In the modern fauna, there are more than 20 thousand species of fishes and fish-like - more than mammals, birds, reptiles and amphibians combined.
In the waters of Russia there are about 1500 species, 300 species are inhabitants of fresh water.
A tenth of all species of fish are fish. They provide many valuable products: food, medicines, pet food, fertilizer for the fields, technical fats and raw materials for light industry. There are freshwater, anadromous and marine fishes.
Freshwater fish spend their lives in rivers, ponds and lakes.
Anadromous and catadromous fish fatten and grow in the sea and breed in the rivers. Anadromous fish do far journeys from feeding to breeding grounds in the upper reaches of the river, with a length up to several thousand kilometers; catadromous usually does not extend far into the sea and do not rise high in the river.
Among marine fishes distinguish pelagic - living in the upper layers of the sea surface (e.g., herring and mackerel), demersal and benthic fish in the bottom waters or on the bottom (for example, cod and flounder).
