Things to consider when electrofishing
Electrofishing is an effective and safe tool for rapidly collecting a community of fish in freshwater environments. It is imperative that certain environmental conditions be taken into consideration in order to be most effective. Here we discuss some of those variables and provide insight on how you can effectively utilize electrofishing equipment for your objectives.
To collect fish by electrical means we must create an electrified zone in the water of sufficient amplitude to overcome the escape capabilities of the fish. An electric field in water can be created between positive and negative electrodes. If a fish is within that electric field, some current will flow through the fish. The conductivity of the water and that of the fish are the main factors that determine how much current will flow through the fish, and ultimately, the affect it will have.
The conductivity is a measure of the ionic content of the water, and depends on the quantity of dissolved salts and minerals in the water. The effective conductivity range of most successful electrofishing samples ranges from 20 to 2,000 µS/cm. Within this range, sufficient electrical current will flow at reasonable voltages and realistic power levels that can ultimately can flow through water and fish to allow someone to electrofish successfully. Outside of this range of conductivity, efficiency begins to be limited and ultimately becomes very difficult because the voltage, current, and power requirements become difficult to achieve, plus the fish’s conductivity limits the ability to overcome its ability to escape.
According to United States Fish and Wildlife Service training materials, the American Fisheries Society has adopted 115 µS/cm as a standard value for fish conductivity. The point is that effective fish conductivity is basically constant, no matter the environment, whereas water conductivity can vary tremendously. This I because fish osmoregulate their internal conductivities to a constant level while the environment around them changes.
A fish will receive the maximum shock through its body when the conductivity of the water is the same as the conductivity of the fish's body. Therefore, ambient water conductivity of 115 µS/cm is ideal. Any deviation from this value will either require higher voltage (in lower water conductivity situations), or higher current (in higher water conductivity situations).
It should be made clear that while there is an adopted value for internal conductivity of fish, each species would naturally have variable internal conductivity and resistance (scaling, nerve shielding, swimmbladder, etc.) which affects their susceptibility to electric current.
Low Conductivity Water
As water conductivity decreases at a fixed voltage (electrical pressure), the total current (flow of electricity) decreases and the electrical power delivered to the fish decreases. If the power is too low, fish will most likely not be stunned enough to capture. There are two ways to compensate for this situation. The first is to increase the applied voltage, although there are practical limits based upon operator safety and possible damage to the fish when they are close to the electrodes. The second is to increase the size of the electrodes which more efficiently transfers power to the water and reduces the intensity of the electric field close to the electrodes. In high mountain streams, where lower conductivity waters are generally found, it can be difficult to keep larger electrodes completely submerged. Therefore, finding a balance of an appropriately larger electrode in combinations with increasing the voltage is essential. Additionally, electrodes can be moved closer together to overcome the resistance associated with low conductivity water, but one needs to be careful to not let fish get too close to electrodes or stay in the electric field for too long of a period of time.
There are some low conductivity situations where electrofishing is going to be incredibly inefficient and frustrating. Distilled water has a conductivity range of 0.5 to 5.0 µS/cm. If typical electrofishing voltages (100 to 1,200V) are applied to water of this low of conductivity, very little current will flow. Too little power is transferred to the water and fish to be effective for electrofishing because there are very few ions to carry the electrical current through the water between the electrodes.
At higher voltages, safety is reduced for the operators should they touch an electrode. Similarly, conditions can quickly become lethal for fish close to electrodes at higher voltages.
High Conductivity Water
Ocean water conductivities are generally between 30,000 and 50,000 µS/cm. Whereas when electrofishing, high water conductivity is anything over 2,000 µS/cm. In high water conductivity conditions, most current will flow through the water in a small electric field and the fish will hardly be affected. The electric current follows the path of least resistance and almost completely bypasses the fish. Therefore, optimally, we use low voltages (not a lot of electrical pressure is needed) and high currents (simpler to get high flow with low pressure when there are a lot of ions in the water to carry the electricity). In these scenarios, currents as high as 60 to 80 amps are common. Therefore, the factor limiting your ability to succeed in these conditions is the rating of the electrofisher and generator. One needs to consider the Current and Power output capabilities of an electrofisher when working in higher water conductivity environments.
Among fish of the same species, the larger fish absorb more power than the small fish and have a larger voltage gradient difference across the length of their body. Therefore, larges fish receive a much greater shock than smaller fish given the same conditions and orientation in the field.
Certain bottom substrates will conduct electrical current. These substrates will attract electric current flow under the substrate, and therefore weaken the electric field in the water, making fish capture less effective. A similar phenomenon can happen with a freshwater lens over the top of a saltwater wedge. Effectiveness in the freshwater lens is minimized because of the current flowing down and through the saltwater wedge.
Very High Conductivity Water
Some brackish water and industrial waste water have conductivities over 10,000 µS/cc. Here, smaller generators are unable to deliver enough voltage gradient to stun fish. Waters in this range can only be electrofished effectively with specialized electrodes and configurations, or another gear may be more appropriate.
There are two measurements for water conductivity: Ambient and Specific. Essentially, ambient water conductivity is what we are interested in when we are electrofishing because it is an indication of the conductivity at this exact moment. Whereas, specific water conductivity is a measure of the water conductivity if the water were 25 degrees Celsius. Therefore, when you are measuring water conductivity, it is important to measure ambient conductivity in order to properly determine what voltage (electrical pressure) to use for the given conditions. It is useful to note that as water temperatures go up, ambient water conductivity also goes up, whereas specific water conductivity would not change.