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Estimating Electrofishing Thresholds…Without Fish??

Electrofishing thresholds are the minimum settings (volts, watts, amps) needed for successful fishing. We teach biologists to aim for thresholds so that they can acquire the samples they need for research or for management and yet avoid negative impacts on the fish or other aquatic organisms which could be affected. Normally, we help develop conservative goal settings for a given situation and ask biologists to begin there and to make minor changes while fishing so as to determine those thresholds. But is there another way to estimate such thresholds? This blog explores an attempt at estimating electrofishing thresholds using electrical measurements made at the boat ramp.

Dr. Jim Reynolds invited me to help him evaluate the Electrofishing Project of the Upper Colorado River Endangered Fish Recovery Program (UCR-EFRP or Program). We went to Grand Junction, Colorado in August 2014 to put on a workshop to evaluate their electrofishing protocol, specifically their applied current goals. We tested four rafts and five boats both in a 65 ha (160 acre) recreational lake, Highline Lake (shown above), and in the Colorado River. This blog will only cover a portion of the work at Highline Lake. There, we made resistance measurements for each vessel, and the crews determined fishing thresholds for each.

Each electrofishing vessel was powered with a generator connected to an ETS Electrofishing MBS 1D 72 amp pulsator. The standard waveform was a 60 Hz, 20% duty cycle square-wave pulsed direct current. Anodes were 23 cm (9 inch) diameter metal spheres suspended so that they were half immersed. Rafts had one anode sphere suspended from a single boom forward of the bow whereas boats had two identical spheres, each suspended from its boom. Cathodes were three (four in one case) stainless steel droppers. The goal of the Program had been to standardize the rafts and boats so that they would fish similarly.


Metal spheres used on rafts and boats.

Also at Highline Lake, we took abbreviated voltage gradient (V/cm) profiles for each vessel. Time was limited, so we only measured voltage gradients at 70, 90 and 120 cm from the center of the anode spheres. We made those measurements fore, aft and lateral to the anodes for boats but only fore and lateral for the rafts because the short distance from the bow to the anode for rafts would not allow an aft measurement. For this blog, we are only reporting the lateral voltage gradient measurements. For other, typical, electrofishing boats, the voltage gradient at about 90 cm from the anode center is key, so the distances chosen here bracketed 90 cm. Conditions at the boat ramp that day were not ideal for measuring voltage gradients. Vessels were being launched and retrieved, and recreational boating was causing wave action at the boat ramp. The half immersed spheres were bouncing vertically, and the voltage gradient readings were not steady. This seemed to be more of a problem later in the day and especially for the rafts which had only one anode sphere. An attempt was made to fit a power regression to the three voltage gradient measurements so as to estimate the 90 cm reading.

VG profile


Example of actual voltage gradient profile for a vessel.

We used Miranda’s value of 60 μW/cc for game fish at match, i.e. at 115 μS/cm. That equates to 0.72 V/cm at match. The ambient conductivity at Highline Lake was 750 μS/cm, so the target voltage gradient at 90 cm lateral to the sphere center was 0.42 V/cm. We used the actual voltage gradient readings at the applied voltage, plus the resistance measurements, to estimate the fishing threshold settings of voltage, current and power based solely on physical measurements. Only the current thresholds will be shown here; they were more consistent than were the voltage and power thresholds. The current thresholds from the physical measurements for each vessel were compared to the corresponding thresholds made by each crew based on their observed fishing success. I calculated the threshold currents based on the physical measurements, and Jim independently calculated the threshold currents based on the fishing results. One was objective and made without any fish; the other was based on the subjective thoughts of fishing success by each crew. So how did the results from the two methods compare?

Raft Boat current jpg

Comparison of threshold current goals for the four rafts (on left) and for the five boats. Thresholds from physical measurements are open red squares for rafts and closed blue triangles for boats. Corresponding thresholds from fishing results are closed black circles. Note that boats had two anode spheres whereas rafts had one anode sphere. Thus, the applied current should be approximately double for the boats with two spheres, and it is.

The overall purpose of these measurements was to evaluate whether target threshold power settings (using current as the metric in this case) could be estimated solely from physical measurements, i.e. before going out fishing. Those estimates were then compared with actual estimates based on fishing. By observing the last graph, you can see that the electrical and fishing result thresholds agreed quite well for the five boats. There was more variation between the estimates for the rafts. As I recall, the voltage gradients for the rafts were done later in the day when the activity at the boat ramp was higher and the wave action more pronounced. Also, rafts only had one sphere, so it is likely that the voltage drop for the one sphere, which was suspended so as to be half immersed, fluctuated more in the wave action at the boat ramp than did the same for the boats with two spheres. Perhaps having two spheres in parallel results in a more stable voltage drop at the anode when there is wave action because one sphere may be bouncing up while the other is sinking down in the wave; thus, one is immersed more than the other at any given instant, so perhaps there is a compensating effect. In any event, the voltage gradient readings were more stable when the water at the boat ramp was calm. It is interesting to note that the variation among estimates for rafts using the objective voltage gradient approach was less than for the subjective fishing approach.

This exercise was done to answer the question of whether voltage gradient measurements could be used to closely estimate initial pulsator settings before going out fishing. For me, the answer appears to be yes. Voltage gradients are independent of water conductivity, so if you can measure the voltage gradient profile for a given electrode configuration once, then you can use that profile to predict fishing settings needed for another water conductivity situation. I won’t take space here to fully explain the procedure, but it is evident to me that it can be done. I appreciate the patience of the workshop crews as these measurements were made.

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© 2015 Thread One Page. Imagery: Tom Rayner, Alan temple, Richard Pearson, Paul Godfrey, Roger Scott, John Rayner.