Hudson to spawn. Often, the distribution of a species is determined by watershed boundaries such as the Delaware and Hudson River drainage systems.

Fish were the first vertebrates to evolve more than 500 million years ago. The earliest forms measured only an inch or two and lacked jaws, teeth, bone, and fins. Over tens of millions of years, primitive jawless fish (survived today only by hagfish and lampreys) became more complex, evolving bone, fins, and finally jaws. Modern cartilaginous fish (sharks and rays) are descendants of an early group of jawed fish while other groups of early jawed fish such as the placoderms and acanthodians became extinct.

Bony fish evolved later into the lobefins which gave rise to land vertebrates as well as modern lungfish and coelocanths and the ray-finned fish which compose almost all modern fish. After several primitive lineages of ray-finned bony fish (survived today only by sturgeon, gars, and bowfin), the advanced teleost fish evolved in the Mesozoic Era (the "Age of Dinosaurs"). Slowly they diversified into the enormous richness of species on earth today.