Overall resistance of an electrofishing system is determined by the combination of ambient water conductivity and the electrodes. The electrode shape, number, length or size, diameter of stock material, configuration and spacing affect both their overall resistance and the profile of the electrical field in the water around the electrodes. Rarely are there data available for assessing the effects of electrode changes on resistance or on fish catch. This blog presents some empirical results from field measurements of electrode resistance before and after some electrode changes.
Brad Vollmar operates Vollmar Pond and Lake Management out of Fredericksburg, Texas USA. He conducts fish surveys with an electrofishing boat, and he wanted to improve its performance, so he contacted me in 2012 about making some electrode changes. We have never met, but Brad has been willing to make incremental changes to his boat electrodes and to provide the resistance results at each step. Those results are summarized in a table below.
Brad’s boat is a fairly small (4.3 x 1.2 meter) flat-bottom aluminum jonboat. That size works well for sampling small ponds (2 hectares or less) in his area; it allows good maneuverability in and out of the water. The cathode is the boat hull, which was painted when we started the process of changing electrodes. He did all of the work; I just gave some recommendations. The anodes are droppers suspended from a T-bar fore of the bow. The T-bar is 166 cm wide with six droppers equally spaced 33 cm apart on center. The anode droppers enter the water 2 meters fore of the bow. A set of cathode droppers, called a cathode skirt, was added to the bow to decrease resistance and to increase the percent of power to the anode. The anode droppers were of 6.4 mm dia stainless steel; the cathode droppers were of 6.4 mm dia aluminum.
According to Ohm’s Law, resistance is the ratio of voltage to current. Thus, ambient system resistance in ohms is measured by dividing volts by amps. Brad’s boat is powered by a generator connected to an ETS MBS pulsator. That is fortunate because the MBS pulsator has excellent metering for output peak voltage and peak current, so the resistance measurements were easy for Brad to obtain. He measured specific conductivity and temperature with an Extech EC 500 conductivity meter. It has an automatic temperature compensation of 2% per C and a specific temperature of 25 C, so it was easy to calculate ambient conductivity from the displayed values.
The data below are from multiple ponds or lakes in which Brad was conducting fish sampling. These are actual field results measured with care but with varying conditions of conductivity and wave action, the latter which can affect electrode immersion depth and thus the accuracy of voltage and current measurements. These data were not taken under strictly controlled artificial conditions. The disadvantage is that the data may vary somewhat in uncontrolled or confounded ways. The advantage is that these are actual field measurements which need no extrapolation to the real world.
The anodes include six droppers of varying lengths exposed to the water. These immersion depths were controlled by partially covering the droppers with non-conductive sleeves which could be adjusted up and down as desired. The actual exposure or immersion length is given in the table as 109 or 61 cm, etc.
The R100 values are derived from the overall system resistance (volts/amps) in ambient conditions then adjusted to resistance at 100 uS/cm based on the ambient water conductivity. R100 values are used to compare electrode resistance before and after electrode changes, regardless of the ambient water conductivity where the measurements were made. The last three R100 values in the table were based on an assumption because we lacked an accurate measure of ambient conductivity that day.
The %Anode is an estimate of the percent of power to the anode. Power is distributed to both the anode and cathode based on the voltage drop across each, and that is a function of the relative resistances of anode and cathode. My experience is that a power distribution to the anode of about 60 to 75 percent is desirable. A low (30-40%) percent to the anode means that more power is going to the cathode, an undesirable situation. The preferred fish catching point is near the anode, for a couple of reasons. It is easier for a dipper on the boat bow to catch fish in a concentrated area near the anode, and some waveforms produce a fish attraction to the anode. Thus, one desires most of the power at the anode for typical electrofishing. The calculation of percent power to the anode is typically done by measuring volts and amps with two anode booms (two anode arrays used) and also with only one boom anode array used. Those results are used in a simple formula to estimate percent power to the anode. The technique used here was different because there was only one anode boom. Resistance measurements were made using all six anode droppers and again when using five anode droppers. The estimate of percent power to the anode may be less reliable than for the typical two-boom, one-boom situation, but they appear to indicate the expected pattern of power to the anode after making electrode changes.
So, what do these results tell us about the effect of changing electrodes? First, the length of anode droppers is very important. Shorter anode droppers (less length exposed to the water) result in increased overall resistance. For instance, the R100 value increased from 37 to 52 ohms when the anode dropper length was reduced from 109 to 61 cm.
Secondly, shorter anode droppers result in more percent power to the anode. Shortening the anode droppers from 56 to 38 cm caused a major increase in percent power to the anode; the estimated percentage value increased from 31% to 62%. Note that the R100 changed very little, only from 60 to 58 ohms. Evidently, the effect of shortening anode dropper length on resistance is not a simple linear relationship. Relatively larger decreases in overall resistance are associated with reductions down to about 60 cm of anode dropper length, with smaller overall resistance changes thereafter. It appears that the major effect of further shortening anode dropper length is the increased percent of power to the anode.
Adding a set of cathode droppers at the bow, called a cathode skirt, resulted in little if any change to the overall resistance, but the estimated percent power to the anode increased to 73-75%. Shortening the cathode droppers from 88 to 46 cm had no appreciable effect on resistance or percent power to the anode.
The effect of removing paint from the boat hull was difficult to assess because the anode dropper length was decreased somewhat at the same time. The R100 value remained basically unchanged (54 vs 52 ohms) when both electrode changes were made. Shortening the anode droppers from 61 to 52 cm should have resulted in somewhat higher resistance, whereas removing the paint from the hull should have resulted in lower resistance. Likely, these two electrode changes cancelled each other to some degree.
Overall, it appears that shortening the anode droppers, and possibly removing the paint from the hull, had beneficial effects on overall resistance and on the power distribution to the anode. However, removing paint from the boat hull apparently had less of a positive effect on the power distribution than expected based on the low (31%) percent power to the anode upon first estimate. Brad was pleased with the effect on his catch effectiveness after the electrode changes. Even though the cathode skirt increased the percent of power to the anode (from 62 to 73-75%), he considered the skirt unnecessary and removed it after the tests.
These results document actual measures or estimates of resistance parameters as they changed due to modification of the electrodes for an electrofishing boat under actual field conditions. It is rare to have such a data set. I hope you find these results useful for your electrode design changes. Many thanks to Brad for compiling all of this information.