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.
However, rather than pass electrodes, in an optical counter the fish interrupt some of a number of vertically arranged beams of light.
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 key disadvantage of optical counters is the small penetration of the beams through the water, restricting their use to narrow river features or in-river structures, an example being fish ladders.
Generally used for short-term or seasonal studies, some situations require a long-term count which is accurate in absolute terms, not only in relative change (for example, no hydroacoustic sensors are routinely used in the detection of Scottish Atlantic salmon).
Such methods usually require significant habitat modification, such as construction of a weir to funnel the fish through the counter.
These systems include intelligent monitoring and real-time data processing, ensuring proper operation and publication of status and results (e.g. fish counts) on a routine basis.
The results of automatic fish counters can be supplemented, confirmed, or replaced by a number of alternative techniques, varying in accuracy, cost, complexity, and skew effects.