This description concerns a production unit for fish including a tight cylinder module (3) submerged in the sea with a float module (2) at the top and a tight top roof (1). The production unit includes in addition a bottom module (4) installed in the extension of the cylinder module (3) downwards in the sea, where the bottom module (4) consist of two flat rings (6, 7) in such a way that it forms a rigid construction, where the ribs function as vanes to lead the water stream inn and out of the production unit. The main reason for the tight construction is to prevent lice and disease microbes moving in the upper level of the sea, to enter into the fish area. Fishing escapes is eliminated as all the materials in the construction have quite a different strength than the fish farm cages with net bags in an open construction. The open bottom construction is acting in such a way that the fish excrements natural are distributed with the sea stream.
