7784

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

SUMMARY

Electrofishing, a valuable sampling technique in
North America for four decades, is in a state of flux.
The field is expanding with new equipment,
applications, and understanding but contracting with
increased concern for comparability of data and safety
of operators and fish. Researchers and managers in
the Colorado River Basin are especially concerned
about potential adverse effects on endangered fishes.
This report is a review of literature and available
unpublished information on the effects of electric fields
on fish. It includes recommendations for interim
policy and research to determine if, how, and to what
extent electrofishing adversely impacts the species of
concern and means for minimizing those impacts.
Recent investigations in several states have
documented substantial injury to the spines of fish
using modem electrofishing equipment with pulsed
direct current. The injuries result from powerful
convulsions of body musculature (epileptic seizures
according to one authority) and include fractured
vertebrae; spinal compressions, breaks, and
misalignments; and associated hemorrhages and
damage to muscles, nerves, and other tissues. The
problem is real, wide-spread, and not simply a matter
of too much power or close proximity to the anode.
Spinal injuries have been observed in fish netted
outside the high-intensity zone of tetany and might
occur at electric-field intensities below the threshold
for taxis. The injuries are often not externally obvious
or fatal. Most spinal injuries can only be detected by
X rays or necropsy. When external signs such as
brands or bent backs are present, serious injuries are
indicated. Unless the injury is severe, most fish
experiencing spinal injuries heal, survive, and appear
to behave normally. Accordingly, concern by some
biologists is shifting from effects on individuals to
effects on populations.
The specific electrical, environmental, and
biological factors associated with spinal injuries remain
uncertain, in part because, the results of pertinent
investigations are often inconsistent or contradictory.
However, sudden changes in voltage differential, as
when current is switched on or off, appear to be the
most likely cause. Use of low-frequency pulsed direct
current ( 30 Hz), specially designed pulse trains, or
continuous direct current effectively reduces the
incidence of spinal injuries, but does not eliminate the
problem. The severity of the problem varies widely
with equipment, technique, environment, species, and

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probably size and condition of the fish. Reported
incidences of spinal injury range from none detected to
over 90%. Among the few species studied thus far,
the Salmoninae (trout, char, and salmon) are by far the
most susceptible species. Endangered Colorado River
Basin fishes are not immune, but neither they nor close
relatives have yet been studied to determine whether
spinal injuries are a significant problem.
Other harmful effects of electrofishing are also of
concern. Mortality, usually by asphyxiation, is a
common result of excess exposure to tetanizing
currents near the anode or poor handling of captured
specimens. Tetany can be minimized by prudent
selection of electrical parameters such as power output
and electrode size. Bleeding at the gills or vent might
not be associated with either spinal injuries or the
stresses of tetany. Effects of electrofishing on the
reproductive behavior of near ripe or spawning fish
remain unknown, but there is evidence that
electrofishing over spawning grounds might harm
developing embryos. Adverse effects of electric fields
on fish larvae have not been investigated.
Until proven otherwise, it is recommended that
Colorado River Basin researchers assume that presently
used and available electrofishing techniques can cause
significant injury to endangered or other native fishes.
Alternatives to electrofishing should be considered in
ongoing programs and use of electrofishing in new
programs should be minimized or delayed until the
necessary research on electrofishing impacts is
completed and defmitive policy can be established. If
electrofishing remains the only reasonable capture
technique for data critical to endangered species
recovery, then electrofishing equipment and procedures
should be continuously monitored and adjusted to
assure least harm to the fish.
Many biologists across North America now
acknowledge that spinal injuries may be a serious
consequence of electrofishing, at least for some
species. It is time for a concerted, well-funded,
national or international effort to document the factors,
thresholds, and mechanisms involved and determine
means for minimizing injury while maintaining
adequate capture efficiency. Where electrofishing
injury is a problem and cannot be adequately reduced,
the technique must be abandoned or severely limited.
As fishery biologists, this is our ethical responsibility
to the fish, the populace we serve, and ourselves.

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