FLUCTUATION IN TRAP-NET CATCHES
IN THE UPPER MISSISSIPPI RIVER

if;

Marine Biological Labofdiury

LIBRARY

WOODS HOLE, MASS.

1

SPECIAL SCIENTIFIC REPORT: FISHERIES No. 101

UNITED STATES DEPARTMENT . •[ INTERIOR
FISH AND WILDL

Explanatory" Note

The series embodies results of investig'jtions, 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 the official use of Federal, State
or cooperating Agencies and in processed form for economy and to avoid
delay in publication.

Representatives from the States of Illinois, Iowa, Minnesota,
Missouri and viTisconsin and from the United States Fish and Wildlife
Service, meeting in December, I9I1.3, formed the Upper Mississippi River
Conservation Committee for the purpose of carrying on scientific
investigations of the fishery and wildlife resources of the Mississippi
River from Caruthersville, Missouri, to Hastings, Minnesota. Field
operations were started in l?!;!;.

r

y

Wasl-Jington, D.C.
Ma.y, 19^3

United States Department of the Interior, Douglas McKay, Secretary
Fish and Wildlife Service, Albert M. Day, Director

FLUCTUATION IN TRAP NET CATCHES IN THE UPPER MISSISSIPPI RIVER

By D. W. Kelley, Aquatic Biologist
Fisheries Research Unit, Minnesota Department of Conservation

St. Paul, Minn.

Special Scientific Reports Fisheries No. 101

CONTENTS

Page

Area fished 1

Gear used 3

Methods 5

Statistical considerations 5

Season trends in catch of trap nets 6

Black crappie catch trends 7

Summary of black crappie catch trends 1$

White crappie catch trends 17

Summary of white crappie catch trends 2k

Blue gill catcfi trends 2U

Summary of bluegill catch trends 30

Catch trends for other species 32

Summary and discussion 35

Acknowledgements 3^

References cited 38

ILLUSTRATIONS
Figure Page

1. Pool 8 of the Mississippi River immediately below

La Crosse, Wisconsin 2

2. Mississippi River trap net used for test netting h

3. Length-frequency distribution of black crappies for the

nine netting peii.ods 9

U. Fluctuation in black crappie catch showing effect of

"recruitment" 12

5. Fluctuation in water levels and mean water temperature in

Pool 8 of the Mississippi River during summer of I9U8 ih

6. Length-frequency distribution of white crappie for the

nine netting periods 19

7. Fluctuation in white crappie catch showing the effect of

"recruitment" 21

8. Length-frequency distribution of bluegills for the nine

netting periods 27

9. Fluctuation in bluegills catch showing effect of

"recruitment" 29

10. Percentage composition of total catch expressed as

numbers of fish in netting Periods III, V, and VII 37

The upper Mississippi River supports an important sport and commer-
cial fishery. To obtain better management of this fishery as well as the
other ra.ldlife resources of the river, the Upper Mississippi River Con-
servation Committee was organized in 19U3- Represented on the committee
were the official conservation agencies of the states of Missouri, Illinois,
Iowa, Wisconsin and Minnesota, and the United States Fish and Wildlife
Service.

Under guidance of the committee a project was set up for gathering
information about the fish population of the Mississippi River by a
series of test-netting surveys. These were carried out during the years
19iili-Ii8 and consisted (1) of netting by traveling crews at stations be-
tween Caruthersville, Missouri and Hastings, Mnnesota and (2) of netting
throughout the open-water season at permanent stations near La Crosse,
Wisconsin. The latter work was done in I9I18 and it is with the fluctua-
tions of trap net catches at these permanent netting stations that the
present paper is concerned.

Quantitative interpretation of net catches presents many difficulties.
As hac' been pointed out by Hartly (19U7) and Moyle (19^0), the catch of
passive fishing gear such as hoop, fyke, gill and trap nets depends not
only on the abundance of fish large enough to be taken and held by the nets
but is also influenced by the rate of activity of the fish. These factors
as well as others related to them must be considered if net catches are to
be used as a quantitative measure of the s"'ze and structure of fish popu-
lations. This paper attempts to evaluate some of the factors found to
influence the catch of trap nets in the Mississippi River during the sum-
mer of 19lt8.

