Special scientific report

APPLICATIONS OF SALT
IN ELECTROFISHING

iNlarine Biological Laboratory

LIB

55.A.K.Y

WOODS HOLE, MASS.

SPECIAL SCIENTIFIC REPORT- FISHERIES No. 280

UNITED STATES DEPARTMENT OF THE INTERIOR
FISH AND WILDLIFE SERVICE

EXPLANATORY NOTE

Hie series embodies results of Investigations, usually of restricted
scope, Intended to aid or direct management or utilization practices and as
guides for administrative or legislative action . It is Issued in limited quantities
for bfflcial use of Federal, State or cooperating agencies and in processed form
for economy and to avoid delay in publication .

United States Department of the Interior, Fred A . Seaton, Secretary
Fish and Wildlife Service, Arnie J. Suomela, Commissioner

APPLICATIONS OF SALT IN ELECTROFISHING

By

Robert E . Lennon and Phillip S . Parker

Fishery Research Biologists
Leetown (P.O. Kearneysville), W. Va.
Bureau of Sport Fisheries and Wildlife

Special Scientific Report- -Fisheries No. 280

Washington, D. C.
November 1958

The Library of Congress has cataloged this publication as
follows:

Lennon, Robert Earl, 1018-

Api)lii'iitions of suit in electrofisliing, by Robert E. Lennon
and Pliillip S. Parker. AVashington, IT. S. Dejjt. of the In-
terior, Fish and AVildlife Service, 1958.

11 p. iliiigi-., tables. 27 cm. (Special scientific report — fislieries
110.280)

Bibliograpliy : p. 11.

1. Electric fisliliiK. 2. Salt. i. Farker, Phillip Slieridaii, 192t>-

.joiut author, ii. Title. ( Series : IT. S. Fish and Wildlife Service.

Special scientific report : fisliei'ies, no. 280)

SH11.A335 no.>280 6:59.2 59-60273

Library of Coiit;ress

The Fish and Wildlife Service series, Special Scientific Report -
Fisheries, is cataloged as follows:

V. S. Fish and Wildlife Service.

Special scientific report : fisheries, no. 1-
iWashingtonj 1949-

no. ill\is., maps, diagrs. 27 cui.

Supersedes in part the Service's Special scientific report.

1. Fisheries — Research.
SH11.A335 639.2072 59-60217

Library of Congress

ABSTRACT

The use of cattle blocks of salt is an effective and economical means
of reducing high resistivities and improving electrofishing in large and small,
high and low, cold and warm streams in the southern Appalachian Mountains .
Electrofishing trials were conducted in salted and salt-free sections of high
resistivity streams . Using salt improved the effective range of the electrode
system . A greater percentage of available fish was taken on initial passes
through test sections; larger numbers of fish were taken per section due to
the extended effective range of the electrodes; and the fish were more thor-
oughly stunned and easier to net. The rate of mortality among trout taken in
salted section was less than 1 percent greater than among fish collected in
salt-free sections. The high yield of fish obtained provides more accurate
population estimates.

CONTENTS

Page

Methods 1

The effects of salt upon resistivity 2

The effects of salt in electrofishing 8

Conclusions 10

Literature cited 11

APPLICATIONS OF SALT IN ELECTROFISHING

Success in electrofishing is largely de-
pendent upon the electrical resistivity of the
water. Resistivity, or specific resistance, is
defined as the electrical resistance of a cubic
centimeter of any material and is measured in
ohms. Measurements can be expressed in
ohms resistivity or in its reciprocal ohms con-
ductivity. In water, resistivity varies inverse-
ly to a great extent with the quantity' and quality
of dissolved solids and to a lesser but import-
ant degree with temperature. It, in turn,
influences the strength and range of an electric-
al field in water and it must therefore be over-
come in electrofishing to reach and narcotize
fish. Failure to take the factor of resistivity
into account often predisposes the application
of electrogear in certain waters to mediocre or
poor results.

The wide variety of AC and DC electrode
systems in use today with power inputs of 115
to 500 volts, represents more or less success-
ful means for collecting fishes in waters of
various resistivities. Research and development
continue to result in improvements on electrode
systems, power sources, and methods of ap-
plication to increase the efficiency of electro-
fishing in very high and very low resistivity'
waters. Little has been done, however, to alter
resistivities to improve electrofishing. Such
alteration would be unnecessary on most waters
and impractical or impossible on some. How-
ever, it has proven practical to reduce the
extremely high resistivities of trout streams in
the southern Appalachian Mountains to levels at
which all- season electrofishing can be done
efficiently .

