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.

Fish counters can be divided into three principal types: resistive counters, optical counters, and hydroacoustic counters.

A resistive counter is associated with an in-river structure, such as 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.