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.
VIDEO: CONTROL FISH
VIDEO: EAGLE CLAW TRAP
VIDEO: GEE TRAP