AREA FISHED

The catches considered here were all made in the upper third of
Pool 8, formed by the dam at Genoa, Wisconsin. This part of the Miss-
issippi consists of a large number of backwater lakes and interconnect-
ing channels all directly or indirectly connected with the main naviga-
tion channel. The area fished is shown in Figure 1.

Most of the backwater lakes have no current in them except during
high water periods in the early spring. There is usually a slight cur-
rent in the connecting channels and there is always current in the main
navigation channel, hihere current is rapid the bottom is usually of
shifting sand. Where there is little or no current the bottom is usually
of mud or silt.

At most places aquatic vegetation is not abundant, being limited by
shifting bottoms, turbidity, current, and fluctuating v.ater levels. Sub-
mersed aquatic plants, mainly coontail (Ceratophyllum demersum) , were

LOCATION OP
MISSISSIPPI

NQTTING STATIONS
ISIVEH • POOL 8

PI ICE LAICE 4. ZeiSLER'S SL.

Z. DEAD SLOUGH 5. SAND SL. CUT

3. POCICC-T

SCALC •• 1 INCH

150O rc-GT

Fig. 1 — Pool ^. of the Mississippi River immediately below La Crosse, 'Visconsin
showing; the locations of trap-netting stations during the summer of
194B. Crosses locate numbered channel markers. Broken Arrow Slough
in which some buffalo nets were set is 1,500 feet south of Marker 696.

common only in the inore isolated backwaters. Emergent aquatic plants
were more generally distributed, the common kinds being lotus (Nelumbo
pentapetala) and arrowhead (Sagittaria latifolia) .

Pool 8 supports a commercial fishery vrhich took 8^0,000 pounds of

fish during the I9I1.8 season. 1/ The important commercial fishes are carp,
sheepshead, buffalo and catfish. Parts of this pool are popular fishing
grounds for anglers from southwestern Wisconsin and southeastern Minne-
sota. Good catches of walleye?, northern pike, panfishes and catfish are
frequent. In the area netted, however, angling pressue is so light that
the effect of angling take on the fish population and on net catches may
be considered negligible.

GEAR USED

The trap net was used to sample fish populations at the permanent
netting stations in 19lj8 and for other netting surveys conducted north
of Dubuque, Iowa. This net is illustrated in Figure 2. It takes most
of the species of fish present in the river and appears to be less selec-
tive than most other kinds of entrapment gear. It is easily handled by
two men and in an emergency can be raised and reset by one man. Since
the trap net offers considerable resistance to current it does not
operate well in fast waters.

The fore-part of the net is a box-like frame, six feet wide, three
feet high, and two and a half feet deep, to which a 5ri-foot lead is
attached. The rear of this frame is fastened to a hoopnet of seven hoops.
In the frame is a heart which serves as the first throat of the net and
in the hoop-net portion are two throats, the first square and the second

tapered or "fingered". Mesh of the webbing on the frane is l-l/Ii inch
bar measure and on the hoops is 1-inch bar measure. The trap nets used
in this study were set at right angles to the bank with the lead extended
from the center of the frame to the shore.

To gain some additional information in fast-water areas, large-mesh
commercial buffalo nets were also fished. The buffalo net is a large
hoopnet, about 13 feet long, with 7 to 10 hoops that are 3 to I4 feet in
diameter. Mesh of the webbing varied from 1-3/U-inch to 2-inch bar
measure. Buffalo nets were fished without leads or wings and were
anchored in the current to intercept fish moving up stream. Buffalo
nets are used by commercial fishermen to take buffalo, sheepshead and
carp from the Mississippi below Dubuque, Iowa, but are illegal in
Wisconsin-Minnesota boundary waters.

