Counting Fish
by
Karen Christopherson
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Have you ever wondered how fish numbers are
calculated, as when someone says, "There are 1000 trout per
mile in this stream"? Do you think the wildlife people
stand on bridges wearing polarized glasses, try to spot fish, and count, "1 trout, 2
trout, 3 trout" while someone tabulates their yelled counts
on a
pad?
There are actually several methods for
"counting fish" or doing fish population surveys. One
of the methods used on creeks and rivers is electrofishing.
In October, 2000, several member of Evergreen
Trout Unlimited (including the author) assisted the Division of Wildlife
with the fish count on Bear Creek, our local water. We
helped with the sampling of three reaches, which had been sampled
seven times previously since 1988.
The process of electrofishing is very
interesting and involves several steps. Basically, a portable generator is used to provide a pulsed, DC current through an
insulated wire which is several hundred feet long. The generator
produces about 250 volts, causing a current of about 1.5 amps to
be transmitted. This wire is placed in the river, with the
negative ends of the wire (cathode) at the upstream end. All of the
surveyors proceed downstream to the start of the reach. There are
three or four people with electrodes (a probe which is placed in
the water and provides the positive end of the current, the
anode) - these
people will be walking in a line across the stream and placed
about five to 10 feet apart. Each probe person also carries a
large dip net on about a six-foot pole. The electrodes are connected
together by insulated wire, which in turn is connected to a wire
running back to the generator.
Standing behind the lead line of probe-holders
are backup samplers also carrying nets. Their job is to catch
the fish that escape the front line, and also to pass fish from
the front to the rear. At the rear of the group is the fish
holding tank, which is a flooded net in a sturdy surround that
can be dragged upriver.
 |
 |
| The crew of surveyors |
Electrode probe, placed in
water and held in left hand
and net in the right hand (probe has been outlined for
visualization) |
The process starts with the probeholders
sweeping their electrodes back and forth through the water in
front of them. The current in the water will affect fish from
one to six feet away from the probe, with smaller fish affected at
closer range. As the current reaches the fish, electrotaxis
effects the nervous system, and thus the muscular system, causing
the fish to be attracted towards the positive probe, slightly
stunned. The samplers have to be very quick in their
wading boots to catch the fish. The fish are
caught in the net and passed back to the holding tank.
 |
 |
| Searching for fish in a
nice pool |
The live net placed in the
stream (used for
holding the fish before counting) |
The group proceeds upstream to the end of the reach, just downstream from the negative end of the current wire.
If the holding tank is full, these fish will be placed in a live
net, stationed about midway through the reach. Else, at the end
of the reach, all of the fish are placed in the live net.
One person immediately starts to measure the
fish. Each fish is singularly measured in millimeters, weighed
in grams, and then placed back in the river. Another person
tabulates these measurements and also records the species of
fish. On this day of sampling in Bear Creek, four species were
sampled - brown trout, rainbow trout, longnose daces, and
longnose suckers.
 |
 |
| Measuring a rainbow |
Weighing a brown trout |
While the measuring and weighing is started, the rest of the group
return downstream to do a second pass of sampling. The procedure
is exactly the same as before; but with far less fish being
found. To be successful, an electrofishing survey should sample
about 65% of the estimated population on the first pass.
 |
One of the fine rainbows
sampled |
After the sampling is done, and the second pass fish have been counted, the crew moves on to the next
sampling area. The tabulated data of fish species, length and
weight will be compiled in the office. Computer programs and
statistical analyses (specifically Seber-LeCren estimations) are
used to determine the probable distribution of species and size
of fish in the sampled reach. Several statistical estimates can
be determined such as number per acre, number per mile, pounds
per acre for specific species or (for example) trout population
as a whole.
These numbers acquired from electrofishing are
useful for numerous reasons. Obviously, we flyfishermen and
women like to know how many trout are in a particular stream.
Fish count information is used by wildlife divisions to label
rivers as "Gold Medal", "Blue Ribbon", and
others. Electrofishing data are also used in environmental assessments
of rivers, in determination of regulation changes, in stocking
analysis, and many other ways by wildlife and fishery groups.
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Evergreen Trout Unlimited thanks
the following Colorado Division of Wildlife employees for their
hard work and a fun day:
