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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Final Electrofishing Field Blog in the Series

This is the sixth and final blog in the series on electrical fields. Much material has been presented on the basics of electrofishing fields, the rationale for using applied current as the electrical measure for determining field size, a description of how to determine field size and specific examples for anodes made of Wisconsin rings or spider arrays, of spheres and of loops. This blog will simplify the calculations and summarize the information in a couple of tables. First, more will be explained about the field intensity (voltage gradient) profile decay equation and what it is telling us about the field shape.

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Electrical Net Size for Loops

Prior blogs have described electrical fields from spheres and anodes such as Wisconsin rings and spider arrays. Most backpacks and push/tow barges employ anodes of a loop attached to a pole carried by the pulsator operator. A backpack loop may be a round rings (torus), a diamond or some other shape. Pointed loops allow getting into rock crevices or into brush cover where a round ring could not reach. However, the field is more intense from points versus from a round ring. Prod poles are used from boats in some situations where flooded timber or other obstructions would prove difficult to maneuver a boat with typical electrofishing booms. Prod poles are larger versions of backpack loops in both rod length and in loop diameter; they are held and maneuvered by someone on the boat bow while another person dips the stunned fish. They can be quite effective in tight spots. A caveat is that the operator is holding the anode while standing on the cathode boat hull. The human injury potential is greater than with a typical boom electrofishing boat, and fishing requires at least a three-person crew. This blog will discuss electrical fields associated with loops and estimates of their field size.

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Electrical Net Size for Spheres

A series of three blogs about electrofishing fields has just been posted. The first one dealt with basic physics of fields around spheres. Let’s now build upon that one in this blog. Time to review some formulas.

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Electrical Net Size: A Two-Boom Boat Example

This information is from an electrofishing workshop held at Table Rock Lake, Missouri in June 2012 for the Missouri Department of Conservation. The aluminum electrofishing boat was 16 ft (4.9 m) long, and its hull was used as the cathode. There were two booms, each with Wisconsin rings of 83 cm diameter, and each ring had 11 metal droppers 22 cm long x 1.3 cm diameter. Distance from the center of the booms to the nearest boat hull waterline was approximately 250 cm.

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How Big is Your Electrical Net?

The featured image is an electrical field intensity map for a Missouri Department of Conservation stream electrofishing boat. The boat is depicted as the white area; the two anode array fields are shown in red. Many thanks to Andy Turner of MDC for providing this graphic.

In electrofishing classes, Alan Temple often uses the term electrical net when discussing standardizing by power. The analogy is that a gill net, for example, can be of a fixed size – length, height, bar mesh – and construction and can be deployed the same way for standardized fishing. When we standardize by power in electrofishing, the objective is to produce the same size effective fishing zone for any water conductivity. That requires adjustments to the applied voltage, current and power in waters of different conductivity so that the same electrical power density in the water enters the fish and causes the desired fish capture response. But how large is the electrical net? This blog presents a method of calculating the size of the electrical net based on hypothetical but realistic values for a typical two-boom electrofishing boat with the boat hull as the cathode and with either Wisconsin rings or spider arrays for the anodes. Be aware; there will be formulas and calculations. Hang on, I think it will be worth it.

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Rationale for Using Applied Current for Standardizing Electrical Fields

The featured image is a representation of an electrical field. It is from the Smith-Root, Inc. GPP manual and is used with permission from SRI.

Prior blogs have mentioned and discussed electrical fields including their measurement and how to visualize them. Let this be the first in a series of blogs which delve deeper into the topic of electrical fields and provide useful equations for describing and predicting field intensity and field size. The primary purpose of this blog is to explain the rationale for using applied current – instead of voltage or power – for standardization across water conductivity.

Electrofishing is the use of electricity to capture fish. This is accomplished by generating an electrical field in water to produce in fish a capture-prone response such as forced swimming (including taxis or attraction to the anode), inhibited swimming or immobilization. According to the power transfer theory of electrofishing, a threshold level of electrical power must be transferred from the water to the fish to produce such a response in the fish (Kolz 1989). The power measure in the water and in fish is termed power density in μW/cc (= μW/cm3) or microwatts per cubic centimeter.

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Instructional Video Links

There are a number of useful instructional videos housed on a server at the National Conservation Training Center (U.S. Fish & Wildlife Service).  The videos, while still valid, are a few years old now, and likely will be updated or supplemented in the next year or two.

Some of these videos are the same as those housed in Vimeo with links listed elsewhere on  Depending upon your connection, the Vimeo versions may have better resolution.  However, to access videos on this list, you don’t need an account or password.

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