Resistivities measured in 50 streams in
Great Smoky Mountains National Park in North
Carolina and Tennessee, and in Shenandoah
National Park in Virginia, ranged from 28,500 to
207,000 ohms with the majority exceeding 50,000
ohms. These are among the highest readings
obtained in natural waters in North America .
Measurements made recently on 15 trout streams
in the northern Appalachian Mountains in New
Hampshire ranged from 22,000 to 122,000 ohms
and indicate that the high resistivit>' condition is
perhaps typical of streams draining the entire

Appalachian chain of mountains. In contrast,
spring water in production pools at the Leetown,
W.Va., fish -cultural station has resistivities of
2,460 to 2,600 ohms at 54° to 62° F. Samples
of single-distilled water at the same station
ranged from 90,000 to 110,000 ohms at 80° F.

The use of an alternate -polarir>- electrode
system enabled successful electrofishing in streams
with resistivities up to 100,000 ohms but efficiency
declined sharply above that level . The necessity
of sampling fish populations in as many of the
streams of the parks as possible, often during
cold seasons when low temperatures increased
the electrical resistance of waters, led to experi-
ments with blocks of cattle salt to reduce resistiv-
ities and increase the efficiency of the shocker
equipment .

METHODS

A portable, batter\' -powered, 1,000-cycle
conductivit%- bridge (Model RC- 7, Industrial Instru-
ments, Inc.), with K-0.1 probe was used to
determine the resisti\'ities of waters in the labor-
atory and field. Readings are obtained quickly and
directly over a wide range in ohms resisti\"it>-.

The salt used in laboratory and field trials
was 50-pound blocks of white cattle salt, common-
ly available at about one dollar each . No appreciable
differences were noted in trials made with plain and
mineralized blocks of salt that would justify the
slight additional cost for the latter. In field tests,
one to several blocks of salt were placed 25 to 50
yards upstream from seine -blocked sections 100
yards long in streams of 5 to 50 cfs flow.

The portable, gasoline -powered generators
used for electrofishing were of 230-volt, AC, 600-
and 2500-watt capacities. The electrode systems
employed were Petty-type, alternate -polarity units
(Petty 1955). Improvements incorporated into this
electrode system for park work included trailers
to expand the electrical field in high-resistivity
waters and the substitution of No. 8, 440-wire, elec-
tric welding cable for the braided copper shielding-
wood dowel electrodes (Lennon & Parker 1955).
The welding cable has proven easier to use on
rough streams and is much more durable than
shielding.

The welding cable electrodes were
made as follows: two equal lengths of cable
were clamped together. The insulation was re-
moved from the first 30 -inch portion of one
cable, then a 6 -inch gap left, and a 30 -inch
portion of the other cable was bared. Thus the
insulation was removed from alternate 30 -inch
portions of the cables, always leaving 6-inch
gaps of insulated cable between the bared por-
tions, until the desired number of electrodes
was obtained . Electrode systems of 12, 18,
and 24 feet in length are used most frequently,
depending upon the size of the stream to be
shocked. Longer systems could be used effec-
tively on larger streams.

the resistivity increased from 104,000 to 148,000
ohms when the temperature was reduced from
64 to 40° F.

The results cf applying salt to selected
samples showed that relatively few ppm of the
salt are required to reduce resistivities by 50
percent, particularly among the higher resistiv-
ity waters. For example, the resistivity of a
sample at 52,000 ohms was reduced 50 percent
by 11 ppm of salt; at 76,000 ohms by 8 ppm; at
110,000 ohms by 5 ppm; at 220,000 ohms by 3
ppm; and at 1,000,000 ohms by less than 1 ppm
of salt. The progressive reduction in resistivity
was slower in water samples at 40° than at 60° F.

Solderless terminal lugs are fastened to
cable tips on one side of an electrode system.
Terminals of this type can be easily and quickly
bolted to alternate -polarity terminals on the
bottom of the switch -brail. The cable tips at
the opposite end of the system are insulated
from one another with electrical tape. Dog-
chain clasps are used to fasten electrode sys-
tems to the brails and thereby systems of differ-
ent lengths can be readily substituted on the
brails.

