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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.

One question was, “Is the electrical field around the anode different in front of and to the side of, i.e. fore and lateral to, the anode ring?” Class members took measurements of voltage gradients at various distances from the centers of anode rings when deployed in typical fishing orientation in the Poudre River. Graphs of the voltage gradient, aka electrical field intensity, values by distance in the two axes strongly overlapped. Therefore, the voltage gradient profiles were similar fore and lateral to the anodes. This indicates that fish should be affected at similar distances in front of and to the sides of the anodes.


Class participants measuring voltage gradients at distances from a backpack anode ring. Poudre River near Ft. Collins, CO. A PVC pipe, suitably marked, was used to determine distance from the center of the ring. Voltage gradients were measured using a voltage gradient probe and a portable oscilloscope. More photos follow.






Comparison of lateral and fore voltage gradient profiles for two of the teams, all in the Poudre River. Strong overlap in the profiles indicate similar electrical fields in the fore and lateral directions from the anode ring. The fore direction is away from the person operating the backpack.

The other question was, “Does water conductivity affect the shape or size of the voltage gradient field around the anode?” We have taught that voltage gradient is independent of water conductivity. But is that really true? We were finally able to address that question in this class because we were able to measure the voltage gradient profiles in streams of differing conductivity. The Poudre River has clear water and a rocky substrate; its ambient water conductivity was 246 μS/cm, a moderate value. Spring Creek is narrower and has a mud bottom and more turbid water; its ambient water conductivity was 545 μS/cm, a moderately high value about double that of the Poudre River. Graphs of voltage gradients by distance from the anode centers, all in the fore direction, were similar for the two streams. Thus, voltage gradient profiles were independent of water conductivity, as expected.


Comparison of fore voltage gradient profiles from two streams with different water conductivity. Graphs from three different teams. Ambient water conductivity in Spring Creek was 2.2 times that in the Poudre River. Similar voltage gradient profiles between the two streams suggest little to no effect of water conductivity, as expected.

I should point out that all of the voltage gradient profiles in this blog were based on an applied voltage of 100 peak volts. Also, according to the power transfer theory, it is believed that fish respond to power density rather than just to voltage gradient. Power density in μW/cc does vary with water conductivity. It is voltage gradient squared times ambient water conductivity. One must adjust applied voltage according to the power transfer theory model to compensate for changes in water conductivity so that the size of the effective electrical field around the anode remains constant. Refer to the blog on the power transfer theory for more on that. The purpose of this blog was to address the two questions about voltage gradient profiles. The graphs supplied here answer both questions.

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