A method and system for registration of predominately freely moving objects in a fish farm netpen are described, in which a camera housing (2) with a number of underwater cameras, is submerged in the netpen suspended from a cable (20). Each camera (1a, 1 b, 1c) takes individual images, at respective angles, of objects that freely pass the camera housing and sends said images to a computer processing unit (5) and the computer processing unit (5) analyses the images taken at different angles to determine the main pattern of the objects, by grouping each pattern element based on relations between shape, size, contrast and/or colour, and to determine mutual distance between given pattern elements for identification of the main pattern and determine, based on the main pattern, whether the objects are fish (10), pellets (11), faeces (12) or other unknown elements.
