24
GPP ELECTROFISHER
USER’S MANUAL
2016
voltage gradient around the
cathode.
Figure 8 shows variation of
voltage, as a function of the
distance from the fishing anode,
for three types of cathode.
The required voltage is
reduced by diminishing the
resistance of the cathode
field. This compensates for
the reduced resistance so that
the current does not vary. The
power consumption is directly
proportional to the voltage used.
One advantage of a large
cathode is that the risk of
accidental electrocution is much
reduced. A large cathode has
very low potential with respect
to the soil and the water around
it. The resistance between the
cathode and the water is halved
each time the surface of the
cathode is doubled. For example,
a 100 square foot cathode would
need another 100 square foot
added to pass from 9 to 4.5
ohm. However a cathode larger
than 100 square feet would be
inconvenient to handle for shore-
side electrofishing.
Figure 9 compares small and a
large cathodes. With a standard
grid cathode, the anode voltage
4 volts for the smaller electrode
at the same distance. Note
also that the voltage the fish
receives closer to the electrode
is less for the larger electrode
(100 volts instead of 144 volts).
Larger electrodes thus offers two
advantages: greater range, and
lower maximum gradient.
One drawback is that a larger
electrode also has greater
circuit loading, and thus draws
more current for the same
voltage (twice as much for the
double size electrode). Thus,
a larger electrode requires a
larger generator. This dictates a
practical upper limit on electrode
size for a given generator and
water conductivity. Except for
this limitation, the larger the
electrode, the better the fishing
effectiveness and the easier it is
on the fish.
Figure 6. Comparison of effects of two
sizes of anode.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
volts
1r
2r
3r
4r
5r
200
100
66
50
40
33
200 100 66 50 40 33
1r 2r 3r 4r 5r 6r 7r 8r 9r 10r
28 25 22 20 18
100
44
16
10
7
134
4
volts
Distance from electrode centers (meters)
10cm
20cm
Figure 7. Larger anodes increase the
fishing area.
10cm
20cm
35cm
60cm
Electrode diameter
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
16.63m
2
12.57m
2
10.18m
2
6.16m
2
Distance from electrode centers (meters)
Figure 8. Variation of voltage for three
different kinds of cathode
2
4
6
44
46
48
Distance from center of anode (meters)
00
100
150
200
250
300
350
400
450
500
550
50
Total voltage
Suf
ficient gradient zone
Cathode Indentical to anode: 600V, 6.3kW
0.5m2 grid cathode: 350V, 3.7kW
10m2 wire netting cathode: 310V, 3.2kW
anode
cathode
Figure 8. Variation of voltage for three kinds of anode.
Figure 7 shows that larger
electrodes increase the fish
collection area. The shaded areas
have a voltage gradient between
0.12 and 1.2 volts per cm, and are
suitable for electrofishing. The
applied voltage is 300 volts.
ELECTRODE BEHAVIOR
• Larger electrodes have lower
resistance, need more current
at given voltage, reach out
farther, and have lower
maximum voltage gradient.
• Small electrodes pose a hazard
to fish because of high current
density and voltage gradient.
• Electrodes placed farther
apart use less current, but the
savings are not large.
• The resistance of an electrode
varies in direct proportion to
water resistivity.
RING ELECTRODES
• Once spacing exceeds 10
radii, the distance between
electrodes is insignificant.
• The region affected by the
electrode is limited to 5 to 10
radii.
• Electrode resistance is primarily
dependent on electrode radius,
and varies in inverse proportion
to radius.
• For ring electrodes, the cross
section diameter of the ring
material is of little importance.
If the ratio of cross section
diameter to ring radius is held
constant, resistance varies
inversely with ring radius.
CATHODES
In electrofishing it is desirable
to have a high voltage gradient
around the anode, and a low
