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. I selected two minnow traps of similar size characteristics but made of different material (Table 1). The Gee-Feets G-40 trap is made of bare galvanized steel mesh, therefore a Faraday cage and the Eagle Claw trap is constructed of “coated metal”; it has no exposed metal and the coating is a smooth, non-conductive, black plastic (Figure 1).

Table 1. Size measurements of the Gee and Eagle Claw traps.


Measurement                                      Gee trap                                  Eagle Claw trap


Length (cm)                                         40.5                                         41.5

Mid-diameter (cm)                              22.5                                         22.5

End diameter (cm)                               19.0                                         17.7

Cone length (cm)                                 11.4                                         8.6

Cone opening (cm)                              2.2                                           2.5

Mesh size (mm)                                  5×5 square                               8×11 diamond


Figure 1. The Gee-Feets G-40 trap (left) and Eagle Claw trap (right).

I conducted the test in a 100-L cooler equipped with two electrodes (parallel metal plates) 59 cm apart covering a cross section of water 38 cm wide and 30 cm deep. I applied 120 V RMS AC to the electrodes to produce a homogenous electric field with an expected voltage gradient of 2.0 V/cm (120 V/59 cm). I created a voltage profile of the water volume (Figure 2) by attaching one lead of a digital multi-meter to one electrode and using an insulated wire with an exposed tip (1 mm diameter, 2 mm length) to probe the water to mid-depth along the center line at 5-cm intervals. The resulting profile was a straight line with near-perfect correlation (r = 0.999), an intercept of 0.587 and a slope of 2.015, confirming the expected voltage gradient.

Figure 2. Voltage profiles for control, Gee trap and Eagle Claw trap. Trap measurements refer to the Gee trap.

The voltage profiles through the centers of the two traps were quite different (Figure 2). The profile of the Eagle Claw trap was essentially identical to the profile in the empty cooler (r = 0.999, intercept = 1.439 and slope = 1.977), indicating that the trap had almost no effect on the electric field. A fish inside the trap would experience a 2.0 V/cm gradient. The Gee trap behaved as a Faraday cage, producing a flat profile (zero V/cm) inside the trap between the openings of the two cone-shaped entrances; slight variations in voltage were due to measurement error. Voltage gradients were high (>5 V/cm) outside either end of the trap and decreased inside the cone entrance. Just inside the cone entrance, voltage gradient was 1.4-1.6 V/cm and halfway inside the gradient was 0.5-0.7 V/cm.

My test indicates that a metal-mesh minnow trap, like the Gee trap, would act as a Faraday cage and produce an interior voltage gradient of zero V/cm; fish inside the trap would be protected from an electric field. However, just outside the trap entrances, voltage gradient is higher that expected (>5 V/cm) because the Gee trap acts like a highly-resistant cylinder in the electric field, compressing the voltage lines near the trap entrances, thus increasing voltage gradient. Traps made of non-conductive material, like the Eagle Claw trap, would provide no such protection.

To further test the theory, I used facilities at the Nampa Hatchery of the Idaho Department of Fish and Game where the raceway water was 18.8 C with an ambient conductivity of 547 µS/cm. I exposed Rainbow Trout fingerlings (11-13 cm) to pulsed DC (25-Hz, 25% duty cycle) in the same cooler. Three groups of six fingerlings were separately given one of three treatments by being held in the cooler (1) with no trap, (2) inside the Eagle Claw trap, and (3) inside the Gee trap. The voltage was increased steadily at about 3 V/s until all fish in each group were immobilized (no swimming, loss of equilibrium) resulting in a threshold voltage value. Videos (below) of fish in the control group and inside the Eagle Claw trap showed similar results; both groups were immobilized at about 50 V or 0.8 V/cm. However, when voltage was increased to a maximum 100 V (1.7 V/cm), fish in the Gee trap were not immobilized. They began agitated swimming at 70-80 V and continued doing so up to 100 V. As voltage increased, the Faraday effect diminished causing the agitated swimming. The Gee trap acted as a Faraday cage, shielding fish from the field outside the trap within limits of voltage.

Thanks to Dylon Kieffer, Nampa Hatchery, for assistance with video of the fish tests.








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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Size Matters

Yes, size matters…and that includes fish size when electrofishing. Large fish are immobilized with less field intensity or power density than are small fish.  Large fish sustain a higher total dose of electrical energy than do small fish; this is sometimes referred to as whole body voltage. An excellent paper on this topic is Dolan, C.R. and L.E. Miranda. 2003. Immobilization thresholds of electrofishing relative to fish size. Transactions of the American Fisheries Society 132:969-976. This short blog provides results of a simple study with various sizes of alligator gar.

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