To collect fish by electrical means we must create an electrified zone of sufficient amplitude to stun fish. A fish between submerged electrodes forms part of a closed circuit, and some current flows through it. The conductivity of the water and that of the fish's flesh are the main factors that affect electrofishing.
The conductivity of water depends on the quantity of dissolved salts and minerals in the water. The conductivity of potable waters in the United States ranges from 20 to 2,000 µS/cm. Sufficient current at realistic power levels will flow through water in this range to electrofish successfully.
Low Conductivity Water
Distilled water has a conductivity range of 0.5 to 5.0 µS/cm. If typical electrofishing voltages (100 to 1100V) are applied, very little current will flow. Too little power is transferred to the water (or fish) to be effective for electrofishing.
As the water conductivity decreases (with a fixed voltage) the total current decreases and the electrical power delivered to the fish decreases. If the power is too low, fish may 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.
If a higher voltage is used, up to 1,200 volts may be necessary. Safety is reduced for the operators, and conditions are lethal for fish close to electrodes. It is better to keep the resistance constant by increasing the size of the electrodes. The only limitations are the weight that can be handled by the operator.
Among fish of the same species, the larger fish absorb more power than the small fish. The large fish receive a much greater shock than the small fish.
Certain bottom substrates will conduct electrical current. These weaken the electric field in the water making fish capture less effective.
High Conductivity Water
High water conductivity is over 2,000 µS/cm. If a high voltage is applied, most current will flow through the water and the fish will hardly be affected. The electric current follows the path of least resistance and bypasses the fish completely. Therefore use low voltages and high currents. Currents as high as 60 amps are common, the limiting factor being the rating of the generator. The 7.5 and 9.0 GPPs can stun large fish in the interface between fresh and salt water as, for example, in taking Striped Bass for brood stock.
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 the larger model GPPs.
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 flesh. Unfortunately, this is rarely the case. Each species has a different conductivity which affects their susceptibility to electric current.