Scientists regularly use a specialized and heavily regulated technique to capture large numbers of fish in freshwater environments. This technique is called electrofishing. Fish scientists need to understand the environmental variables that they are working in order to set appropriate electrical settings to safely catch fish. Here, we provide an easy to understand analogy in order to understand how voltage, current, power, and conductivity play a role in successfully and safely capturing fish while electrofishing.
Have you ever tried blowing air through a coffee stirring straw? How about a soft drink straw? Now how about a 1 inch (~3cm) diameter PVC tube?
Imagine you want to get a ball across a table by blowing air through the three different sized tubes.
With the coffee stirring straw, it requires a lot of pressure from your lungs to get even a small amount of air to flow through the straw. Therefore, you would need a lot of lung pressure to get enough air to go through the straw to push the ball.
With the soft drink straw, you need a moderate amount of pressure to get a moderate amount of air flow to get the ball going. This straw would allow for an efficient outcome of moving the ball because your lungs can create pressure and the flow is appropriate for controlling the movement of the ball.
Finally, with the PVC tube, you are unable to build much lung pressure because there is very little resistance. You are able to get a lot of air, but it is not focused and isn’t as useful at moving the ball before you exhaust your lungs.
This little example is a great analogy to the electrical terms associated with electrofishing: voltage, current, power, and conductivity.
Voltage is electrical pressure, and in the straw example, is analogous to the pressure you apply with your lungs.
Current is the rate of flow of electrons past a point and is analogous to the rate of flow of air through the different straws.
Power is simply the product of voltage multiplied by current and is analogous to the combined effort of how much pressure you apply with your lungs and the air flowing through each of the straws.
Finally, water conductivity is a measurement of the capability of the water to carry the electrical charge from your cathode (boat hull or rattail) to your anode (boat booms or anode pole) and is analogous to the different diameters of the three straws.
The coffee stirrer with a small diameter represents low conductivity water (<70 microSiemens/cm). In order to get a moderate amount of air to flow through the straw, one must push very hard with your lungs. Similarly, in order to get a moderate amount of electrical current to flow in low conductivity water, one must apply a tremendous amount of voltage.
The soft drink straw with a moderate diameter represents ideal conductivity conditions for electrofishing. A moderate amount of lung pressure gets a moderate amount of air to flow. Similarly, in order to get a moderate amount of electrical current to flow in moderate water conductivity, one must apply a moderate voltage.
Finally, the PVC tube with a large diameter represents high conductivity (>700 microSiemens/cm) water. Very little pressure from your lungs will put a lot of air through the tube. Similarly, even a small amount of voltage will allow a lot of electrical current to flow through the water.
See the two images below. The first gives you an indication of how much effort it takes to get the ball to move across the table (green (slowly) and red (fast)) using straws of different diameter. The second image depicts the amount of power it takes to effectively catch fish at different water conductivities.
As you can see, water conductivity and voltage are inversely related to each other, or stated more simply, when water conductivity is low, turn the voltage up, and when water conductivity is high, turn the voltage down.
Conversely, water conductivity and and current are positively correlated, or stated more simply, when water conductivity is low, current is low, and when water conductivity is high, current is high.
Take home messages:
Using an inexpensive conductivity meter prior to electrofishing can play a very important role in helping identify the target voltage settings. Here is a great conductivity meter to take in the field that provides readings of ambient water conductivity.
A moderate water conductivity is most efficient for electrofishing (see graph above) because it uses a moderate amount of current and voltage to successfully catch fish. Further, the conductivity of a fish is in this moderate part of the range, and therefore, electrical current flows through the water and fish very efficiently at these conditions.
As you can see from the graph and when thinking of blowing air through straws, operating at extreme water conductivities can be taxing on the equipment and cause an overload because the electrofisher cannot produce the voltage, current, or power (combination of voltage and current) required to allow you to successfully catch fish. Additionally, operating at the extremes can be harmful to the fish because too much electricity can pass through the fish.
Now that you know all this, here are two simple formulas to demonstrate how all four of these variables relate to each other:
Current = Voltage x Conductivity
Power = Current x Voltage
Of all of these variables, the only one you have direct control over on your electrofisher is Voltage. Having a knowledge of how all of these variables impact each other, along with understanding the role that water conductivity plays in this whole thing, should give you a much better ability to be successful in the field.
Written by Patrick Cooney