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Challenges of Conductive Substrates

Sometimes, things just don’t work as expected. You go to a new sampling site, determine ambient water conductivity, estimate a good starting point for standardizing by power and conduct a quick pretrial to fine tune the voltage, current or power levels for successful fishing.  But, either the starting point is incorrect, or the sampling begins successfully and then something changes to reduce effectiveness.  Below are three examples of unusual circumstances in electrofishing.  Two of these were my direct experiences; the other was reported to me.

The first example was at Whitney Lake on the Tensas National Wildlife Refuge in Northeast Louisiana. It is a small pond used for public fishing and can be especially popular for kid’s fishing events. It had rained immediately before we got to the pond, so just getting into the site was a challenge. We had to disconnect the boat trailer from the truck and use a four-wheeler all-terrain vehicle to launch the boat. The small aluminum boat used a 6500 watt generator and an ETS MBS boat electrofisher. Ambient water conductivity was 330 µS/cm, so the initial setting was for 21 amps total current.  The water was turbid, muddy, from the recent rain, so we set the duty cycle to 40% in the hopes of producing some fish attraction to the anode for effective capture. That was successful at first while in the lower part of the pond where the water was deeper. When we moved to the upper end of the pond, which was very shallow, the situation changed. The outboard motor began stirring the bottom mud, and the pulsator began to overload and shut off. The first time or two, I wasn’t sure what was happening. Had I bumped a switch or something? What was causing it to shut off? Then I realized that the bottom muds in the shallow upper end of the pond must be highly conductive so as to cause a reduced electrical resistance and therefore a suddenly large current draw for the applied voltage. The water was so shallow that we had to trim the outboard motor up to move around in much of the upper end of the pond. We had driven a few hours to be there, and we needed to get the sample that day. Therefore, the duty cycle was decreased from 40% to 20% which cut the average current in half while retaining the same peak voltage, current and power for fish response. That allowed the generator and pulsator to function even in the situation of a highly conductive pond bottom, and we were able to capture the fish needed to assess the pond’s fish community.

The next situation was at a pond at a sports complex in Pineville, Louisiana. We were asked by someone from the Louisiana Department of Wildlife and Fisheries to help with a fish project at that pond. It just so happened that Dr. Alan Temple of the Fish and Wildlife Service National Conservation Training Center in West Virginia had come to Louisiana so that we could work together on a scheduled revision to the Department of Interior electrofishing safety policy. The area had significant rainfall just prior to our going to the site. Yes, more rainfall. It rains a lot in Louisiana. The water level had risen substantially such that typical shoreline vegetation was flooded. The ambient conductivity was lower than we were expecting for ponds in that area; it was only 36 µS/cm. We were using an ETS ABP-3 backpack electrofisher with a diamond anode and a rattail cathode. We set the voltage to provide the current and power that we thought should have provided successful fishing based on that low conductivity. Alan wore the backpack and began fishing but was having poor success so he kept asking for more voltage. The effective field size still seemed too small, and fishing success was poor. Fortunately, we had brought another ring which was a cathode. We switched from the rattail cathode to the ring cathode. That seemed to improve the fishing field size and allowed us to continue fishing with some success. We did notice that the current draw increased when the rings touched the substrate. Initially, we started with a 40% duty cycle to increase the likelihood of fish attraction to the anode for capture. The water was turbid, muddy, and much of the shallow shocking area around the pond was flooded vegetation, so we wanted to draw fish from the cover and from the turbid water to the anode for capture. Before we were finished, we had consumed one of the two batteries, so we switched from the 40% to a 20% duty cycle to continue fishing while reducing the drain on the last battery.

Here is what we surmised was happening in that pond. We are not sure about the watershed for that pond, especially of incoming streams, but the heavy rain event raised the water level substantially and quickly. Evidently, the rain water lowered the pond water conductivity relative to the highly organic and conductive bottom mud. Our guess is that the electrical current was flowing mostly vertically between the ring electrodes and the conductive substrate rather than the typical pattern of mostly horizontal flow around the electrodes, where the fish are. Thus, the effective fishing field size was reduced severely, and that substantially reduced the fish catch at the expected power level based just upon water conductivity. To compensate for the reduced effective field size, we had to increase the voltage much higher than expected.  That allowed some fishing success but drained the batteries quickly. The calculated resistance for the backpack electrodes was half of what we would have expected at that water conductivity. The low resistance must have been due to the conductive substrate.

The next situation was reported to me by Anthony Strokoff and Matt Kulp based on sampling a stream in the Great Smoky Mountains National Park of Eastern Tennessee. The streams there typically have low ambient conductivity (20 µS/cm or less). Fish sampling is done with Appalachian Aquatics or Aquashocks Solutions backpack electrofishers using two rings for electrodes and operated on 60 Hz alternating current. There is one place in the stream where the current draw on the electrofishers increases substantially. The issue is that electrofishing efficiency diminishes as the units tend to overload when they approach that section of the stream, and that allows fish escapement.  Water depths in the general area may be 0.2 to 1 meter, but they sometimes are only about 15 cm, and the ring electrodes contact the rocky substrate. The bedrock in the area is known to have iron deposits. For instance, there is high iron sulfide in the pyritic shales. There may be other metal deposits in the bedrock, too. It has been mentioned that there may be tin and aluminum in the area, but at least the iron deposits are evident.  To make it through that area of the stream, the biologists adjust the voltage down and then back up some so as to lower power output, but that is often inconsistent. The rocks containing metal are acting to short-circuit the electrical current which should be flowing through the water and the fish.

In all three cases mentioned in this blog, it appears that the substrate had a substantial negative effect on fish sampling because the substrate was more conductive than the water and fish. The electrical current will follow the path of least resistance. If more current flows to and through the substrate to complete the circuit, then less current is available to attract or immobilize fish, and the effective fishing field size may be substantially reduced. The result is lower fish capture efficiency and a deviation from the concept of standardizing by power.  This blog mentions those challenges and some actions which allowed successful fishing despite conductive substrates.

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