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.

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Measurement                                      Gee trap                                  Eagle Claw trap

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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

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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.

VIDEO: CONTROL FISH

 

VIDEO: EAGLE CLAW TRAP

 

VIDEO: GEE TRAP

 

 

Course Announcement: Principles & Techniques of Electrofishing

Opportunity knocks! The National Conservation Training Center (U.S. Fish & Wildlife Service) is offering an electrofishing course in Carterville, Illinois during October 23 – 27, 2017.  Please see course description, course flier, and registration process below.

Course Description

Often electrofishing sampling is unsatisfactory (low effectiveness, high variance) due to reasons that include equipment limitations, insufficient understanding of equipment function, inadequate electrode design, and a lack of guidance regarding proper settings given prevailing water conditions and target species.  This class addresses these factors and builds skills in participants that will enable them to tackle sampling issues and increase the efficiency and standardization of electrofishing.

In addition, participants learn how to evaluate gear performance and select suitable equipment, trouble-shoot equipment, assess likelihood of fish injury and use approaches to minimize the potential for stress and injury, and provide a safer operating environment for their crews.

This course covers all types of electrofishing gear types including boats, rafts, tow-barges, shore-based, backpack, electric seine, and pre-positioned.

Participants are encouraged to bring their equipment for evaluation which includes analysis of outputs, calibration check, electrode design, and a safety workup. Gear also is used for standardization exercises.

Course Flier:

Electrofishing Training Announcement Illinois Fall_2017

Registration process for non-Department of Interior Biologists:

doi-learn-request-account-and-register-for-class-electrofishing

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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Two Backpack Electrofishing Questions Answered

Alan Temple and I taught an electrofishing course in Ft. Collins, Colorado last October. The course was unusual in that there were only backpack electrofishers, so that allowed us to have two field trips, in different habitats, using only backpacks. That permitted us to examine two questions – one pertinent to electrofishing in general, and one more specific to backpack units. This blog examines both questions and what we found.

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Field Tool Kit

 

Things happen in the field, particularly when we are using complicated sampling gears (= electrofishing equipment).  Malfunctions can occur in the electrodes, branch circuits (conductors running from the control box or power source to the electrodes), the power source, or the control box.  Biologists build a lot of their own equipment, wiring boats, electric seines, and the like.  From what I’ve seen, most problems appear in our wiring or construction, but generator problems, and sometimes control box issues do occur.  Fisheries biologists are a resourceful lot.  If given direction, they usually can diagnose the problem at hand and get back to sampling.  With this in mind, Midwest Lake Electrofishing Systems, based in Polo, Missouri, has developed a list of items for a basic field tool kit (see below).  Although the intent is a tool kit for electrofishing boats, this kit applies to any electrofishing gear type.

Many thanks to the crew at MLES.

suggested-basic-field-tool-kit-for-an-electrofishing-boat-11_2016

To Kill a Fish Egg

How much electricity does it take to kill a fish egg, or unhatched embryo? We have mentioned that in classes but have only discussed it in generalities. The purpose of this blog is to look more closely at the question and to quantify it as best we can with the limited information available. It makes sense to look at this from the perspective of egg, or embryo, diameter. Bohl et al. (2010), Electroshock-induced mortality in freshwater fish embryos increases with embryo diameter: a model based on results from 10 species, Journal of Fish Biology 76:975-986 is the source of information for this investigation.

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New Boat Pulsator seen at Kansas City AFS Meeting

While attending the Annual Meeting of the American Fisheries Society in Kansas City in August, I visited the vendors, especially the electrofisher manufacturers. It is good to visit with those I know and to see new products. I don’t attend a lot of such meetings, but this was the first time for me to see a booth by ETS Electrofishing Systems LLC.  Burke O’Neal of ETS Electrofishing retired and sold the company to his sons.  Mark O’Neal now operates the company, which had a slight name change, and moved it to Madison Wisconsin. Burke and I never met, though we had corresponded by email and had talked on the phone over the years. We even collaborated on some testing of voltage gradient probes and of backpack electrofisher anodes.  In August, it was my privilege to meet Mark.

ETS now manufacturers an 82 peak amp, high-conductivity version called the MBS-82. It is an upgrade from their former 72-amp high-conductivity version and also has a larger internal circuit breaker. The 82-amp version has been their standard high-conductivity model since August 2015. The new development is a high-voltage version for use in lower conductivity water. This new version, which was in beta format for the AFS meeting, has three voltage ranges – 1000, 600 and 300 volts – to cover a very wide range of water conductivity.  The expected current outputs were reported as 30, 40 and 82 peak amps for the 1000, 600 and 300 volt ranges, respectively. While advertised performance is somewhat informative, I wanted to see actual performance data. So, I talked Mark into doing some extensive testing of his new unit over a wide range of resistance to simulate a wide range of water conductivity. This blog shows those results.

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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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Electrofishing and Pacemakers: A Personal Experience

On behalf of a fish biologist who wishes to remain anonymous, we are publishing this blog. The biologist has an implanted heart pacemaker and faced concerns about participation in electrofishing operations. The text has been authored in first person and lightly edited by us with approval by the author. We believe that this factual investigation will provide useful information to employees and supervisors alike.

~Colleen Caldwell, New Mexico Cooperative Fish and Wildlife Research Unit and Jim Reynolds, Alaska Cooperative Fish and Wildlife Research Unit (retired)

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