Electrofishing apparatus and method

To minimize injury during electrofishing while inducing a high degree of electrotaxis, a packet of DC pulses having sharp leading edges and preferably an exponential or square decay are transmitted under water between an anode electrode and a cathode electrode several times each second to stimulate contraction of the red muscle tissues and bring about electrotaxis without inducing epileptic seizure of the white muscle tissues of fish and thereby avoiding injury to the fish.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electrofishing and, more particularly, to 
apparatus and method for minimizing the likelihood of injury to fish while 
inducing electrotaxis and narcosis. 
2. Description of the Prior Art 
Electric fishing has been known since at least 1863 when a British patent 
directed to the subject was granted. However, it was not until after World 
War I that serious attention began to be given to electric fishing or 
electrofishing. As a result of various studies and investigations, certain 
conclusions reached by various researchers have been taken to be truisms. 
Some of these conclusions are based upon inadequate data, unsubstantiated 
assumptions or insufficient inspection and analysis of the fish caught and 
are therefore suspect. 
There are many uses for electrofishing which are carried out to the extent 
the equipment available permits and without inducing an unacceptable 
mortality rate. Electrofishing permits the capture and removal of fish 
population from one locality to another. It permits surveys or population 
estimates to determine the type and number of fish present and their size 
range. Such estimates may also uncover natural fluctuations in population 
and assess the impact of channelization. Electrofishing permits guiding 
the movement of fish such as keeping predators away from freshly planted 
fry, keeping fish away from electric power plant water intakes, keeping 
migrating fish away from specific areas en route and trapping fish (such 
as the sea lampreys in the Great Lakes). Biological sampling may be 
accomplished through electrofishing to collect brood stock, determine 
species composition, tabulate size and age characteristics. Food habits 
can be determined by collecting fish at feeding areas. Tagging and marking 
studies are readily carried out by acquiring fish by electrofishing, 
provided that the stresses of handling and shocking do not injure or kill 
too large a percentage. The prospect of injury and death is of particular 
concern when one is dealing with endangered species of fish. 
Electrofishing permits a determination of seasonal migrations. Fish can be 
readily counted by recording changes in underwater field strength due to 
passage of fish therepast. Electrofishing can be used as an anaesthetic to 
assist in treating sick fish, such as parasite control, and to quiet fish 
for handling. Electrofishing can be used for collecting floating 
invertebrates and electrofishing can be used to quickly and efficiently 
electrocute fish without degrading the commercial value of the catch since 
the fish scales remain intact. 
During electrofishing with pulsed DC electric current, a fish will have 
several reactions, depending upon the field strength or density in which 
it finds itself and upon the frequency, shape and width of the pulses. The 
first reaction is that of frightening the fish. A second reaction is 
electrotaxis, the involuntary exercise of swimming muscles to draw the 
fish toward the source of electric current. The third reaction is narcosis 
when the muscles go limp and the fish rolls on its side; this permits 
netting and acquisition of the fish. The fourth reaction is tetanus which 
is an involuntary contraction of the muscles without interleaved 
relaxation and can result in death. A fifth reaction can occur if the 
white muscles of the fish are stimulated to the point of an epileptic 
seizure, thereby causing morphological trauma. 
Since the inception of electrofishing for scientific purposes, there have 
been reports of injuries to fish due to exposure to electric stimuli. The 
injuries include compression of the spinal column, torn supportive tissues 
around various organs and broken blood vessels (hematomas). In general, 
these injuries have been thought to be the result of high current 
densities which may be encountered by the fish near an electrode. 
In normal electrofishing practice, direct current or pulsed direct current 
is used because aquatic animals will move, in general, to the anode 
electrode. In the case of fish, this movement, electrotaxis, involves a 
pseudo swimming reaction. As a fish approaches the anode electrode, it 
encounters an exponentially increasing field strength. At some critical 
value of field strength, depending upon many physical factors, such as the 
water conductivity, the fish may cease electrotaxis action, enter a state 
of narcosis and then tetanus, a few feet from the anode electrode or very 
near it. Often, the critical state occurs a few feet from the anode 
electrode or very near it. In either case, the fish almost always drifts 
near to or may actually touch the anode electrode. The field strength 
within this zone causing tetanus is very high and a significant flow of 
electric current through the fish occurs. This electric current is 
generally believed to stimulate and then overwhelm the neuromuscular 
system of the fish. It is believed that the overwhelmed neuromuscular 
system causes the above referenced trauma. 
