Fish farming is the principal form of aquaculture, while other methods may fall under mariculture. It involves raising fish commercially in tanks or enclosures, usually for food. A facility that releases juvenile fish into the wild for recreational fishing or to supplement a species' natural numbers is generally referred to as a fish hatchery. Fish species raised by fish farms include salmon, catfish, tilapia, cod and others. Basically, there are two kinds of aquaculture: extensive aquaculture based on local photosynthetical production and intensive aquaculture, in which the fishes are fed with external food supply.