THE EFFECTS OF SALT UPON
RESISTIVITY

The effects of cattle salt applied in 1 ppm
amounts to 1 liter samples of water were ob-
served in the laboratory. The resistivities of
the samples ranged from 48,000 to 1,000,000
ohms. With the exception of the latter sample,
the resistivities were considered typical of
those encountered in soutliern Appalachian trout
streams. Measurements of resistivity were
made successively following 1 or 2 ppm applica-
tions of salt until the accumulated salt in a water
sample amounted to 30 ppm (table 1).

The samples were held in water baths
throughout the trials to preserve near -constant
temperatures in the test solutions. The influ-
ence of water temperature on resistivity was
exhibited during preliminary trials wherein the
resistivity of one sample increased from 53,000
to 57,000 ohms when the temperature changed
from 61 to 59° F. In another, the resistivity in-
creased from 48,000 to 56,000 when the temper-
ature dropped from 64 to 56° F . In a third sample

These trials demonstrated that decreas'
ing advantage is gained by the addition of more
salt beyond 12 ppm (figure 1). Regardless of
initial resistivities and temperatures of the samples,
the application of 30 ppm of salt put final resistiv-
ities within a range of 8,000 ohms (14,000 to 22,000
ohms).

The effects of salt on resistivities in large
and small streams were observed in both parks
during the winter of 1956-57. This season of the
year was chosen since water levels and tempera-
tures were relatively stable . The more elaborate
trials were made on Indian Creek in the Great
Smokies since it is a small and rapid trout stream
with a closely parallel truck road which facilitated
movements from one trial station to another. Salt
points and stations were located at convenient
sites and immediately preceding a trial, the vol-
ume of stream flow, the normal resistivity, and
water temperature were measured at each station .

Typical of the results obtained in Indian
Creek and other streams were those observed
when a 50-pound block of salt was placed in the
water for 15 minutes (table 2). The stream at
this point was flowing at 34.4 cfs, the resistivity
was 207,000 ohms, and the water temperature was
44° F. Sixteen pounds of salt were dissolved from
the block in the 15 -minute period. At Station I,
100 yards downstream, the resistivity dropped in
11 minutes to a low of 54,500 ohms or 26 percent
of the original level . It increased abruptly to
150,000 ohms 5 minutes after the salt was removed
from the stream but did not return to the original
level until 27 minutes later.

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8 12 16 20

CONCENTRATION OF SALT

(ppm)

Figure l.--The effects of cattle salt on the resistivities of various water
samples. The results in Samples K and L; I and J; F, G, and
H; and C and E listed in table 1 were averaged. Test N was
made with a sample of deionized water for comparison

Table 2;- Resistivities measured at 3 stations below point where 50-pound block of cattle salt was

placed 7J\ stream for 15 minutes, A total of 16 pounds of salt w=s dissolved from the block.
Elapsed time is listed from introduction of s-^lt block into stream to t're last measurement
of resistivity at Station III, Stations were located downstream from salt point as follows;
Station I, 100 yards; Station II, 2 miles; and Station III, 3.5 miles.

Elapsed

station I

Elapsed
time

Station

II

Klapsed
time

Station III

time

Hesistivity

Salt '

Resistivity

Salt ■

Resistivity
(in thousands)

Salt

(minutes)

(in thousands)

(ppm)i/

(minutes)

(in thousands)

(ppm)i/

(minutes)

