A primary aim of electrofishing is to produce an electrical field in the water of sufficient intensity to enable the capture of fish within the field. The field intensity is highest near the electrodes and decreases with distance from the electrodes. Miranda and Kratochvil (2008; TAFS 137:1358-1362) used a floating grid around the anode arrays of an electrofishing boat to measure field intensity, or voltage gradient (V/cm), in x,y coordinates so that a map of the field intensity could be constructed. This blog includes graphs which show the effect on the field of changing the distance between the anode arrays. What is new from the article is the use of color graphs made using R code for spline interpolation.
The boat was a flat-bottom aluminum one 5.5 m long x 1.8 m wide. The boat hull served as the cathode, and anode arrays were suspended from a boom attached to the hull. Each array contained six droppers of 0.5 cm stock diameter cables suspended equal distance around a 0.9 m diameter ring. Each dropper was 1.0 m long and immersed 0.8 m. Voltage gradient measurements were made at 30 cm depth using a voltage gradient probe connected to an oscilloscope.
The grid was used to locate the voltage gradient probe at 36 known positions around the anode arrays. Anode arrays were arranged in four configurations: a single array and double arrays spaced 1.3, 1.9 and 3.2 meters apart at the distal ends of the booms. The black and white voltage gradient maps in Figure 2 were made using a kriging method. New color graphs were made using R code for spline interpolation originally written by Jahn Kallis of the FWS Columbia Fish and Wildlife Conservation Office in Columbia, Missouri. The data were graciously furnished by Steve Miranda of the USGS Mississippi Cooperative Fish and Wildlife Research Unit, Mississippi State, Mississippi.
For the maps, the applied power was kept nearly constant. The anode array arrangement affected the resistance, so the voltage and current differed for each configuration. Thus, focus on the pattern of the field intensity contours and not on the absolute values.
Position of the x,y coordinates on the floating grid for voltage gradient mapping by Miranda and Kratochvil (2008).
Voltage gradient (V/cm) contours for the four anode array configurations of Miranda and Kratochvil (2008). The contours were created by the kriging method from the voltage gradient data.
Spline interpolation graph of the voltage gradient contours for the single anode array. The array was centered on the boat midline at about 2.8 m from the bow waterline, i.e. from the nearest point of the cathode. The red area indicates the highest field intensity, and the bright green could indicate the immobilization field intensity, depending upon the electrical current applied to the array. The bright green on the lower part of the graph (near the boat hull) indicates a relatively strong cathodic field when using a single anode array; this can cause fish immobilization or inhibited swimming near the boat hull and thus out of the reach of the fish dipper on the bow. The result can be a lowered fish capture effectiveness.
With a boom separation of only 1.3 m, there is a strong interaction of the fields between the two anode arrays. This is called field coupling and usually is to be avoided for typical boat electrofishing.
With a 1.9 meter boom separation, the fields from the two anode arrays were electrically separated, or isolated, but they were close to one another and should result in a wide fish catching area fore of the bow where the fish dippers can operate effectively, resulting in successful fish capture. This anode configuration was considered the most ideal of all four tested by Miranda and Kratochvil (2008).
The 3.2 m boom separation configuration also resulted in anode field isolation. They may have been isolated too much; this resulted in an area between the arrays of such low field intensity that fish could possibly recover and escape.
Both types of electrofishing field intensity graphs are informative. The color graphs may indicate effective fishing zones around the anode arrays in a way that is easily and quickly understood. Thanks to Jahn Kallis and to Steve Miranda for making these graphs and this blog possible.