ELECTROFISHING
Key words: Current, Fish Injury, Mortality
INTRODUCTION
Electrofishing can be defined as a fish sampling technique using electric currents and electric
fields to control fish movement and/or immobilize fish, allowing the capture of fish. Scientists
use electrofishing to survey fishes and monitor the size of populations and determine the species
in a community. It is an efficient capture method that can be used to obtain reliable population
estimates, length-weight relationships, and age and growth on most streams. Whether using a
boat or a backpack electroshocker, the basic principle involves generating an electrical field in
the water to stun fish. When fish are stunned, they often times float near the surface of the water
and can be removed from the electrical field. When exposed to the field, most fish become
oriented toward the anode and as the density of the electric field increases as they swim toward
it. In close proximity to the anode, they are immobilized. The actual sequence of responses to the
electric field is more complex and varies depending upon the type of current applied (AC, DC,
pulsed DC), the initial orientation of the fish with respect to the field and field density.
Electrofishing is a technique whereby electrical energy is put into the water and fish, intercepting
this energy, are drawn toward the probes and incapacitated in such a way that they can be
captured with nets. The movement of fish toward the source of electricity is called galvanotaxis
(uncontrolled involuntary muscular convulsion that results in the fish swimming toward the
anode) and is believed to be a result of direct stimulation of the central and autonomic nervous
systems which control the fish‘s voluntary and involuntary reactions.
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, which is
determined by the concentration in the water of charge carriers (ions), influences the shape and
extent of the electric field in the water and thus affects the field's ability to induce capture-prone
behavior in the fish.
HOW DO FISH GET STUNNED?
When a fish swims into a weak electrical field, it may not be affected at all. There is a threshold
of electrical charge that must be emitted into the water in order to affect the fish. When the
electrical charge in the water is sufficient to allow transport of the charge across the nerve cells
in the body, then the fish‘s muscles will undergo involuntary contraction. The contractions will
lead to increased exercise of the muscle and a buildup of lactate in the blood stream. This process
is very similar to what happens to the muscles of a runner or a swimmer who exerts a lot of
exercise. The runner or swimmer may eventually get a cramp in the muscle and cannot move it
effectively. When the fish cramps up, it floats to the surface and can be removed from the
electrical field. The process to stun a fish is usually 5 – 10 seconds.
Fisheries scientists have long studied the behavior of fish in electrified water. However, the
science of electrofishing evolved independently of other fields of science whose knowledge
could have explained the behaviors of fish to electric shock. Theories to explain the behavior of
organisms in electric fields developed out of two paradigms: ―classical stimulus–response‖ (S–
R) theory, and ―local action‖ of electrical energy on nerves and muscle fibers. Stimulus–response
theories dominated in the late 19th Century; although they were abandoned by the early 20th
Century by animal behaviorists they persisted in work with fish. An alternative theory, the ―local
action‖ paradigm, arose soon after 1900. Attributing galvanotropisms to the action of electricity
on local nerves and muscles fibers, the local action theory remains in one form or another, the
main explanation for galvanotropisms today. In a more recent attempt to better understand and
explain the inter-action between fish and electric fields, electrofishing has been treated as a
power-related phenomenon. According to this ‗‗power-transfer theory for electrofishing,‘‘ the
relationship between electrical power in the water and in the fish is a function of the ratio of
water conductivity to the effective conductivity of the fish (Kolz and Reynolds, 1989; Kolz et al.,
1998).
Factors
Electrofishers tend to collect larger fish more easily than smaller fish but variable control
electrical transformers allow adjustable control of voltage, pulse, and electrical frequency
thereby reducing size selectivity. Electrofishing efficiency can also be affected by stream
conductivity, temperature, depth, and clarity of water. Each condition must be considered to
ensure a reliable population estimate. Electrofishing can be more efficient than other methods of
population estimates such as seining and underwater observation. Boulder-rubble substrate,
turbidity, aquatic vegetation, and undercut banks can bias other population estimation methods.
The literature describes four classic zones of effect of the electric field each occurring at
differing distances away from the source (Vibert et al. 1960, Regis et al. 1981, Snyder 1992).
Some zones are common to all electric current types and some are specific to one type.