(ppm)i/

1

207.0

0.0

69

180.0

0.1

128

las.o

0.5

2

207.0

0.0

70

175.0

0.2

129

lao.o

0.7

3

150.0

1.0

71

171.0

0.3

130

136.0

0.9

h

70.0

5.0

72

165.0

0.5

131

I32.U

1.0

5

62.0

6.0

73

158.0

0.6

132

129.0

1.1

6

58.0

7.0

7U

150.0

0.8

133

125.0

1.2

7

56.0

7.3

75

1U2.0

1.0

13a

123.0

1.3

8

56.0

7.3

76

13U.0

1.2

135

119.0

i.a

9

55.0

7.5

77

127.0

l.h

136

117.0

1.5

10

55.0

7.5

78

120.0

1.6

137

115.0

1.5

11

5U.5

7.6

79

nh.o

1.8

138

113.0

1.6

12

51. 5

7.6

80

108.0

2.0

139

112.0

1.6

13

5)4.5

7.6

81

lOti.O

2.2

lao

111.0

1.7

Hi

51i.5

7.6

82

100.0

2.a

lai

110.0

1.7

15

55.0

7.5

83

97.0

2.6

ia2

liO.O

1.7

16

58.0

7.0

8a

95.0

2.7

U3

111.0

1.7

17

58.0

7.0

85

93.0

2.8

Dili

111.5

1.6

18

66.0

6.0

86

92.0

2.8

ia5

112.0

1.6

20

150.0

1.0

88

93.0

2.8

ia6

112.5

1.6

22

180.0

O.U

90

96.0

2.6

ia7

llli.O

1.6

2lt

190.0

0.2

92

102.0

2.3

Ui8

116.0

1.5

26

195.0

0.1

9h

110.0

1.9

ia9

118.0

i.a

28

200.0

0.1

96

1?0.0

1.6

150

120.0

i.a

30

205.0

0.1

98

129.0

l.U

151

122.0

1.3

35

205.0

0.1

100

138.0

1.1

152

125.0

1.2

UO

205.0

0.1

102

lli6.0

0.9

153

127.0

1.2

U5

205.0

0.1

IOI4

I5lt.0

0.7

15a

129.0

1.1

L7

207.0

0.0

106

159.0

0.6

155

131.0

1.0

108

162.0

0.5

156

13a. 0

0.9

UO

165.0

O.U

157

136.0

0.9

112

168.0

o.h

158

138.0

0.8

llii

171.0

0.3

159

lao.o

0.8

116 /
1182/

172.0

0.3

160

ia2.o

0.7

173.0

0.3

165

152.0

O.U

175

161.0

0.9

185

165.0

0.1

187

165.0

0.1

226

170.0

0.0

1/ Concentrations of salt in ppm estimated from curves in figure 1

2/ Observations at Station II terminated in order to intercept bolt of salt at Station III

5

Station II was located 2 miles down-
stream from the salt point. The volume of
flow was 43.4 cfs, the resistivity 188,000 ohms,
and water temperature 44° F. The salt reached
this point in 69 minutes and the resistivity
dropped to 92,000 ohms or 49 percent of the or-
iginal level. After 50 minutes of observation
at this station, the resistivity had increased to
173,000 ohms and readings were discontinued
in order to intercept the salt at the next station.

At Station UI, 3.5 miles downstream from
the salt point, the volume of flow was 51.6 cfs,
resistivity 170,000 ohms, and water temperature
44° F. The salt reached this station approx-
imately 10 minutes before the observer did or
about 118 minutes after the block was intro-
duced into the stream at the salt point. The 10-
minute error was closely estimated from back-
calculations and from results of other trials
when only Stations I and III were observed. The
salt was in the area of Station III for 110 minutes
and the resistivity dropped to a low of 110,000
ohms or 65 percent of the original.

The data obtained at the salt point and at
Stations I and III were related to the concentra-
tion of salt in ppm by two methods . First, the
concentration of salt added to the stream at the
salt point and the average concentrations of salt
as it passed Stations I and UI during the periods
of observation were calculated on the basis of
the 16 pounds of salt dissolved from the block
during the 15 -minute period, the stream flow
rates measured in cfs at Stations I and III, and
the lengths of time in minutes that the salt was
in the station areas. Accordingly, the salt dis-
solved into the stream at the salt point at an
average rate of 8.3 ppm for the 15-minute period.
Its average concentration at Station I during the
44 minutes it influenced resistivity was 2.8 ppm.
The average concentration at Station III for 110
minutes was 0.8 ppm. In the second method, the
concentrations of salt at the various levels of re-
sistivity measured at Stations I and III were
estimated from the curves shown in figure 1 .
The concentrations of salt at Station I ranged
from 0.1 to 7.6 ppm as the resistivity dropped
from 207,000 to 54,500 ohms but the average con-
centration for the 44-minute period was 2.6 ppm.
The concentrations at Station III ranged from 0. 1
to 1.7 ppm as the resistivity dropped from 170,000

to 110,000 ohms and the average concentration
was 0.6 ppm. Despite the dilution of the salt
and the stretchout of the bolt over the 3.5 miles
between the salt point and Station III, the re-
sistivity was appreciably lowered.

