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 pass through embedded electrodes that disrupt the resistivity field near them, changing inter-electrode resistance. Using three electrodes, these disturbances can be detected by measuring with a Wheatstone bridge or other methods to determine the fish's travel size and direction.