The invention provides an arrangement for measuring the biological mass of fish, the arrangement comprising a measurement unit and a processing unit operatively connected to or integrated in the measurement unit. The measurement unit comprises one of: a camera comprising an integrated autofocus function or an external distance measurement device providing data on distance between fish and camera, and a stereovision film camera； the measurement unit or the arrangement comprises sources of in substance monochromatic light, and the processing unit comprises: a pattern recognition functionality and function for creating an outline of said fish after a positive finding, the outline comprises length and at least one transverse dimension across the length of the fish being measured, or an area inside the outline and at least one of length and a transverse dimension across the length of the fish being measured, all other data are eliminated and the outline is used to calculate or find the biological mass of the fish. The invention also provides a method for measuring the biological mass of fish.
