The present invention provides a fish farm seawater circulation and oxygenation system comprising a subsea arrangeable ejector device (1) comprising a pipe (2) and at least one ejector (3) arranged inside the pipe, the pipe (2) comprises a pipe inlet (4) and a pipe outlet (5)； the ejector (3) comprises an ejector housing (6) having an ejector inlet (7) and an ejector outlet (8), and a nozzle (9) arranged inside the ejector housing, the nozzle is connectable by a conduit (10) to an air pump (11) for supply of pressurized air to the nozzle； such that seawater is sucked into the ejector inlet (7) and a mixture of air and seawater is ejected through the ejector outlet (8) and the pipe outlet (5), during use.