This view overlooks or disregards facts relating to muscle cells and proven 
in laboratory experiments by biology researchers. A neuromuscular response 
requires a minimum threshold level of external electrical stimulus before 
response. Once the existence of a response is established, the following 
principal factors determine the type of response: frequency of pulse, 
duration (or width) of each pulse and the shape of each pulse. 
It is known and accepted that a nerve cell responds best to an almost 
instantaneous rise from zero to a maximum value in less than 1 ms. When a 
cell responds or "fires", contraction is initiated, which contraction will 
proceed without regard to further external stimulus. If the external 
stimulus is brief, the cell will relax in approximately 1/300 of a second 
and be ready for the next pulse stimulus. In the event the initiating 
pulse has a long on time, the cell becomes stressed due to lack of 
relaxation and it may remain contracted. Tetanus is the constant 
unrelieved contraction of a muscle cell. 
Preferably, the leading edge of the pulse is sharp enough to fully 
stimulate the nerve cell. Should the leading edge represent a sloping 
gradual rise to the firing point of the cell, traumatization and tetanus 
will result. The trailing edge of the pulse can be either square or 
exponential in decay time. Should the slope of the pulse be sinusoidal or 
a linear decay, the cell might not relax and such lack of relaxation will 
lead to trauma and tetanus. 
In a paper entitled "Influence of Electrofishing Pulse Shape on Spinal 
Injuries in Adult Rainbow Trout" (1988), Sharber and Carothers evaluated 
the effect of three wave forms of pulsed DC on spinal injury to 300 to 560 
mm rainbow trout captured by electrofishing. The overall injury incidence 
was 50% with a significantly higher incidence (67%) in fish stunned with a 
quarter sine wave than those captured with either exponential or square 
waves (44% each). Smaller fish or fish in less conductive water appear to 
have a lower incidence of injury, as reported in other studies. In a 
recent study by the Alaska Department of Fish and Game, the incidence of 
spinal injury to rainbow trout having a fork length of greater than 400 mm 
was 50%. As the result of such high incidence, further electrofishing for 
population studies of large rainbow trout has been suspended in Alaska 
until an acceptable solution is found. The pulse rate employed was 60 
pulses/second. 
Based upon criticism from authorities in the field of electrofishing, it 
was suggested that a pulse frequency of 500 pulses/second having a width 
of 0.2 ms. would prevent the injuries caused by a pulse rate of 60 
pulses/second. The basis for this argument resided in a contention that 
the muscle cells would not respond to the repetitive pulsation with their 
limited 300 contraction cycles per second and, without response, would not 
be traumatized to the extent that injuries occur. Based upon a further 
study performed and reported in a paper entitled "Electrofishing Induced 
Spinal Injury in Rainbow Trout" (Sharber, Carothers, Sharber), various 
experiments were performed changing the pulse frequency through a range of 
15 to 512 pulses/second and a correlation between injury rate and 
frequency was established. Based upon this study, less than 3% of the 
trout were injured at a frequency of 15 pulses/second with the incidence 
of injury rising to 24% at 30 pulses/second. From 60 to 512 pulses/second, 
the injury rate occurrence was 42% to 61%, respectively. Accordingly, the 
experimental results did not support an authoritarian viewpoint. 
Further authorities in the field have postulated that the intensity of the 
field attendant a relatively small electrode results in overwhelming 
electric currents in the fish and thereby traumatize the muscles of the 
fish leading to broken backs. As discussed in further detail in the above 
identified paper entitled "Influence Of Electrofishing Pulse Shape On 
Spinal Injuries In Adult Rainbow Trout", experimental results indicate 
that electrode size and shape, even though producing substantially 
different current gradients, had little or no influence on the rate of 
spinal injury. 
SUMMARY OF THE INVENTION 
A packet of DC pulses having sharp leading edges and, preferably, 
exponential or square decays are transmitted under water between 
electrodes to stimulate contraction of the red muscle tissues without 
substantially affecting the white muscle tissues of the fish. The number 
of individual pulses and their pulse width may be varied. Similarly, the 
number of pulse packets per second may be varied in an effective range of 
approximately 5 to 25 packets per second. These variables permit 
optimizing the performance of equipment for different species of fish and 
various physical features attendant the habitat being electrofished and 
relating to water conductivity, temperature, depth and size of fish even 
within a species. A high frequency oscillator generates the high frequency 
pulses which are counted by a counter to limit the number of pulses per 
packet. A low frequency oscillator controls the number of packets of 
pulses per second transmitted and means are provided to permit variation 
in the pulse width. The output is a burst of high frequency pulses having 
square wave leading edges repeated at a low frequency high voltage direct 
current. 
