Automatic fish counters are automatic devices for measuring the number of fish passing along a particular river in a particular period of time. Usually one particular species is of interest. One important species studied by fish counters are Atlantic salmon. This species is of interest owing to its ecologically vulnerable status and anadromous lifestyles. Methods of operation Fish counters can be divided into three principal types: resistive counters, optical counters, and hydroacoustic counters. Resistive counters A resistive counter is associated with an in-river structure, an example constituting a Crump weir. The resistivity of a fish is lower than that of water. So, as fish cross this barrier, they pass embedded electrodes, and the difference in resistivity disturbs the field established in the vicinity of the electrodes, altering inter-electrode resistance. With three electrodes these disturbances can then be measured by a Wheatstone bridge, or other means, to detect the size and direction of travel of the fish. Fish counters of this type are used widely in Scotland to census populations of Atlantic salmon, where comparison with closed circuit television shows around a 97% detection rate. Optical counters An optical counter is also associated with an in-river structure. However, rather than pass electrodes, in an optical counter the fish interrupt some of a number of vertically arranged beams of light. The pattern of beam-breaks can be used to determine the size, profile, and direction of motion of the fish. Infrared light is used for minimizing the disturbance of the fish as they will not see the light when passing through the counter. When a fish swims through the net of light beams, the resulting silhouette image is used for counting as well as estimating the size of each fish. Each individual image is memorized in the control unit so that the counting can be verified afterwards. Some systems such as the Riverwatcher use the infrared scanner to trigger a digital camera to capture between 1 and 5 photos or a short video clip of each fish. The computer then automatically links the images to other information contained in the database for that individual fish such as size, passing hour, speed, silhouette image, temperature etc. The camera is installed in a special tunnel that contains both the camera and lights providing constant light, and same distance from the camera for the fish. That way, it is possible to get good images of the