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.
As fish cross this barrier, they encounter embedded electrodes that cause a difference in resistivity, which affects the field around the electrodes, altering inter-electrode resistance. With three electrodes, these changes can be measured using a Wheatstone bridge or similar techniques to assess the size and direction of the fish's movement.