It is therefore a primary object of the present invention to provide 
apparatus for managing the injury to fish to an acceptable level during 
electrofishing. 
Another object of the present invention is to provide apparatus for 
inducing electrotaxis with a low injury rate. 
Yet another object of the present invention is to provide apparatus for 
significantly stimulating only the red muscle groups during electrotaxis. 
Yet another object of the present invention is to provide apparatus for 
generating a burst of high frequency pulses repeated at a low frequency 
high voltage direct current. 
A further object of the present invention is to provide a method for 
inducing electrotaxis and narcosis in fish with minimal injury. 
A yet further object of the present invention is to provide a method for 
stimulating the red muscle tissues without inducing injury causing 
epileptic seizure in fish during electrofishing. 
These and other objects of the present invention will become apparent to 
those skilled in the art as the description thereof proceeds.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
It is well known to biologists that fish have two groups of muscles. A 
first group, known as red muscles, are disposed along opposed sides of the 
body of the fish. These muscles are the swimming muscles. They are 
alternatively contracted to effect undulation and swimming motion by the 
fish. These muscles are relatively small sized but have great stamina and 
can be continuously active for long periods of time. The remaining muscles 
in a fish are referred to as white muscles. These muscles perform all the 
remaining body functions. They are generally large sized, very powerful 
and rapidly responding to a triggering impulse. Generally, they are pairs 
of muscles located on opposed sides of a fish. When paired groups of 
muscles are triggered, a contracting reaction occurs. Normally, cessation 
of the triggering impulse whether internally or externally induced, 
permits relaxation of the muscles. When the paired groups of muscles are 
overwhelmed by triggering impulses which preclude alternating relaxation, 
epileptic seizure occurs. Because of the substantial power of the white 
muscle groups, severe damage to the fish occurs during such seizure. The 
power of the muscles is sufficient to break or dislodge the spinal column; 
this injury is often fatal. A tearing of secondary muscles and tissues 
occurs which produce massive hematomas which are often fatal. Epileptic 
seizure of the white muscles can cause both short term and long term 
damage or injury to organs and may result in immediate or delayed death. 
Many of these injuries are not immediately visually apparent. Usually, 
X-ray films must be analyzed or autopsies must be performed to learn the 
extent of the injuries. 
For years it has been thought that the red muscle tissues, which are 
responsible for the sustained swimming motion, were overwhelmed by 
electrical impulses due to a high field strength proximate an electrode 
during electrofishing. That is, it was believed that the degree of injury 
to fish was a function of the power level used during electrofishing and 
that the damage and injury was due to contractions of the red muscle 
groups. Based upon information supplied by biologists, it is generally 
accepted that the red muscle tissues are insufficiently powerful to break 
the spinal column or cause massive organ and tissue damage which has been 
found in fish injured by electrofishing. The white muscle tissues are, 
however, sufficiently powerful to cause the fatal or near fatal injuries. 
One must therefore conclude that the white muscle tissues are responsible 
for a significant part of the injuries caused during electrofishing. 
Since field strength, for any given power input, is to some extent a 
function of the anode size, it was believed that modification of the 
electrode configuration would have a salutary effect in reducing the 
injury rate from approximately 50% to a very acceptable rate of less than 
10%. Experiments indicate that a reduction in field strength due to a 
particular electrode configuration had little effect upon reducing the 
injury rate. The main difference appears to be that the distance at which 
electrotaxis resulted was substantially reduced. 
As fish transition from electrotaxis to narcosis, the muscles relax and the 
fish essentially stop swimming. Depending upon the direction of the water 
current and other factors, migration of the fish toward the anode 
electrode may occur. Such migration will subject the fish to more intense 
or higher power level field strength. Tetanus may result with attendant 
severe or fatal injuries. 
