7116 ‎(2)‎

• UCREFRP Catalog Number: 7116
• Author: Baxter, G. and J. Simon
• Title: Editor
• USFW Year: Series
• USFW - Doc Type: 1970
• Copyright Material: YES

36

eases and parasites, qualified persons should con-
trol and identify release specimens.
The electrofishing apparatus is extremely effi-
cient for catching trout and salmon parent fish in
smaller rivers and streams. Highest efficiency is
attained by using a boat. In deep waters it may be
necessary to lengthen anodes and cathodes. High
voltage should be avoided as large fish are easily
damaged or killed when exposed to long lasting or
strong electrical current.

Electrofishing in practice

There is no principal difference between electro-
fishing in lakes and streams, and sampling design
is similar. The design of equipment and per-
formance of fishing is often slightly modified
depending on differences in habitat and fish
behaviour.

Electrofishing in streams

The fishing operation should be performed by a
team of at least 2 persons. The fisherman handles
the anode and is usually equipped with a hand net
for catching. The cooperator is responsible for the
handling of the catch and may also take part in
collection. The fishing efficiency, both with
respect to time and accuracy, is closely related to
the experience of the team, and quantitative fish-
ing should only be carried out by qualified person-
nel.
The fishing is carried out in an upstream direc-
tion. If the removal method is used, it is essential
to work in a standardized manner. The time spent
is oflittle relevance; the main thing is to cover the
whole area in a systematic way. A time lapse of
at least half an hour between the removals is
recommended.
In large streams and rivers the area possible for
electrofishing is restricted by depth and rate of
flow to certain parts, e.g. along the banks. As
discussed in Chapter 4, this has consequences on
the possibility of estimating population size. One
reason is that the catchability of fish within a

sampling area gradually decreases with increasing
depth, which may yield a negative bias. Further,
flight effects may cause displacement of fish from
the area. If the time lapse between fishings is
prolonged, fish may also immigrate, which further
complicates the evaluation.
The use of blocking nets to avoid some of the
above effects is a matter of some controversy. In
a small stream where the cross section is much
smaller than the length of the area, the use of
blocking nets is probably not necessary when
fishing for territorial salmonids, as we have no
indications offish concentrations in the vicinity of
the nets. In large streams blocking nets are some-
times used. The transportation and installation of
these is usually laborious, and the advantage may
be counterbalanced by the fleeing effect of fish
while installing the nets. Further, in large streams
the object is usually by necessity restricted to
estimation of population change, and this change
is probably not estimated most accurately by the
use of blocking nets. In most cases, therefore, the
use of nets is not recommended. In large streams
when blocking nets are not used, the study area
must be marked to ensure that the same area is
fished each time.
Other complications frequently encountered in
large streams are caused by a varying water level.
After a rapid rize it may take some days before
near shore areas are reoccupied by fish. Further,
the physical conditions (depth, substrate, flow) of
a stream section may vary widely with the water
level which may effect both the population and its
catchability. We therefore recommend quantita-
tive fishing only after a period of stable flow, and,
especially when following population changes, at
similar water levels.
In addition to the water level there are other
environmental factors influencing catch ability or
accuracy which may vary with time, e.g. tempera-
ture, turbidity etc. When monitoring population
change the fishing should therefore be carried out
under similar conditions in these respects as well.

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