An enclosure (1) for breeding fish, said enclosure at its upper side wall portion (2a) is secured to an upper buoyancy or floating frame (3a). The lower side wall portion (2b) of enclosure (1) is secured to a liftable and lowerable frame (5) for lifting the bottom (1a) of said enclosure (1) from its immersed position in the water to an upper position in the water, to facilitate catching operations of bred fish, and maintenance of the sheet/net material of said enclosure. Guide means (6) for centration of frames comprise guide stay means (6a) attached to lower frame (5) and passing through guide sleeves (6b) in said upper floating frame (3a). The bottom (1a) of said enclosure consists of a rigid plate having a central portion (1a') being lower than the marginal portions to collect sediment that is removed by the aid of a device (7) to a purification plant.