To prevent injury to fish during electrofishing, apparatus must be 
developed which affects the red muscle tissues to induce electrotaxis 
without simultaneously or subsequently causing contraction of the white 
muscle tissues. Based upon substantial experimentation and testing, a 
pulse train has been developed which includes high frequency pulses for 
stimulating the red muscle tissues and a sufficiently low repetition rate 
to permit the white muscle tissues to relax between contractions and 
thereby prevent epileptic seizure. A circuit for developing such a pulse 
train is illustrated in FIG. 1. Operation of this circuit has been 
effective at various power levels tested. At different power levels, only 
the distance at which electrotaxis is first induced varies; the injury 
rate does not seem to be a function of the power level. Moreover, 
different electrode configurations to modify the maximum field strength to 
which the fish may be subjected has had no significant effect upon the 
injury rate and has only affected the distance at which electrotaxis first 
occurs. 
The pulse train transmitted by the circuit illustrated in FIG. 1 includes a 
plurality of packets, each packet having a plurality of high frequency 
pulses, which packets are repeated at a low frequency rate. The high 
frequency pulses will stimulate the red muscle tissues. The low repetition 
rate permits any contracted white muscle tissues to relax between 
contractions. With such relaxation, the white muscle tissues will not 
suffer epileptic seizure. Without epileptic seizure, significant damage to 
the fish will not occur. 
Referring to FIG. 1, there is shown a circuit 10 for generating a plurality 
of packets of high frequency pulses repeated at a low frequency rate to 
produce a high voltage direct current for use in electrofishing. The 
circuit includes three sections. A power supply, designated by block 12, 
provides plus and minus 12 volts DC to the various circuit components. A 
high voltage DC section, designated by block 14, provides the output 
signal to a load R.sub.L representative of the anode electrode used in 
electrofishing. A timing/SCR drive section, designated by block 16, 
controls the shape and frequency of the pulse train present at the 
electrode anode (R.sub.L). 
Power supply 12 may be a conventional power supply wherein the primary coil 
18 of transformer 20 is connected across an alternating current power 
supply 22. Secondary coil 24 provides power to a bridge rectifier BR2. The 
DC output of the bridge rectifier is smoothed by capacitor C.sub.4 and 
regulated by voltage regulator VR.sub.1. A +12 volt DC output is provided 
between conductor 26 and ground 28. A second bridge rectifier BR3 is 
connected across secondary coil 30 of transformer 20 to provide a DC 
output. The DC output is smoothed by capacitor C5 and regulated by voltage 
regulator VR2. A -12 volt DC voltage is provided between conductor 32 and 
ground 28. 
Referring to high voltage DC section 14, a full wave bridge rectifier BR1 
is connected across power supply 22 to provide a high voltage direct 
current between conductor 40 and ground 28 and across series capacitors 
C1, C2. When silicon controlled rectifier SCR1 is turned on, power is 
applied across the load R.sub.L. Silicon controlled rectifier SCR2 turns 
off SCR1 via commutating capacitor C.sub.3. The voltage level at the anode 
electrode (R.sub.L) is adjusted by a phase control circuit. That is, 
potentiometer P.sub.1 is adjusted to fire diac D.sub.1 at a selected point 
of the 60 cycle alternating current input. When diac D.sub.1 fires, triac 
TR.sub.1 is turned on. Choke L.sub.1 provides di/dt protection for triac 
TR.sub.1. 
The timing cycle produced by timing/SCR drive section 16 begins with low 
frequency oscillator U.sub.1. The frequency of this oscillator controls 
the repetition rate of the high frequency burst (packet of pulses). A high 
output at pin 3 of U1 is transmitted to pin 6 of flip flop U2 via 
conductor 45. The wave form of this pulse is illustrated in FIG. 2A. The 
output of flip flop U2 at pin 1 is an extended pulse, as represented in 
FIG. 2B. It enables, via conductor 46, high frequency oscillator U3. An 
output at pin 2 of flip flop U2 enables counter U5 via conductor 48. The 
frequency of oscillators U1 and U3 can be adjusted by potentiometer 
P.sub.1 connected to pin 7 of oscillator U1 and by potentiometer P.sub.3 
connected to pin 7 of oscillator U3. The output of oscillator U3 on pin 3 
is transmitted along conductor 50 to the trigger input at pin 2 of a one 
shot multi vibrator U4; the pulse train is depicted in FIG. 2C. The output 
pulse width of one shot multi vibrator U4 is controlled by potentiometer 
P.sub.4 connected to pins 6 and 7. As shown in FIG. 2D, the pulse width of 
the pulse train at pin 3 of one shot multi vibrator U4 and conveyed on 
conductor 52 is, in example, approximately 2.9 milliseconds (ms). The 
repetition frequency of the pulses is approximately 4.17 ms (250 Hz), as 
is also indicated in FIG. 2C for the output of oscillator U3. The output 
of one shot multi vibrator U4 is conveyed to pin 14 of counter U5 via 
conductors 54, 56. The leading edges of the pulses transmitted to the 
counter initiate the counting. In the example illustrated in FIGS. 2A to 
2H, the number of pulses per packet has been set at 3. Accordingly, the 
output of counter U5 occurs on conductor 58 at each third count, as 
illustrated in FIG. 2G. The output pulse from counter U5 is transmitted 
via conductor 58 to pin 4 of flip flop U2. When pin 4 goes high, the flip 
flop is reset. The resetting disables the pulse train at output pin 1 and 
oscillator U3 is disabled. The commensurate output at pin 2 of flip flop 
U2 is transmitted via conductor 48 to pin 15 of counter U5 to reset/enable 
the counter. It may be noted that the number of pulses in each frequency 
burst is a function of the selected output pin of counter U5 to which 
conductor 58 is connected. 
