The present invention discloses a system for the registration of numbers and/or weight distribution of marine organisms (7) in movement, comprising a registration container (2) manufactured at least partly from a transparent or translucent material； a transfer duct (15) connected in fluid communication to the registration container (2) and adapted for the transfer of marine organisms (7) from at least one reservoir (8) to the container (2)； and a registration unit (1) comprising at least one camera (3), wherein said registration unit (1) is performing an optical registration of marine organisms (7) passing through the registration container (2)； and further comprising at least one outlet duct (27) connected in fluid communication with the registration container (2)； and at least one injection duct (28) connected in fluid communication with said registration container (2) in a position mainly upstream of the at least one outlet duct (27)； wherein the at least one outlet duct (27) and the at least one injection duct (28) are connected mutually such that the fluid flowing through the transfer duct (15) is directed to the area of the registration container (2) giving the optimal optical registration of marine organisms (7).