The close agreement at Stations I and III
between the calculated average concentrations
of salt (2.8 and 0.8 ppm respectively) and the
estimated average concentrations (2.6 and 0.6
ppm respectively) derived from curves in figure 1
supports the validity of the curves and indicated
that they can be used in the interpretation of data
on resistivity collected in streams.

Additional, detailed observations were
made on Indian Creek and 8 other streams (table 3).
One or two blocks of salt were placed in them for
varying periods of time and resistivities were
greatly reduced. The degree to which the resistiv-
ity was reduced in a trial area was roughly con-
trolled by placing the salt block in either fast,
slow, shallow, or deep spots in the stream . In
Roaring Fork (Stream G, table 3), 31 pounds of
salt were dissolved at Station I during a 2 -hour
period to reduce the resistivity from 106,000 to
a range of 37,000 - 66,000 ohms. The stream
flow was 12.5 cfs and water temperature 49° F.
At Station III, 32 pounds of salt were used in 1.75
hours to reduce the resistivity from 103,000 to a
range of 40,000 - 52,000 ohms when the flow was
11.9 cfs and temperature 50° F. At Station IV,
50 pounds of salt were used in 2.5 hours by placing
the block in fast water. The resistivity was re-
duced from 106,000 to 20,000 ohms, the flow was
12.5 cfs, and water temperature was 50° F. The
salt used at the rate of 20 pounds per hour at
Station IV had greater effect in reducing resistiv-
ity than did the rate of 15.5 pounds per hour at
Station I. It was found, too, that a block of salt
would last from 1 to 4 hours, depending on the
size of the stream and the location of a block
placed in it .

The effects of salt on an electric field in
water were tested at one of the stations on Roaring
Fork. The stream at this point was 33 feet wide,
its flow was 25.8 cfs, and its temperature was 50°
F. A 6-electrode, alternate -polarity system was
stretched across the stream and the relative
strengths of the electric fields in the water were
measured at input voltages of 230 and 330 volts AC

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and at resistivities of 50,000 ohms in salted
water and 100,000 ohms in salt-free water.
Voltage readings were made in the water at
measured distances from the electrode system
by means of a probe with 5 -inch gap and a volt-
meter . The readings indicated that the electric
field was extended as well as stronger in the
salted water, particularly in a lateral direction
from the electrodes. For example, at 330 volts
input, the 1 -volt isovolt line extended 4 feet off
the end of the electrode system and 4.3 feet
downstream as compared with 2.4 feet off the
end and 4 feet downstream in salt-free water.
The extension of the field laterally in salted
water has proven a great advantage in rough and
brush lined streams.

The alternate -polarity electrofishing
gear operated well and was found to be safe to
use in salted water. The risk of shocks is min-
imized by wearing rubber boots and rubber-
coated work gloves .

THE EFFECTS OF SALT
IN ELECTROFISHING

A series of trials to determine the ef-
fects of salt and reduced resistivities on electro-
fishing were run on 9 small streams which
contained rainbow trout, eastern brook trout, or
both (table 3). The persistently high water lev-
els which prevailed in both parks through the
late winter and spring of 1957 restricted the
choice of streams to those which were small,
accessible, and which consequently had relative-
ly few fish per station.

Fourteen, 100-yard survey sites were
selected and divided into 50 -yard test sections
which were as nearly alike in every respect as
possible. It was likely, however, that some
sections contained more fish than adjacent ones.
The test sections were blocked upstream and
downstream with 3/4-inch stretch mesh, nylon
seines to prevent movements of fish into or out
of the sections.

The salt blocks for salted test sections
were placed in the stream 25 to 50 yards up-
stream, usually in or above a series of falls or
cascades so that thorough mixing and even dis-
tribution of the salt would occur before it reach-
ed the test section. The blocks were weighed

before and after each trial to determine the total
amount used. The original resistivities ranged
from 80,000 to 176,000 ohms and were reduced
in salted sections to a range of 17,000 to 56,000
ohms. Water temperatures at the 14 stations
ranged from 42° to 50° F.

