A method for counting fish comprises providing at least one detector with a transmitter and a receiver in each grid element in a counting grid and connecting the detectors to a control unit which is coupled to a display unit, together with detecting the passage of a fish through a grid element on the basis of a recorded digital level change and the duration thereof in a signal transferred between the transmitter and the receiver, recording the passage of a fish as one count and transferring the result of the count to the display unit for display. A counting apparatus for fish for the implementation of the method comprises a surface part with a display unit and an underwater part with a counting grid formed from separate grid elements, each of which constitutes an opening in the counting grid. In each grid element there is provided a detector for detecting the passage of a fish and the detector comprises a transmitter and a receiver. There is further provided a demultiplexer connected to the transmitters and a multiplexer connected to the receivers, and a control unit connected to the demultiplexer and the multiplexer and also coupled to the display unit in the surface part. In a method for counting fish swimming in an enclosure and by means of the counting apparatus for fish, the enclosure in which the fish are to be counted is connected with an additional enclosure, the underwater part is lowered and placed in the enclosure net at the connection point, the counting grid thus constituting a connecting opening for fish between the enclosures. When counting takes place the bottom of the enclosure in which the fish are to be counted is raised towards the surface, thus forcing the fish to pass through the counting grid and into the additional enclosure.
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