Updated 6 Jun 2009

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Photo 066


Fishing census, 4 June 2009

Electrofishing uses electricity to stun fish before they are 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 2 minutes after being stunned.

There are three types of electrofishers: backpack models, towed barge models, and boat mounted models, sometimes called a stunboat. All models rely on two electrodes which deliver current into the water to stun fish. The current runs from the anode to the cathode, creating a high-voltage potential. When a fish encounters a large enough potential gradient, it becomes affected by the electricity. Usually pulsed DC current is applied, which causes galvanotaxis in the fish. Galvanotaxis is uncontrolled muscular convulsion that results in the fish swimming toward 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.

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, 2 meter pole and is usually in the form of a ring.

The cathode is a long, 3 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 incapacitates 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.

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.

When boat electrofishing, the boat itself is the cathode, and the anode(s) are generally mounted off the bow. The stunned fish swim toward the anode, where they are caught alive using a dip net.

66a. The stunboat arrives at Bear Mead from upstream.
66b. The crew relax on the river bank for lunch

66c. Setting off again, the anode rings are held forward of the stunboat.
66d. The gas cylinder powers the generator driving the anodes forward.

66e. The young lady with the dip net transfers the stunned fish to the holding tank amidships, for later length and weight measurements.
66f. Catching a beauty

66g. The team in action, anodes for'ard, paddles aft, dip net at the ready.
66h. Disappearing down river towards their next measurement stop.

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