Four to five passes were made througji
each of the salted and salt -free test sections
with 230-volt gear during the electrofishing trials.
The brail -handlers and scap-netters started at
the upstream end of a section and worked down-
stream to the check seine. This downstream
technique was superior to the upstream approach
on rough waters in respect to numbers of fish
caught and ease of operation. The trout captured
during each pass were maintained separately in
livecars until fishing in a test section was
completed.

There were immediately apparent differ-
ences observed in salted and salt-free sections.
A larger proportion of the fish available were
taken on the first pass through salted sections
(78.1 percent) than in salt-free sections (64.2
percent). In two passes, 90.6 percent of the
available fish were removed from salted sections
in contrast with 80. 1 percent from salt-free
sections . The combined take in fourth and fifth
passes in salted sections amounted to only 2.3
percent of the total fish whereas those captured
in salt-free sections amounted to 8.9 percent.

The term available fish used in respect
to totals listed refers to those specimens which
are in such location, position, condition, and
size that they are collectable by the shocker
method. This qualification is applied since it is
seldom possible to remove all fish from a section
of stream by any collecting mechanisms or meth-
ods despite sincere attempts to do so .

More trout were taken in salted sections
than in salt-free sections in 9 of the 14 stations.
This was due in part to the very high resistivities
of salt -free waters and to the fact that the lateral
field of the electrode system was greater in
salted water. Fish were therefore shocked and
taken from under banks and boulders where they
otherwise would be unobtainable .

The disparity in numbers of trout taken
in salted and salt -free sections would have been

greater had not the test sites been blocked off
with check seines. Many trout in the high re-
sistivity, salt-free sections could not be cap-
tured until driven or frightened downstream to
the check seine and then surrounded and stunned
there. On the other hand, most trout in the
salted sections were taken throughout the sec-
tions and before the check seines were reached.

One important advantage conferred by
salt is that the trout and other species were
much easier to capture than in salt-free waters.
The reduced resistivities were reflected in
more thorough and prolonged stunning of the
fish. Escapement from the electric field, par-
ticularly in swift cascades, occurred less
frequently in salted sections since most speci-
mens were immobilized rather than addled as
was typical in salt-free waters. The use of
salt therefore made the job of scap netters
much easier.

There was doubt that these data obtained
on 9 small streams could be considered repre-
sentative since water conditions were poor and
the numbers of fish available were small. The
trials were therefore extended through the sum-
mer and fall of 1957 to Include a total of 100
salted sections on 28 streams and 40 salt -free
sections on 16 streams (table 4). A wide variety
of conditions relative to weather, water, re -
sistivity, and crew skill was included.

As many as 6 passes were made with
electrofishing gear through seine -blocked sec-
tions of 50 to 100 yards in length. Up to 189
trout were taken in individual salted sections
and 122 trout in salt-free sections. The per-
centages of trout taken per pass differed, how-
ever, by less than 1.5 percent from the prelim-
inary data obtained on the 9 streams (table 3).
The first passes through salted sections yielded
77.5 percent of 6,421 trout in 28 streams as com-
pared with 78. 1 percent of 297 trout in 9 streams.
In contrast, the first passes through salt-free
sections yielded 64.7 percent of 2,247 trout on
16 streams and 64.2 percent of 226 trout on the
9 streams.

The same advantages noted in electro -
fishing in salted sections on the first 9 streams
held throughout the expanded trials . Again there

were consistently more trout taken in salted
than salt-free waters. The trout were more
thoroughly stunned in salted waters which im-
proved the pickup rate in very swift, high, and/or
turbid waters .

It was presumed that the reduction of
resistivities might result in an increased rate
of mortality of fish but it proved only slight.
The total mortality of trout in salted sections
from the combined effects of shocking, holding
in livecars, anesthetizing, measuring, and weigh-
ing amounted to 4 percent in comparison with 3
percent in salt-free sections. Under optimum
shocking conditions when resistivities are re-
duced by salt to a range of 30,000 to 40,000 ohms,
many stations have been worked with electrofish-
ing gear with no losses among trout. The rate
of mortality of fish tends to increase, however,
when resistivities are reduced to 20,000 ohms or
lower .