1. The indifference zone is the area where the electric field has no influence upon the fish.
2. The repulsion or fright zone occurs on the periphery of the field where the fish feels the
field but it is not intense enough to physiologically attract the fish. The fish instead reacts
as to any reactive stimulus; this may include escape or seeking refuge (hiding in weed
beds or burrowing in bottom depending on species). Intelligent use of the anode can limit
a fish‘s probability of encountering this zone.
3. The attraction zone (dc and pdc only) this is the critical area where the fish is drawn
towards the electrode. This occurs due to either anodic taxis (normal swimming driven by
the electric field effect on the fish‘s CNS), or forced swimming (involuntary swimming
caused by direct effect by the electric field on the ANS). In the latter case swimming
motions often correspond with the initial switching of dc and the pulse rate of pdc. This is
the zone fishing equipment should seek to maximise.
4. The tetanus (ac, pdc and some dc fields) and/or narcosis (dc fields) zone is the region
where immobilisation of the fish occurs. In ac, pdc and very high dc fields this results
from tetany. Fish in this state have their muscles under tension and respiratory function
ceases. Fish may require several minutes to recover from this state. In normal dc fields
however immobilisation results from narcosis. In this state the fish muscles are relaxed
and the fish still breathes (albeit at a reduced level). When removed from this narcotising
field the fish recover instantly and behave in a relatively normal manner. Tetanus can
harm fish and thus this zone should be minimised in gear design or fish removed quickly
from it.
Electrical current types:
The current types used for electric fishing can be divided into two main types:
1. Bipolar or Alternating Current (ac), characterised by continually reversing polarity
2. Unipolar or Direct Current (dc) characterised by movement of electrons in one
direction only.
Dc can be further sub-categorised into continuous dc (cdc) and Pulsed Direct Current (pdc).
For all types of current the pattern of voltage and current around the electrodes conforms to the
pattern shown in Figure 1. When the fish are aligned along the current lines they will experience
the greatest voltage potential, when aligned along the voltage lines they should experience the
least voltage difference. However they will experience some lateral voltage gradient across their
body.
Fig – 10.1
Alternating Current (ac)
The current direction reverses many times a second thus there is no any polarity to the current
(one electrode being successively positive and negative many times a second). Ac may be single
phase or multi (usually 3) phases. Figure 2 shows these forms of current.
Fig - 10.2 Single phase (A) and multi-phase (B) ac current pattern
This waveform has the advantage of being able to be produced easily from small generators and
suffers little variation in effectiveness due to physical parameters of the stream (stream-bed
conductivity, temperature etc.). The voltage gradient required to provoke a reaction is also quite
small. When fish encounter an Alternating Current (ac) field they experience:
Oscillotaxis – the fish are attracted to the electrodes (but not to the same extent as with dc and
pdc).
Transverse oscillotaxis – The fish quickly take up a position across the current and parallel to
the voltage lines in order to minimize the voltage potential along their body.
Tetanus - Once so aligned the fish muscles are in strong contraction and the fish are rigid.
Breathing is also often impaired by the fixation of the muscles controlling the mouth and
opercular bones. The effect is more violent than with dc or pdc and at high voltages muscular
contractions may be so severe that the vertebrae are damaged. The recovery time can be
significant.
The disadvantages of ac are predominantly that it has minimal attraction effect and its effect
upon fish is to tetanise the fish with its muscles in a cramped state. This tetanus quickly restricts
the fish‘s ability to breathe and renders them unconscious. If not removed quickly from the field,
death may occur quite soon from asphyxia. Delayed mortality may also occur due to acidosis
resulting from the oxygen debt generated by the contracted muscles. Kolz (1989) found that even
when applying the same power to the fish, fish immobilised with ac took longer to recover than
fish immobilised with pdc. In addition, with little attraction to the electrode, fish are not drawn
out of cover or deep areas to where they can be seen and caught.
Direct Current (dc)
This is the simplest waveform used and technically is not a true ―wave‖ but a constant voltage
applied over time (Figure 3). The electrical charge flows only in one direction; from negative
(cathode) to positive (anode).
