An Assessment of the Performance of the Resistivity Fish Counter in the Borland Lift Fish Pass at Dundreggan Dam on the River Moriston

INTRODUCTION Central to the management of salmonid fisheries in the UK is assessment of the size and age composition of adult fish returning to fresh water. Assessment is most commonly achieved by mark-recapture studies, full river trapping or by the use of fish counters. In hydroelectric installations in Scotland, the electrical resistivity counter is presently employed to count fish automatically. These counters were first described by Lethlean (1953) after extensive research for the North of Scotland HydroElectric Board (now Scottish and Southern Energy) into methods of monitoring the movements of salmon through the fish passes situated in their dams. All resistivity counters are based on the principle that a fish has a lower electrical resistance than the water in which it is swimming and when swimming over electrodes will increase conductivity between each electrode pair. The original electrode arrangement described by Lethlean consisted of three stainless steel hoops mounted on the inner surface of an electrically insulated submerged tube. This type of counter is still employed in some orifice type passes today, but has largely been superseded by an open channel array of three stainless steel electrodes located at right angles to the direction of the river flow. Such an arrangement overcomes some of the problems of the submerged tube, such as blockages by debris, but must always be sited where the water surface is relatively smooth in order to reduce the likelihood of false counts (Dunkley, 1984). The most recent resistivity counter produced by Scottish and Southern Energy, referred to as the Mk10, is derived from the designs developed by Lethlean in the 1950s, but differs in the principle of its operation (Stephen, 1998). Traditional resistivity counters were based on monitoring differences in the voltage across three electrodes produced by the presence of a fish, with the electrodes forming two pairs and the water resistance between each forming one half of a Wheatstone bridge. The direction of fish movement is given by the sequence of change in voltage amplitude. The Mk10, however, does not use a Wheatstone bridge to measure the 'out of balance voltage' produced by a fish, but instead employs a microprocessor to monitor the conductivity between electrode pairs directly several hundred times a second. As a fish swims over a given pair of electrodes it is detected by the microprocessor as an increase in the numeric measure of conductivity between that electrode pair. The direction of …