Electrofishing uses direct current electricity flowing between a submerged cathode and anode. This affects the movement of the fish so that they swim toward the anode, where they can be caught. Electrofishing is a common scientific survey method used to sample fish populations to determine abundance, density, and species composition. When performed correctly, electrofishing results in no permanent harm to fish, which return to their natural state in as little as two minutes after being caught.
Method
Electrofishing relies on two electrodes which deliver direct current at high-voltage from the anode to the cathode through the water. When a fish encounters a large enoughpotential gradient on this path, it becomes affected by the electricity. Usually pulsed direct current is applied, which causes galvanotaxis in the fish. Galvanotaxis is uncontrolled muscular convulsion that results in the fish swimming towards the anode. At least two people are required for an effective electrofishing crew: one to operate the anode, and the other to catch the stunned fish with a dip net.
Types
There are three types of electrofishers: backpack models, tote barge models, and boat mounted models, sometimes called a stunboat. Backpack electrofisher generators are either battery or gas powered. They employ a transformer to pulse the current before it is delivered into the water. The anode is located at the end of a long, two meter pole and is usually in the form of a ring. The cathode is a long, three meter braided steel cable that trails behind the operator. The electrofisher is operated by a deadman's switch on the anode pole. There are a number of safety features built into newer backpack models, such as audible speakers that sound when the unit is operating, tilt-switches that incapacitate the electrofisher if the backpack is tilted more than 45 degrees, and quick-release straps to enable the user to quickly remove the electrofisher in the event of some emergency. When boat electrofishing, the boat itself is the cathode, and the anode are generally mounted off the bow. The stunned fish swim toward the anode, where they are caught alive using a dip net. Towed barge electrofishers operate similarly to backpack electrofishers, with the exception that the generator is located on a floating barge instead of on a backpack. Often the barge can be left stationary on the shore and longer cathodes and anodes allow the crew to sample large areas. Barge electrofishers often employ gas-powered generators since a user does not have to carry the extra weight on his or her back. A method of electrofishing using illegally diverted electricity from overhead electric lines is prevalent in parts of the IndianState of Kerala. This can result in accidental deaths due to electrocution.
Effectiveness
The effectiveness of electrofishing is influenced by a variety of biological, technical, logistical, and environmental factors. The catch is often selectively biased as to fish size and species composition. When using pulsed DC for fishing, the pulse rate and the intensity of the electric field strongly influence the size and nature of the catch. The conductivity of the water influences the shape and extent of the electric field and thus affects the field's ability to induce capture-prone behavior in the fish. It can also cause injury to the fish. The electricity causes muscle spasms that damage the vertebrae. For unknown reasons, this is more common and severe in longer fish.
