Comparison of In-Water Voltage Gradients Produced by Electrofishing Boats

The voltage gradients of electric fields produced by electrofishing boats are important in determining sampling efficiency and the potential for injuring fish. We evaluated 10 electrofishing boats and found that 3 boats had malfunctions that could impact sampling or operator safety. The in-water voltage gradients were measured for the remaining 7 boats to make comparisons among boats and to determine the voltage gradients present during electrofishing. For all boats evaluated, the cathode was the aluminum boat hull, and the 2 anode arrays each consisted of 3-11 droppers (cables, chains, or rods; 0.6-1.2 m long) suspended from a boom in front of the boat. A grid (1.5 x 2.0 m) was attached to the anode support booms between the anodes and the bow of the boat; this grid facilitated measurements of voltage gradients in the portion of the electric field where most fish are captured. For 9 locations defined by the grid and for 3 water depths (0.1, 0.5, and 1.0 m), a voltage gradient vector was calculated from the horizontal and vertical voltage gradients measured with a probe connected to an oscilloscope. With applied voltages of 900-1000 V, the mean voltage gradient for sampling locations within 1 m of the bow was 2.6 V/cm (SE, 0.1); means for individual boats ranged from 2.1 to 3.4 V/cm. In addition to measurements at locations defined by the grid, maximum voltage gradients of 16-20 V/cm were measured within 5 cm of anode droppers. Despite differences in equipment, the electrofishing boats produced electric fields with similar voltage gradients when measured at similar locations relative to the electrodes.

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