Fig - 10.3- ―True‖ (A) and ―rippled‖ (B) direct current
Direct current was the first type of electrical waveform to be applied to electric fishing; this is
because it is the type that is produced from a galvanic cell (battery). Generating it needs a
considerable amount of power however, thus requiring large generators or quickly exhausting
batteries. Generators designed to produce dc current are heavier, more expensive, less reliable in
voltage control and less reliable than ac generators with comparable power rating. For these
reasons dc power is usually produced by conditioning power from an ac generator. In the past
this conditioned dc often had a noticeable ripple resulting from inefficient smoothing of the ac
source current (Figure 3.B), modern electronics however should give a good dc waveform. As
the two electrodes (negative charge (cathode) and positive charge (anode)) produce differing
physiological responses, the fish reaction will vary slightly depending upon which electrode it is
facing. In field situations however the cathode field should ideally be very diffuse and thus
should not influence the fish. Reactions to the anodic dc field can be broadly categorized into
five basic phases.
Alignment - With initial electrical introduction the fish align themselves with the direction of the
electrical current. If initially transverse to the anode the fish undergo anodic curvature that turns
the head toward the anode.
Galvanotaxis - Once parallel with the current the fish start to swim towards the anode. This is
achieved through electrical stimulation of the CNS, resulting in ―voluntary‖ swimming.
Galvanonarcosis - When fish get close enough to the anode to experience a sufficient voltage
gradient their ability to swim is impaired. In this state their muscles are relaxed.
Pseudo-forced swimming – as the fish gets even closer to the anode a zone where the fish
begins again to swim toward the anode occurs. This swimming is caused by direct excitement of
the fish muscles by the electric field and is not under the control of the CNS.
Tetanus – At high dc voltages the muscles go from a relaxed state into spasm. This can result in
impaired ability to breathe and possible skeletal damage.
Unless held under conditions of tetanus, when the electricity is switched off, or the fish are
removed from the electric field, they recover instantly. Dc has a far greater attractive effect than
other waveforms (ac and pdc) but it is less efficient as a stimulator and thus will not narcotise /
tetanise the fish so readily. This is because threshold values required to elicit responses are high
with dc compared to ac and pdc. As it also shows great variation in effectiveness for slight
variations in the physical factors that affect it, any physical factors, which may affect the dc field
characteristics, are likely to substantially reduce the effectiveness of the process.
Pulsed Direct Current (pdc)
This waveform is like a hybrid between dc and ac. It is unidirectional (i.e. it has no negative
component) but it is not uniform. It has a low power demand (like ac) but is less affected by
physical variations in stream topography (unlike dc). Voltage gradients required to elicite a
respones are also substantially lower than those for dc. The shape and frequency of the pulses
can take many forms, some of which are better than others with regard to their effectiveness and
the injuries they cause. Figure 4 (A-F) shows examples of a range of pdc waveform types.
Fig - 10.4- Examples of a range of pdc waveform types
The behaviour of fish to pdc is somewhere between that of dc and ac. As with dc the fish react
differently to the anode and cathode field and thus their reaction will vary depending on which
electrode they are facing. There is some debate among researchers as to whether pdc produces
true galvanotaxis and whether narcosis or tetanus causes immobilisation. In general terms
however a fish‘s reaction to a pdc field can be summarised as follows.
Electrotaxis – there is good attracting power but this is due to the electrical effect on the fishes
muscles (the muscles contracting with each pulse of electricity and thus accentuating the
swimming motion) and not, as in dc, by electrical effect on the spinal nerves. This vigorous
effect upon the fish can also increase injury rates.
Tetanus/Narcosis – like dc the fish are immobilised near the anode but at a much lower voltage
gradient, as tetanus may be involved the fish need to be removed from this zone quickly.
Pulse frequency - Frequencies of pulses are measured in pulses per second or Hertz (Hz).
Within the UK only two pulse frequencies are commonly used (50 & 100 Hz). The principal
reason for this is historic in that originally the source of the electricity was a commercial
generator (producing 50 Hz ac) and the pulse box either full wave rectified the ac (producing 100
Hz pdc) or half wave rectified the ac (producing 50 Hz pdc). In the USA however the equipment
used enables a wide variety of pulse frequencies to be used and considerable experimentation has
taken place regarding the most efficient pulse rates to capture different species. Justus (1994) and
Corcoran (1979) finding that optimal frequency even varied between similar catfish species.
