Beyond Infinity with Voltage Gradients!

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.

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Are Captured Fish in Minnow Traps Safe from Electroshock?

While instructing an electrofishing short course in early 2017, I was asked if electrofishing near minnow traps containing fish would be harmful to them. When I asked “Why do that?”, I was told that electrofishing and trapping crews, in this case, work separately but in the same areas and often at the same time. To the question I said, “It depends on the material of construction. Minnow traps made of metal mesh are Faraday cages, but those made of non-metals are not. A Faraday cage in an electric field should protect the fish because there would be no voltage gradient (change in voltage over distance) inside.” If you were in a car struck by lightning, it’s not the tires that offer protection; it’s the metal shell you’re in. Even though a metal-mesh trap has holes in it, the mesh, if small enough, would divert the field over the trap exterior.

However, my curiosity got the best of me and I decided to test the theory. Continue Reading..

Electrical Field Graphs for Electrofishing

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.

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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.

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Electrode Resistance: How Important is Surface Area?

In early 2016, I published a paper, “Spheres, rings and rods in electrofishing: Their effects on system resistance and electrical fields” (Transactions of the American Fisheries Society 145:239-248, 2016). My aim was to elucidate the relative importance of size and shape of common electrodes in determining electrical resistance of electrofishing systems and the intensity and size of the electrical fields they produce. In that paper, I did not cover the relationship of electrode surface area to resistance; instead, I am reporting that information in this blog.

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Grass Carp Effective Conductivity – Part A

This blog is being presented it two parts. Part A involves the lab trials to determine Grass Carp effective conductivity, Cf, and power density at match, Dm. Refer to prior blogs at this site on the power transfer theory and on lab experiments for more information about terms, setup and procedures.

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Electrical Fields from Model Anode Arrays

Electrical fields around electrofishing anodes are critical to fish capture effectiveness. The size, shape and intensity of those electrical fields are determined by the anode design and deployment in the water as well as by the electricity applied to them.  There were two main questions to answer in this little study: (1) Could accurate electrical measurements be made from approximately ¼ scale model electrodes in a small body of water, and (2) Would those measurements provide useful information about the effect of anode ring size on their electrical fields?

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Pulse Shape Affects Fish Immobilization Threshold

Modern electrofishing pulsator (control box) manufacturers as a whole produce a variety of direct current pulse shapes. Which of these are more effective or more efficient for fish capture? We have noticed some differences in fish reaction thresholds and overall behavior when exposed to different pulse. For years, I have wanted to compare various pulses under controlled laboratory conditions. The challenge has been acquiring a suitable power supply that can produce the desired pulse shapes. That opportunity recently became available.

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Power Transfer Theory of Electrofishing, in a Nutshell

Fisheries biologists have known for a long time that many factors affect fishing success. The most important environmental factor is the conductivity of the water, i.e. its ability to conduct an electrical current due to the concentration of ions in the water. Water conductivity has been used as independent variables in multiple regression equations or as covariates to estimate catch per unit effort or some measure of capture efficiency. For decades, biologists made equipment adjustments to compensate for varying water conductivity in an ad hoc fashion without a guiding principle.

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Procedures for Lab Experiments in Tanks

Alan Temple wrote a blog, Setting Doses for Lab Experiments, which I followed with Setup for Lab Experiments in Tanks. That was followed by a short one, Size Matters, on the effect of fish size on the threshold voltage gradient and power density for immobilization or other responses. This blog discusses some aspects of how a tank study is conducted. Specifically mentioned are the fish themselves, the desired response to be assessed, how that response is to be evaluated, and two primary approaches for quantifying the results.

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