The output of one shot multi vibrator U4 is buffered through transistors 
T1, T2 and comparator U6. The rising edge of the square wave present at 
collector 60 of transistor T2 is coupled to gate 62 of silicon controlled 
rectifier SCR1 via capacitor C13. The wave form at collector 64 of 
transistor T2 is depicted in FIG. 2E. Transistor T3 inverts the wave form 
present at collector 60. The resulting pulses at collector 64 of 
transistor T3 are depicted in FIG. 2F. The rising edge of each pulse at 
collector 64 is applied to gate 66 of silicon controlled rectifier SCR2 
via capacitor C14 and conductor 70. When silicon controlled rectifier SCR2 
turns on, silicon controlled rectifier SCR1 is turned off. Similarly, when 
silicon controlled rectifier SCR1 is turned on, silicon controlled 
rectifier SCR2 is turned off. The resulting pulsed DC output across load 
R.sub.L is depicted in FIG. 2H. 
As noted, the individual pulses may be approximately 2.9 ms in width with a 
repetition frequency of 240 Hz. The three pulses occur at the beginning of 
a 66.6 ms pulse train, which is equivalent to a low frequency of 15 
packets per second (15 Hz). Each pulse of the three pulse packet depicted 
in FIG. 2H is a relatively high frequency pulse sufficient to trigger 
contraction of the red muscles in a fish. The relative low frequency pulse 
train repetition rate of 15 Hz has been well documented to be sufficiently 
low to permit relaxation of any contracted white muscles and epileptic 
seizure of such muscles is precluded. 
The number of pulses per packet is believed to be variable in the range of 
2 to 5 pulses per packet without resulting in any substantial injury to 
most fish. The pulse width may be varied at least within the range of 5 to 
25 packets per second. 
To summarize our understanding of the important features of our invention, 
the following conclusions are presented. Spinal compression injury and 
other internal traumas are the result of over stimulation (leading to an 
epileptic seizure) of paired white muscle systems in the fish. Low 
frequency pulses of DC current, e g., 15 pps with a pulse width of 10 
milliseconds, virtually eliminate the problem. However, low frequency does 
not stimulate good electrotaxis. Using high frequency pulses of narrow 
width in packets at approximately 15 packets per second retains the low 
injury rate and causes good electrotaxis response. 
Based upon experience to date, the shape, frequency, width and the ratio of 
the high and low components of the pulse train are all sensed by the 
physiological systems of the target fish More specially, the benefit of 
low injury and effective electrotaxis is dependent upon A) low "packet of 
pulses" frequency; B) shape, width, and number of high frequency pulses in 
each packet; and C) the ratio of the packet frequency to the number of the 
high frequency pulses per packet. If the number of packets per second is 
reduced (say from 20 to 10) then the number of high frequency pulses per 
packet needs to be increased (say from 3 to 4) in order to have the best 
overall results. If the number of high frequency pulses per packet is too 
great, fish will be narcotized too far from the anode electrode for easy 
capture. If the number is too low, the catch efficiency will decrease. If 
the number of packets is too high, the incidence of injuries will 
increase; if too low, the catch rate will be down. That is, there is an 
optimum ratio which our inventions allows the operator to choose to suit 
the type of fish and environment for a given electrofishing effort. 
While the principles of the invention have now been made clear in an 
illustrative embodiment, there will be immediately obvious to those 
skilled in the art many modifications of structure, arrangement, 
proportions, elements, materials and components used in the practice of 
the invention which are particularly adapted for specific environments and 
operating requirements without departing from those principles.