Novotny & Priegel (1974) state that some species selectivity is possible by varying the pulse
frequency of pdc. Halsband (in Vibert 1967) states that the frequencies shown in table 1 are
optimal for tetanising those species. It should be noted however that it may not be desirable to
produce tetanus and it is the frequency that produces the greatest attraction reaction that should
be optimised (Hickley 1985, 1990).
Table 1 Optimal tetanising frequencies for different fish species (Halband 1967)
Species
Minnow
Trout
Carp
Eel
Optimal Frequency (Hz)
90
80
50
20
There are three types of electrofishers:
Backpack models,
Towed barge models,
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, 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.
DOES ALL THE FISH GET STUNNED?
Electrofishing is a widespread tool used for surveying black bass because black bass are
generally easily stunned by the electric field. When stunned, all black bass are removed from the
water. However, not all black bass are likely stunned by the electrofish boat. The electric field
quickly weakens with distance from the boat and with depth, so many fish in the area are never
affected. Some fish that have been stunned may even build up a tolerance for the electric field.
Many species also have evolutionary adaptations that help them avoid the boat, such as a keen
lateral sensory system and eyesight. The black bass may be able to see the boat coming and swim
out of the way of the electrical field. Many other species of fish, such as common carp or
longnose gar, have especially thick scales that protect them from the electrical fields. Small fish,
such as many minnows and killifishes, have such a small body area that the electrical field
doesn‘t affect them.
THE RECOVERY TIME
Generally, fish recover almost immediately after they are stunned by an electrofisher. They
become oriented in the upright position and begin swimming normally within 1 – 2 minutes. To
fully recover from the electrofishing, it can take 4 – 12 hours, which depends on the amount of
lactate in the blood (or the level of stress the fish experiences) and habitat conditions. It may take
also longer for the fish to recover during late summer when dissolved oxygen in the water is low.
DO THE FISH DIE BECAUSE OF ELECTROFISHING?
Whether the fish dies or not depends on the person generating the electrical field and the
handling techniques that follow. Most fish live through the experience, but delayed mortality
following these surveys is not well-studied. Fish may immediately die if they are shocked too
intensively. This is clearly evident because some of the skin tissue begins to turn black. This type
of fatality is one of the easiest to avoid because it is easy to detect and easy to correct. Fish may
experience some delayed mortality because of lactic acidosis, which is the build up and
persistence of lactate in the blood stream. If the lactate is not removed by sufficient respiration of
oxygen, or if the fish cannot adapt to high levels of lactate, then it will die.
Fish Injury/survival
The effects of electrofishing on fish health have been the subject of a considerable amount of
research. Survival rates, injury rates, growth rates, physiological effects and gamete viability
have all been examined. Much of this research has examined the relationship between electrical
characteristics (type of current and wave form) and mortality and injury rates, and most has been
conducted on salmonids. Mortality rates are generally low for DC electrofishing.
The most commonly reported serious injuries to fish from electrofishing are spinal dislocations
and, in extreme cases, vertebral fractures that are apparently caused by strong muscular
contractions. Internal hemorrhaging has also been reported and skin discolourations, referred to
as branding, also occurs. A large proportion of spinal injuries evident on X-rays are not evident
from external examination (Kocovsky et al. 1997). In several studies, fish have been X-rayed to
determine the rate of injury. Both the rate and severity of injury increased with fish size.
Short-term physiological effects induced by pulsed DC current in the absence of injury include
lactacidosis and disturbance of the inter-renal stress response (Mitton and MacDonald, 1994).
Field studies examining the effect of electrofishing on growth and condition of salmonids have
reported mixed results.
However, neurologists have recognized for more than 100 years that electric stimulation of
vertebrates causes epileptic seizures. Similar epileptic seizures are produced by alternating,
direct, and pulsed currents of any shape or frequency. The observed behaviors result from
stimulation of the central nervous system, not from local nerve and muscle responses. Spike–
wave patterns of neural discharge on electroencephalograms, which are diagnostic of epilepsy,
have been recorded in fish.
