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GPP ELECTROFISHER
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An electric field in water can
be considered to have three
separate areas. The outer
peripheral area is a weak field
that the fish is indifferent to.
The next area, closer to the
electrodes, has a stronger
electrical field, but not enough
to stun the fish. In this area, the
involuntary swimming action
will occur and the fish will
swim towards the anode. The
innermost area has the strongest
electrical field, and the fish within
it are immobilized.
ZONE OF POTENTIAL FISH
INJURY
Fish close to the anode receive
a very high head-to-tail voltage.
Most fish injuries occur within
half a meter from the anode. This
is called the Zone of potential
fish injury. We can minimize the
injury by reducing the time the
electricity is turned on .
DUTY-CYCLE
Duty-cycle is the percent of
on-time. It is a product of the
pulse width and the pulse
frequency. The duty-cycle can
be lowered in three ways: by
reducing the pulse width, by
reducing the pulse frequency, or
by using gated bursts, where the
power is off for a period between
each burst of pulses. Fish close to
an anode with a low duty-cycle
are far less likely to be injured
than with a high duty-cycle.
ELECTRODE DESIGN
The way in which voltage
and current distribute around
electrofisher electrodes is
complex. Figure 4 shows the
field pattern created by a pair of
closely spaced ring electrodes,
and the voltage gradient
between them. Note that the
current density and voltage
gradient are highest near the
electrodes.
The dimensions of the
electrodes are very important
in determining the voltage
distribution around electrofisher
electrodes. Figure 5 compares
a 10cm and a 20cm ring anode
carrying 200 volts in open
water. The cathode dimension is
considered to be infinite. Note
that the 20cm anode reaches
out much further, producing a 33
volt potential at 1.2 meter. But the
10cm anode produces the same
potential at only 0.6 meter from
the electrode.
Figure 6 further illustrates
the effect of electrode diameter.
The voltage is applied head-
to-tail to a 20cm long fish. The
applied voltage is 200 volts with
10cm and 20cm diameter ring-
electrodes. Note that the 20cm
electrode reaches out farther,
producing 7 volts head-to-tail
between 1.0 and 1.2 meter from
the electrode; as opposed to only
Figure 4. The field pattern, and the variation of gradient
between two electrodes.
Distance
Electrode V
oltage
Gradient
Current lines
Constant voltage lines
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Distance from electrode centers (meters)
volts
1r
2r
3r
4r
5r
200
100
66
50
40
200 100 66 50 40 33
1r 2r 3r 4r 5r
33
10cm
20cm
volts
Figure 5. Comparison of two sizes of
anode.
Figure 4. The field pattern, and the variation of gradient
between two electrodes.
Distance
Electrode V
oltage
Gradient
Current lines
Constant voltage lines
Figure 4. The variation between
between two electrodes.
The field pattern.
INTRODUCTION TO ELECTROFISHING
