Biological field and laboratory methods for measuring the quality of ...

2.2.3 Electrofishing
Electrofishing is a sampling method in which
alternating (AC) or direct (DC) electrical current
is applied to water that has a resistance different
from that of fish. The difference in the resistance
of the water and the fish to pulsating DC
stimulates the swimming muscles for short
periods of time, causing the fish to orient
towards and be attracted to the positive electrode.
An electrical field of sufficient potential
to immobilize the fish is present near the positive
electrode.
The electrofishing unit may consist of a
llO-volt, 60-cycle, heavy-duty generator, an
electrical control section consisting of a
mod ified, commercially-sold, variable-voltage
pulsator, and electrodes. The electrical control
section permits the selection of any AC voltage
between 50 to 700 and any DC voltage between
25 to 350 and permits control of the size of the
electrical field required by various types of
water. The alternating current serves as a standby
for the direct current and is used in cases of
extremely low water resistance.
Decisions on the use of AC, DC, pulsed DC, or
alternate polarity forms of electricity and the
selection of the electrode shape, electrode
spacing, amount of voltage, and proper equipment
depend on the resistance, temperature, and
total dissolved solids of the water. Light-weight
conductivity meters are recommended for field
use. Lennon (1959) provides a comprehensive
table and describes the system or combination
of systems that worked best for him.
Rollefson (1958,1961) thoroughly tested and
evaluated AC, DC, and pulsating DC, and discussed
bask eledrofishing principles, wave
forms, voltage- current relationships, electrode
types and designs, and differences between AC
and DC and their effects in hard and soft waters.
He concluded that pulsating DC was best in
terms of power economy and fishing ability
when correctly used. Haskell and Adelman
(1955) found that slowly pulsating DC worked
best in leading fish to the anode. Pratt (1951)
also found the DC shocker to be more effective
than the AC shocker.
Fisher (1950) found that brackish water requires
much more power (amps) than fresh-
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FISH SAMPLING
water, even though the voltage drops may be
identical. Seehorn (1968) recommended the use
of an electrolyte (salt blocks) when sampling in
some soft waters to produce a large enough field
with the electric shocker. Frankenberger
(1960), Larimore, Durham and Bennett (1950)
and Latta and Meyers (1961) have excellent
papers on boat shockers. Frankenberger and
Latta and Meyers used a DC shocker and Larimore
et al. an AC shocker. Stubbs (1966),
used DC or pulsed DC, and has his (aluminum)
boat wired as the negative pole. In his paper, he
also shows the design and gives safety precautions
that emphasize the use of the treadle
switch or "deadman switch" in case a worker
falls overboard.
Backpack shockers that are quite useful for
small, wadeable streams have been described by
Blair (1958) and McCrimmon and Berst (1963),
as has a backpack shocker for use by one man
(Seehorn, 1968). Most of these papers give diagrams
for wiring and parts needed.
There are descriptions of electric trawls (AC)
(Haskell, Geduliz, and Snolk, 1955, and Loeg,
1955); electric seines (Funk, 1947; Holton,
1954; and Larimore, 1961); and a fly-rod electrofishing
device employing alternating polarity
current (Lennon, 1961).
The user must decide which design is most
adaptable to his particular needs. Before
deciding which design to use, the biologist
should carefully review the literature. The crew
should wear rubber boots and electrician's gloves
and adhere strictly to safety precautions.
Night sampling was found to be much more
effective than day sampling. Break sampling
efforts into time units so that unit effort data
are available for comparison purposes.
2.2.4 Chemical fishing
Chemicals used in fish sampling include
rotenone, toxaphene, cresol, copper sulfate, and
sodium cyanide. Rotenone has generally been
the most acceptable because of its high degradability;
freedom from such problems as precipitation
(as with copper sulfate) and persistant
toxicity (as with toxaphene); and relative safety
for the user.
Rotenone obtained from the derris root
(Deguelia elliptica, East Indies) and cube root