Even more recently, it has been suggested that the observed responses of fishes to an electric
field, including twitches (in the zone of perception or reactive detection), taxis, narcosis, and
tetany, are essentially aspects of the same phases of epilepsy (automatism, petit mal, and grand
mal) that are observed in humans and other animals subjected to electroconvulsive therapy
(Sharber et al., 1994, 1995; Sharber and Black, 1999) (Figure 1).
Fig – 10.5
Figure 1. Major intensity-dependent electrofishing response zones. The outer boundaries of
response zones for a spherical anode at the surface and sufficiently distant from the cathode are
more-or-less hemispherical shells around the anode that represent field-intensity thresholds for
the associated responses. Actual and relative sizes of the zones are specimen dependent (species,
size, condition, and orientation) and vary with electrical output, electrode size and shape, and
environmental conditions. Labels in italics represent corresponding phases of epilepsy as
suggested by Sharber and Black (1999) except that here the phase of tonic–clonic contractions
(quivering or pseudo-forced swimming) between petit mal and grand mal (narcosis and tetany) is
treated as the initial part of grand mal (partial tetany). Zone of reactive detection is sometimes
referred to as zone of perception. Zones of taxis, narcosis, and tetany represent the effective
range for fish capture using direct and pulsed direct currents. (Reproduced from Snyder (2003),
Figure 11.)
Most of the currently accepted or proposed concepts for explaining or better understanding the
responses of fish to electric fields, and the mechanisms involved, need to be further explored,
validated, refined, and integrated to advance the science and technology of electrofishing. This
might be accomplished best through a well-coordinated, cooperative, interdisciplinary program
for future electrofishing research.
Harmful effects
Stress, injuries, and sometimes mortalities among captured fish are unavoidable consequences of
electrofishing and most other collection techniques. Among the more effective gear and
techniques available for collecting fish, biologists usually select those known to be least harmful,
but comparative data on harmful effects are often lacking or inconclusive. In many cases,
especially prior to the late 1980s, electrofishing had been considered not only the most effective
but also the least harmful means to capture fish, particularly moderate to large-size specimens.
Despite occasional reports of substantial harm to fish, the relatively benign nature of
electrofishing had been assumed because generally fish recovered quickly and few mortalities or
external injuries were observed or reported. Also, the most frequently noted external effects,
brands, were often dismissed by experienced electrofishers as harmless, temporary effects rather
than as indicators of potentially serious spinal injuries or hemorrhages.
But since the late 1980s, many investigators have shown that assessment of electrofishing
injuries based only on externally obvious criteria can be highly inadequate. Sharber and
Carothers (1988) X-rayed and necropsied many large rainbow trout captured by electrofishing,
found spinal injuries and associated hemorrhages in 44–67% of the fish, and concluded that
without such analysis, most of these injuries would go undetected unless they were very severe.
Especially severe spinal injuries or muscular hemorrhages (Figures 2 and 3) can be represented
externally by brands (particularly those that are in fact bruises, Figure 4), bent backs, punctures,
or abnormal swimming, but in most fish even severe injuries are not externally obvious.
When electrofished specimens were similarly examined in subsequent investigations by other
biologists (e.g., Holmes et al., 1990; Meyer and Miller, 1991; Fredenberg, 1992; Newman, 1992;
McMichael, 1993; Hollender and Carline, 1994), they too documented large percentages of fish
with electrofishing injuries for some species, especially salmonids. As a result, new research
focused on the extent of such injuries in specific applications, longer-term impacts, causes, and
modifications to gear and techniques that might reduce harmful effects.
Based on these studies, some programs, agencies, and institutions have been re evaluating their
use of electrofishing and instituting policies or guidelines to reduce the potential for injury. But
we must better understand the problem, the factors involved, and how to minimize injuries.
Fig – 10.6
Figure 2. Dorsal-view X-ray of a rainbow trout (Oncorhynchus mykiss) revealing severe spinal
misalignment and fractured vertebrae caused by electrofishing. (Photograph provided by and
used with permission of N. G. Sharber, Flagstaff, Arizona; reproduced from Snyder (2003),
Figure 16-top.)
