Updated 6 Jun 2009
Planting a new oakwood in Dorset
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
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.
The stunboat arrives at Bear Mead from upstream.
The crew relax on the river bank for lunch
Setting off again, the anode rings are held forward of the stunboat.
The gas cylinder powers the generator driving the anodes forward.
The young lady with the dip net transfers the stunned fish to
the holding tank amidships, for later length and weight measurements.
Catching a beauty
The team in action, anodes for'ard, paddles aft, dip net at the ready.
Disappearing down river towards their next measurement stop.
Compiled, formatted, hyperlinked, encoded,
and copyright © 2009
All Rights Reserved.