The shocker was more effective in salted
than salt-free, high resistivity waters on all sizes
of eastern brook, brown, and rainbow trout, in-
cluding newly hatched, young-of -the -year speci-
mens 0.9 to 1.5 inches long. Of the 3 species,
brook trout and brown trout were more easily
captured than rainbow trout in open waters . Of
the other species encountered in test sections,
the majority of daces, shiners, stonerollers,
sculpins, darters, American eels, basses, and
sunfishes, were removed in 1 or 2 passes when
resistivities ranged between 25,000 and 50,000
ohms. Hogsuckers appeared to be more resistant
to shock and capture at all resistivities than the
other species.

The close agreement in the percentage of
trout collected per pass through salted sections
on the 9 streams and on the 28 streams demon-
strates not only the consistent advantage of using
salt but permits the use of the percentages as
escapement factors when but 1 or 2 passes are
made through a test section . It is seldom possible
to make 4 or 5 passes through a large number of
stream survey sites in order to obtain accurate
estimates of fish populations. It has proven prac-
tical to make but 1 or 2 passes through a good
number of representative stations and apply the
percentages listed for omitted passes when com-
puting population estimates . The validity of this

Table 4: --The total numbers and percentages of rainbow trout and eastern brook

trout captured per pass in 100 salted stations on 28 streams and 40 salt-
free stations on 16 streams in Great Smoky Mountains and Shenandoah
National Parks with 230-volt, alternate-polarity electrofishing gear

Passes

Salted

section

Number

Percent-

of trout

age

4,975

77.5

794

12.4

442

6.9

210

3.2

0

0.0

0

0.0

Salt -free section

Nurriber

Percent-

of trout

age

1,453

64.7

384

17.0

233

10.4

158

7.0

19

0.9

0

0.0

I

n
ni

IV
V
VI

Totals

6,421

100.0

2,247

100.0

approach to population estimates has been
checked and confirmed on a number of the 28
test streams with cresol and with rotenone.

CONCLUSIONS

1. The use of cattle blocks of salt is an
effective and economical means of reducing high
resistivities and improving electrofishing in
large and small, high and low, cold and warm
streams in the southern Appalachian Mountains.

2. One or two 50-pound salt blocks were
usually sufficient in 28 test streams with flows
up to 50 cfs to reduce resistivities from a max-
imum of 207,000 ohms to a range of 25,000 to
50,000 ohms. Increases in the concentration of
salt had proportionately smaller effects in re-
ducing the resistivities below 25,000 ohms.

3 . A block of salt lasts up to 4 hours in
the water. Substantial reductions in resistivit-
ies were measured 3.5 miles downstream. The
placement of a block in fast or slow, shallow or
deep water influenced its rate of dissolution and
thereby roughly controlled the degree to which
resistivities were reduced.

4. Electrofishing trials were conducted
in salted and salt-free sections of high resistiv-
ity streams . The following advantages of using

salt were determined: the 230-volt, alternate -
polarity electrofishing gear performed best
within a range of 30,000 to 40,000 ohms; the ef-
fective range of the electrode system was greater,
particularly in a lateral direction; a greater per-
centage of available fish was taken on initial
passes through test sections; larger numbers of
fish were taken per section due to the extended
effective range of the electrodes; and the fish
were more thoroughly stunned and therefore
easier to scap net.

5 . The rate of mortality among trout
taken in salted sections was less than 1 percent
greater than among fish collected in salt-free
sections. Mortalities tended to increase sharply,
however, in waters in which resistivities were
reduced to 20,000 ohms or lower. On the basis

of both mortality and gear performance, the lower
limit of desirable resistivity was considered to be
25,000 ohms.

6. Extensive trials have shown that the
use of cattle salt in conjunction with alternate-
polarity electrofishing gear provides the best
means for all-season sampling of fish populations
in the extremely high resistivity streams of the
southern Appalachian region. The high yield of
fish obtained by these means facilitates the com-
putation of more accurate estimates of populations .

10

LITERATURE CITED

Lennon, Robert E., and Phillip S. Parker Petty, A. C.

1957. Electric shocker development 1955. An alternate -polarity electrode,

on southeastern trout waters. N. Y. Fish and Game Jour.

Trans. Am. Fish. Soc. 85 (1955): 2: 114-119.

234-240.

IHT.-DUP. SEC. i*SH., D.C. us 1,97
11

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