Fig – 10.7
Figure 3. Necropsy fillet of rainbow trout (Oncorhynchus mykiss) revealing multiple
hemorrhages and associated tissue and vertebral damage caused by electrofishing. (Photograph
provided by and used with permission of N. G. Sharber, Flagstaff, Arizona; reproduced from
Snyder (2003), Figure 17-top.)
Factors affecting injuries and mortality
Factors considered in the literature to affect electrofishing injuries and mortalities include type of
current, field intensity, duration of exposure, orientation of fish relative to lines (net direction) of
current, and for alternating current (AC) and pulsed direct current (PDC), waveform
characteristics such as shape, wave or pulse frequency, and pulse width. Additional factors
considered were fish species, size, and condition. However, data regarding the effects of these
factors are sometimes sparse, difficult to compare, and often questionable. Available data
generally support the contention that of the three types of electrofishing currents, AC is most
harmful, DC (constant direct current) least, and PDC usually somewhere between depending on
the frequency and complexity of pulses. Although there are reports of no mortality or injury for
each type of current, when such adverse effects do occur and comparisons are possible, AC tends
to be more lethal than either DC or PDC, and AC and moderate to high-frequency PDCs tend to
cause more spinal injuries and hemorrhages than DC, low-frequency PDCs, or the only complex
PDC tested to date—Complex Pulse System (CPS, a patented pulse train of 3 square pulses at
240 Hz delivered 15 times per second). The extent of mortality or injury caused by each of these
currents varies considerably with how they are used, other electrical parameters, biological
factors, and environmental conditions. With enough field intensity and duration of exposure, any
type of current can be lethal, and under certain conditions even DC can injure substantial
numbers of fish.
Fig – 10.8- Brands (bruises or dark pigmental discolorations) in rainbow trout (Oncorhynchus
mykiss) caused by electrofishing. Brands are usually temporary external manifestations of spinal
injury, but injured fish often lack brands. (Photograph provided by and used with permission of
W. A. Fredenberg, Creston National Fish Hatchery, Kalispell, Montana; reproduced from Snyder
(2003), Figure 2.)
As for most chemical substances and physical parameters affecting living organisms,
concentration (in this case, field intensity) and duration of exposure are the primary factors
affecting physiological stress and mortality in fish subjected to electrofishing currents. Beyond
lethal threshold levels, increases in electrical-field intensity or duration of exposure typically
result in increased mortality. However, it is not field intensity itself, but the magnitude of voltage
differential generated across fish (usually head-to-tail voltage) or specific nerves or tissues that
causes electrofishing mortalities and most sublethal physiological effects and behavioral
responses. Voltage differential is a function of both field intensity and orientation of the fish
relative to the lines of current. Unlike its crucial effect on electrofishing mortality, field intensity
beyond requisite threshold levels has an unclear, but evidently not critical effect on electrofishing
injuries. Spinal injuries and associated hemorrhages can occur in fish located anywhere in the
field at or above the intensity threshold for twitch in the zone of perception. In the zone of
perception, as many fish, including those injured by the electrical field, are likely to escape the
field as move into the effective zones of the field for capture (taxis, narcosis, and tetany).
Fig – 10.9 - Fractured vertebrae from a rainbow trout (Oncorhynchus mykiss) caused by
electrofishing. (Photograph provided by and used with permission of W. A. Fredenberg, Creston
National Fish Hatchery, Kalispell, Montana; reproduced from Snyder (2003), Figure 18.)
The principal cause of spinal injuries appears to be muscular convulsions (myoclonic jerks or
seizures) induced by sudden changes in field intensity or, more specifically, in voltage
differential across the fish or affected tissues at or above a relatively low threshold in magnitude
of change for twitch. Such sudden changes occur when current is switched on and off or pulsed,
when fish leap frantically out of and back into the electrified water, and when netted fish are
removed from or dipped in and out of the field. Accordingly, duration of exposure in DC should
have no effect on incidences of spinal injuries while fish remain in the water, but in PDC, longer
exposures subject fish to more pulses and thereby increase potential for spinal injury.
