Fish is a kind of aquatic vertebrate with variable temperature. It breathes through gills, swims through the swinging of tail and trunk, and feeds on the upper and lower jaws. It belongs to the vertebrate subfamily of Chordata. Vertebrates are generally divided into fish (53%), birds (18%), reptiles (12%), mammals (9%) and amphibians.

According to the statistics of the late Canadian scholar Nelson in 1994, there are 246 18 species of fish in the world, accounting for more than half of the named vertebrates, and new fish species are constantly being discovered, with an average of about 150 species per year, which should increase by more than 1500 species in more than ten years.

Fish are mainly divided into tropical fish, temperate fish and cold water fish.

Aquatic fish are divided into freshwater fish and saltwater fish.