Fish culture in natural waters is concerned with the replenishment and improvement of the fish stocks of rivers, lakes, reservoirs, and seas. The increasing impact of man on such waters (water pollution and construction) has impeded or disrupted the natural replenishment of fish stocks. Thus, the rearing of fish, that is, fish-farming, is needed to maintain the existing fish life and to improve an ichthyofauna of minor value. Semimigratory (pike-perch, carp, roach) and migratory fishes (sturgeon, beluga, stellate sturgeon, inconnu, Atlantic salmon, chum salmon, pink salmon, Baltic vimba, and kutum) are particularly threatened. Semimigratory fishes inhabit lakes, rivers, and the mouths of rivers; and migratory fishes inhabit seas but travel, often hundreds of kilometers, up rivers for reproduction. However, as a result of construction on the rivers, the spawning grounds are cut off by dams from the feeding areas, and timber floating on the rivers leads to the destruction of the spawning grounds.