However, neither muscular convulsions as the principal cause of spinal injuries in fish nor
sudden changes in voltage differential as the principal cause of the convulsions have been
experimentally documented. Also, the latter is contradicted, seemingly, by the observation of
twitches during uninterrupted DC and occasional documentation of as many spinal injuries (at
least minor ones) in DC with just two sudden change events (when the current is switched on and
later off) as in some simple or complex PDCs with numerous sudden changes in voltage
differential. Increases in spinal injuries with exposure time might be expected as well for AC
with its cyclic changes in voltage differential and direction (effectively alternating half-sine
pulses), but limited experimental evidence suggests otherwise. Perhaps the changes in AC
voltage are not sufficiently sudden (if so, the same would apply to halfsine PDC), or the change
in direction precludes possible consecutive-pulse summation effects that might sometimes be
necessary to achieve the threshold magnitude of change in voltage differential.
Whether the probability or degree of spinal injuries and hemorrhages increases with field
intensity or not, fish in a state of narcosis (petit mal) or tetany (grand mal) may no longer be
subject to the sudden convulsions that are believed to cause most spinal injuries in PDC (and
possibly AC). Injuries might still occur during transition between these states and when fish are
removed from the field. If some spinal injuries do occur during tetany, as has long been
suspected but unproven, the sustained muscular tension would have to be sufficiently strong to
permanently compress one or more portions of the spinal column, burst blood vessels, and
possibly fracture vertebrae (Figure 5). Aside from this possibility, measures to specifically
reduce the intense zone of tetany around an electrode might not have much impact on the
frequency of spinal injuries, but they should reduce incidences of severe stress, fatigue, and
mortality.
Orientation of fish when first exposed to the effective portion of the field is probably as
significant a factor in electrofishing injuries as in other responses and mortality. However, based
on limited evidence, greatest effect appears to occur when fish are perpendicular to rather than
parallel to the lines of current (minimum rather than maximum head-to-tail voltage differential).
If so, experiments to assess the injurious effects of electric currents on fish might be confounded
or biased to minimum effects if fish are held parallel to the direction of current. For PDC, pulse
frequency appears to be a primary factor affecting the incidence of spinal injuries and may be a
significant secondary factor in electrofishing mortalities. As expected if spinal injuries are
caused primarily by sudden changes in electrical potential, the incidence of injuries is generally
lowest for low-frequency currents and increases with pulse frequency. With regard to incidences
of spinal injuries, the CPS pulse train with a primary frequency of 15 Hz appears comparable to
simple low-frequency currents (and DC). It is unknown whether other pulse trains or complex
variations of PDC also result in as few injuries as low-frequency PDCs.
The effects of pulse shape or waveform, pulse width or duty cycle, and voltage spikes on
mortality and spinal injuries have been inadequately investigated and data that are available are
difficult to compare and sometimes contradictory.
Although exponential and half-sine PDCs have been implicated as particularly lethal and halfsine, quarter-sine, and square PDCs as particularly injurious, the effects of PDC waveforms on
electrofishing mortality and injury remain inconclusive. Likewise for AC waveforms, despite
one comparison of sine wave and triangular-wave AC which revealed no significant differences
in incidence of externally obvious injuries but notable differences in the nature and perhaps
severity of those injuries. The little data that exists with regard to pulse duration or duty cycle
suggests no effect on mortality and a tendency for fewer spinal injuries using currents with
longer pulses or greater duty cycles.
A limited-scope investigation suggested that voltage spikes have little or no impact on
electrofishing injuries or mortality. Evidence to date strongly indicates that trout, char, and
salmon (subfamily Salmoninae) are more susceptible to spinal injuries, associated hemorrhages,
and probably mortality during electrofishing than most other fishes. Among other species, burbot
(Lota lota) and sculpins (Cottidae) were reported to be particularly susceptible to electrofishing
mortality, at least under some environmental and electrical-field conditions, whereas goldeye
(Hiodon alosoides), some suckers (Catostomidae), channel catfish (Ictalurus punctatus),
largemouth bass (Micropterus salmoides), walleye (Stizostedion vitreum), and possibly
paddlefish (Polyodon spathula) were reported to be more susceptible to electrofishing-induced
spinal injuries and associated hemorrhages. Electrofished mountain whitefish (Prosopium
williamsoni) have been reported to be particularly susceptible to bleeding of the gills.
