By Jeremiah Smith
Electrofishing has become a widely used sampling technique for detection of invasive carp throughout the Midwest. Fisheries Biologists and Technicians at the Columbia Fish and Wildlife Conservation Office have spent several years refining a new electrofishing technique that incorporates both trawling and electrofishing. The electrified Paupier (Butterfly E-Skimmer Trawl) took on newer heights as we looked to understand electrical field intensities of many different anode configurations.
Figure 1. Butterfly E-Skimmer Trawl (Paupier) with a three linear dropper array and single rearward holding hemisphere configuration.
We used a 7.32 m long X 1.83 m wide semi-v bow flat bottom boat. The hull of this particular boat was coated with a non-conductive paint. A 5.08 cm exposed strip of aluminum metal on the interior side of the trawling frames served as our cathode. The control box we used was a Wisconsin ETS MBS-1D 82-amp high-output electrofishing box with a Honda 7000i (5500 average watt output) generator.
Within this blog, I will present four different anode configurations for simplicity’s sake. The main four anode configurations were a three linear dropper array with a hemisphere (Figure 1); a three linear dropper array with two micro-arrays; a 45.7 cm insulated Wisconsin Spider array immersed .9 meters with two micro-arrays (Figure 2); and a support bridle array 3.05 m in length with two micro-arrays (Figure 4). The single hemisphere and two micro-arrays (Figure 3) are the rearward holding anodes that create an electrical net to contain fish behind the cathodic frame. For electrical field measurements in slack water, each anode configuration was pulled to its operational fishing position using non-conductive material.
Figure 2. Insulated Wisconsin Spider Array drifted to fishing set-up using a wooden dowel rod, and polyproplene rope.
Figure 3. Two micro-arrays (rearward holding arrays) are pulled taut to fishing set-up using PVC pipe and synthetic rope.
Figure 4. The red line represents the support bridle arrays length and position.
Figure 5. Diagram of the electrified paupier showing the position of our voltage gradient measuring grid and the sampling locations within the two quadrants. The darker zero line represents the cathodic frame.
Voltage gradient measurements were taken using a floating probe and a Neolithic grid as described by Luecking (2004). Our Neolithic grid was constructed of two different ropes with marked points to triangulate the needed reference point which comprised of nylon rope, color and number stamped washers zip-tied at the reference points. We measured 36 reference points within a two dimensional plane located at 45.7 cm, 91.4 cm and 182.8 cm depths (Figure 5). We sampled along six grid transects forward and aft of the cathodic frame of the longitudinal axis of the boat at 30 cm, 80 cm, 160 cm, 240 cm, 320 cm and 400 cm. Our measurements were distributed 80 cm apart within each transect and made with the floating probe. Our probe was constructed similar to that of Miranda and Kratochvil (2008); however, our floating probe was supported by a Styrofoam disc, and a 182.8 cm catfish rod with solid core wire pins set at depths of 45.7 cm, 91.4 cm and 182.8 cm (Figure 6 and 7). We painted the top half of the Styrofoam black for the negative banana plug connector (designating cathode side) and the other top half painted red for the positive banana plug connector (designating anode side). This design allowed us to make 216 measurements for each anode configuration without becoming fatigued (Figure 8).
Figure 6. Constructed voltage gradient probe made from extendable paint rod, Styrofoam, hydraulic tubing for 360 degree turning control, a lightweight rod with wire pins set at 45.7 cm, 91.4 cm and 182.8 cm depths.
Figure 7. Close view of the constructed pins along the lightweight rod. Plastic discs were used to secure pins 1 cm apart and epoxied down to secure them in place.
Figure 8. An in-water view of how our probe floats and twists 360 degrees by turning the extendable paint rod.
Figure 9. The Neolithic Grid is comprised of a color-number coated washer system to triangulate a sampling reference point.
Figure 10. The Fluke 124B Oscilloscope was used to record peak voltage readings at each sampling reference point.
With the probe connected to the Fluke 124B oscilloscope, we would rotate the probe to find and record the maximum peak voltage readings (Figure 10). For each day we ventured out to the river, we took ambient water conductivity measures so that we could standardize our electrical net to a targeted invasive carp voltage gradient threshold. We set the current to the targeted threshold using an electrofishing tool phone app which used the following formula for applied peak current to the forward anode: I = 5 x (Ca+90)/180. We checked peak current output to the operating anode zones using the General Technologies Corp CM600 Current Clamp for the four different anode configurations. These voltage gradient measurements were used to generate a field intensity contour map in R using spline interpolation. Such maps can be very useful in understanding our forward and rearward holding electrical net of the paupier.
Miranda, L. E. and M. Kratochvil. 2008. Boat electrofishing relative to anode arrangement. Transactions of the American Fisheries Society 137:1358-1362.