7784

• UCREFRP Catalog Number: 7784
• Author: Snyder, D. E.
• Title: Impacts of Electrofishing on Fish.
• USFW Year: 1992.
• USFW - Doc Type: \
• Copyright Material: NO

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Impacts of Electrofishing on Fish

electrofishing operation (like people working on a
metal boat used as the cathode). The level of
ambient potential relative to the electrodes depends
on the power output, total resistance (sum of anodic
and cathodic resistances), and the ratio of anodic to
cathodic resistance. When cathodes are much larger
than anodes, most of the total potential between
electrodes is associated with the anode, the voltage
differential between ambient potential and the
cathode is relatively small, and voltage gradients near
the cathode are much lower than near the anode.
The size, shape, configuration (e.g., single or
multiple electrode arrays) and relative position of the
electrodes should be selected according to the
available power output, water conductivity, and
desired distribution of current in the water. Novotny
(1990) emphasized that "The most common electrode
problem is that the electrodes are simply too small .
. . ." At the same voltage output, the larger the
electrode, the less its electrical resistance in water,
the lower the maximum field intensity immediately
around it, the smaller the zone of tetany , and the
larger the effective field (greater the range).
Sometimes the zone of tetany can even be
eliminated. Increasing the number of anodes or
cathodes in a system has a cumulative effect similar
to increasing the size of an individual electrode (the
effect is maximized when multiple electrodes are
well separated). Maximum size or number of anodes
or cathodes is dictated largely by practical
considerations (e.g., maneuverability, transportability,
interference with netting). It can also be limited by
the maximum power output of the generator,
especially in highly conductive waters. Electrode
resistance varies inversely with either electrode size
(available surface area) or water conductivity. At
constant applied voltage, reductions in total electrical
resistance result in increased current, sometimes
enough to overload the generator. When water
conductivity is high, the size of the electrodes must
sometimes be reduced to prevent such an overload.
Spherical electrodes are generally considered
superior to other shapes (e.g., cables or narrow
cylinders). Electric fields generated immediately
around spheres are uniform and without the hot spots
(localized regions of higher intensity) produced near
the corners and edges of many other electrode
shapes. Novotny and Priegel (1974) and Novotny
(1990) noted that except near their surfaces, circular

Review I Electric Fields in Water 17

and ring-like electrodes, including dropper arrays,
produce fields similar to those produced by spheres.
In DC and PDC systems, the desired
electrofishing responses are generally produced only
in anodic fields, whereas fish tend to be repulsed by
cathodic fields. However, some adverse effects may
be as great or greater in cathodic fields. Jesien and
Hocutt (1990) found that channel catfish in
homogeneous fields are more sensitive to tetany
when facing the cathode than when facing the anode.
To minimize cathodic effects on fish, cathodes
should be as large as practical. As noted above, this
will also maximize potential in the anodic field and
reduce the overall electrical resistance of the system.
In systems with cathodes much larger than anodes,
the very low voltage differential between the cathode
and soil and water in vicinity reduces the risk of
severe shock or electrocution to people or animals
that inadvertently approach or touch the cathode
(Smith 1989). Because cathodic resistance for well
separated electrodes is halved each time the surface
area of the cathode is doubled, Smith (1989)
suggested that 10 m2 is a practical limit to the size of
the cathode.

Electrofishing Currents and Waveforms

Electrical currents are of two principal types:
bipolar or alternating currents characterized by
continually reversing polarity and movement of
electrons (AC; Figure 9A) and unipolar or direct
currents characterized by movement of electrons in
one direction. More specifically, the term DC refers
to unipolar currents that are continuous and relatively
constant in voltage (Figures 9B, 9C). Both AC and
DC can be periodically interrupted or pulsed.
Although pulsed AC (e.g., Jesien and Hocutt 1990)
is seldom used for electrofishing, several variations
of pulsed DC (PDCs) are very popular and typically
used with boat systems. PDCs are characterized by
frequency (Hz-Hertz, cycles or pulses per second),
pulse width (time power is applied during each pulse
cycle, usually expressed in ms, milliseconds) or duty
cycle (time power is applied per cycle, expressed as
a percent of cycle time), shape or waveform (e.g.,
rectangular, exponential, half sine, and quarter sine),
and pattern (either a uniform frequency or
secondarily interrupted at much slower frequencies to
produce bursts, packets, or trains of pulses). The