because the electrodes of the circuit are always the same. Alternating current changes directions as the anode and cathode switch positions between the electrodes.
The types of electrical current produce different electrical current shapes or wave forms. Alternating current produces a wave form that consists of a sequence of positive and negative waves that are equal, usually sinusoidal, and follow each other alternately at regular time intervals. Direct current produces a unidirectional, constant electrical current. Pulsed direct current, a modified direct current, produces a unidirectional electrical current composed of a sequence of cyclic impulses.
The responses of fish to electricity are determined largely by the type of electrical current and its wave form. These responses include avoidance, electrotaxis (forced swimming), electrotetanus (muscle contraction), electronarcosis (muscle relaxation or stunning), and death. Forced swimming without orientation relative to the electrical current (oscillotaxis) is a typical fish response to alternating current. Alternating current can be damaging to fish, resulting in hemorrhaged tissue, ruptured swim bladders, and fractured vertebrae because of severe electrotetanus caused by the alternating polarity of alternating current. Direct current forces fish to swim with orientation toward the anode (galvanotaxis). The modified pulsed direct current can sustain galvanotaxis longer than unmodified direct current, and with less likelihood of damage to the fish than unmodified direct current or alternating current.
The frequency of the pulses produced when using pulsed direct current can be adjusted by the operator and usually ranges from 15 to 120 pulses per second (pps). High pulse frequencies (greater than 30 pps) have proven to be more effective in collecting fish but appear to cause spinal injuries, particularly in trout and salmon species (Coffelt Manufacturing, Incorporated, written commun., 1991). Pulse rates below 30 pps have caused low incidence of injury, but are generally ineffective in collecting fish. Therefore, a pulse rate range of 30 to 60 pps is recommended to provide maximum collection effectiveness with a minimum potential for damage to fish.
Water conductivity also influences the response of the fish to the electrical field and is the single most important limiting factor in electrofishing effectiveness. Low-conductivity water is highly resistant to the flow of electrical current, thereby reducing the amount of electrical current traveling through the water and passing through the body of the fish. Under such conditions the electrical field is limited to the immediate area of the electrode. Thus, a relatively high output voltage is required to create an electrical field of sufficient size and strength to stun fish. High-conductivity water produces the opposite effect by concentrating a narrow electrical field between the electrodes. In high-conductivity water, output voltage must be re duced to minimize potential damage to the fish. Most electrofishing equipment is designed to operate in water with conductivity ranging from 20 to 2,000 microsiemens per centimeter, and is usually capable of generating output voltages of 100 to 1,000 volts. An electrical field strength meter is used to determine the size and strength of the electrical field generated by the electrofishing equipment. The conductivity of the water must be measured prior to electrofishing to determine the appropriate outp ut voltage for effective electrofishing.