System for grading fish while moving the fish from one place to another, wherein the system comprises at least two fish tanks 1, 2 for receiving the fish after the grading, means for draining off seawater 3A. and a pipe system transporting the fish. The system is operated with reduced pressure. Further, the system is closed and comprises a grader 3. The pipe system 4 is transporting the Fish from a pool 5 offish and water onto the grader, and further from the grader and into the fish tanks. The invention also comprises a method for grading fish while moving the fish from one place to another with a system according to the above. The method comprises the following steps:- arranging one part of the pipe system 4 into a pool offish and water, containing the fish to be graded,- creating reduced pressure in the system by removing air through valve 9, until water and fish is flowing from the pool into the pipe system 4, and controlling the pumps for removing air through valve 9 and water from the water tank 7 in order to maintain the negative pressure.