Because voltage differential across fish or specific tissues increases with size, larger fish have
been expected to be more susceptible to electrofishing mortality and injury than smaller fish.
However, laboratory and field data suggest that increases in electrofishing mortality with size
might only occur with increases in exposure time and some researchers have reported greater
electrofishing mortality among smaller fish. Some data support an increased frequency of spinal
injuries as fish size increases, but other data do not, and so the importance of size remains
questionable.
The physical condition of fish can affect their susceptibility to electrofishing injury and
mortality, but assessment of this factor is based mostly on suppositions and casual observations
rather than specific experiments and data. Fish in poor health may respond less strongly to
electric fields, thereby reducing chances for spinal injury, but they also may be less able to
withstand the stresses of tetany and apnea during narcosis, thereby increasing probability of
death. On the other hand, weakened skeletal systems probably make fish especially susceptible
to spinal injuries.
Impacts on reproduction, embryos, and larvae
Electrofishing can also affect reproduction and early life stages. In addition to or as a result of
injuries, exposure of ripe fish to electrofishing fields can cause significant damage to, or
premature expulsion of, gametes and sometimes reduces viability of subsequently fertilized eggs.
Electrofishing over active spawning grounds can also significantly affect survival of embryos on
or in the substrate if exposed during their more sensitive stages (prior to acquisition of eye
pigment). Exposure of recently hatched larvae might not cause significant mortality but can
reduce growth rates for at least a few weeks. Field intensity and duration of exposure appear to
be the most critical electrical factors affecting embryos and larvae.
RECOMMENDATIONS FOR MINIMISING EFFECTS
Notwithstanding the range of literature detailing negative aspects associated with the capture of
fish using electric fishing, it is generally considered that provided the technique is carried out in
an optimal manner, the technique should continue to be used for sampling fish populations. The
following, whilst detailing ways in which optimal settings can be achieved for fishing, does not
however provide a magic wand for the end of all capture related injury to fish. If applied
intelligently to any particular situation it will however reduce the negative impacts that may be
associated with electric fishing. Where the populations are rare or endangered, methods other
than electric fishing should be considered.
The following is based on the recommendations of Snyder (1992) and Kolz et al. (1998) In order
to lower the possibility of trauma, the following measures should be taken:
Where practical, use smooth dc.
If dc is not practical, use pdc systems with waveforms, pulse frequencies or patterns, and
power levels likely to cause the least damage while still maintaining adequate capture
efficiency.
Whether warranted or not, ac is recognised by many authorities as the most harmful
waveform used in electric fishing. Until proven otherwise, ac should be avoided for most
purposes. Ac should only be considered when fish are to be killed.
Operate electric fishing systems at the lowest effective power setting that still provides
for effective electric fishing. Fish should be observed following capture to ensure that
they recover equilibrium within one to two minutes; if not, power should be reduced.
Use electrodes with the largest effective diameter practical to minimise or eliminate
the zone of tetany around the anode.
Equipment for measuring conductivity and field strength (voltage gradients) in the water
should be available on each electric fishing trip to monitor equipment operation and
adjust settings and electrodes for the desired size and intensity of the field.
Minimise exposure to the field and specimen handling – rapidly net fish before they get
too close to the anode, and quickly, but gently, place them in oxygenated holding water.
While length of exposure to the electric field does not appear to increase rate of trauma
(bleeding or fractures), length of exposure does increase stress levels. Netters should not
allow fish to remain in the net too long or repeatedly dip fish back into an active electric
field.
Change the holding water frequently to ensure adequate dissolved oxygen and avoid
fishing in excessive temperatures. In order to determine the threshold level for electric
fishing with pulsed direct current Temple (pers. com.) recommends the following
procedure. Whilst to many, the procedure will appear to be overly cautious it would only
be required to be carried out a few times for the optimal settings (not just those that catch
fish but those that catch fish with little danger of injury) to be established.
(Epilepsy as a Unifying Principle in Electrofishing Theory: A Proposal; Norman G. Sharber,
Jane Sharber Black; Transactions of the American Fisheries Society 1999; 128: 666-671)