1/ Sixth Progress Report, Upper Mississippi Riirer Conservation Committee.
Technical Committee for Fisheries, 1950

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METHODS

In I9U8, periranent netting stations were selected in Pool 8 at five
locations shown in Figure 1. Beginning on May l5, one trap net was set
at each of the stations. Each net was lifted daily between 8 and 11 A.M.,
the catch removed and the net immediately reset. Fishing in this manner
was continued for 1^ days after which the nets were removed for a "rest
period" of h to 6 days.. During the rest period the nets v;ere repaired
and treated with copper napthenate preservative.

Such 10-day netting periods alternating with h to 6 day idle periods
continued through September 25, making a total of nine netting periods.
In following portions of this paper these netting periods will be desig-
nated by the Roman numerals I through IX. They are dated as follows:

Period I - May iS through May 2k

II - May 31 through June 9

III - June 17 through June 26

IV - July 1 through July 10

V - July 18 through July 27

VI - August 3 through August 12

VII - August 18 through August 27

VIII - September 1 through September 10

IX - September I6 through September 25

During the nine netting periods 325 successful trap-net lifts were
made. The results of 125 other lifts were discarded because of muskrat,
beaver or human interference with the nets. One of the five netting
stations was abandoned during Period IX because of muskrat activity.

All fish taken in the trap nets -were counted, weighed and measured.
After the middle of June scales were taken from a selected sample of each
species of fish. Gross examination was made of the state of gonad develop-
ment of many of the fish.

During the netting a careful record was kept of water temperature and
water turbidity at the time each net was raised. Turbidity readings were
made with a platinum needle turbidimeter. Water level and rainfall records
were obtained from the U. S. Weather Bureau at La Crosse.

Data for each net lift were recorded separately in code suitable for
tabulation on an IBM punch card machine.

STATISTICAL CONSIDERATIONS

The use of trap-net catches for evaluation of size and structure of
a fish population is essentially a sampling problem. Means calculated
from trap net catch dat^ are subject to sampling error and some method

for judging the validity of such means must be used if the mean catches
from the 10-day netting periods are to be compared. An atten^jt was made
to judge sampling errors by standard statistical procedures, but the data
appeared not amenable to this type of treatment. The distribution of the
catches is skewed and random sanpling of the individual fish in the popu-
lation cannot be assumed. As pointed out by Moyle (1950) for the catch
of gill nets, the skewness of the catch pattern and the apparent non-
random nature of the sampling probably reflects the associative patterns
of the fishes themselves.

Instead of standard error a more general and empirical method has
been used for judging the validity of catch means. Mean catch for each
species, in terms of weight and numbers of fish per lift, was calculated
for each of the It or 5 nets used during each netting period. If the mean
catch in at least 7$ percent of the nets - that is, 3 out of U or ii out
of 5 - was higher or lower than in the preceding period, the trend was re-
garded as real and probably not due to sampling errors. Changes of a
lesser degree are considered to be uncertain. All fluctuations discussed
in this paper meet this requirement, unless otherwise specified.

This method of judging reliability of means is similar to that of
Hubbs and Perlrautter (19U2) who in attempting to apply statistical pro-
cedures to fish systematics concluded that "Even when no single sample
of one group can be demonstrated as significantly different from that of
another group, a true distinction between the two categories must never-
theless be accepted, if the difference is shown consistently by enough
small samples of each group."

SEASON TREiraS IN CATCH OF TRAP NETS

During this study 12,810 fish weighing 11,25U pounds were taken in
trap nets. Thirty-five species of fish were caught but only 11 species
were taken in quantities adequate to demonstrate catch trends. Arranged
in the order in ^*iich they -^jill be discussed they are:

Black crappie - Pomoxis nj-gro-maculatus

White crappie - Pomoxis annularis

Bluegill - Lepomis macrochirus

Northern pike - Esox lucius

White bass - Lepibema chrysops

Channel catfish - Ictalurus lacustris punctatus

Flathead catfish - Pilodictus olivaris

Northern redhorse - Moxostoma aureolum

Carp - Cyprinus carpi o
Carpsucker - Carpiodes spp.
Sheepshead = Aplodinotus grunniens

Black crappies, white crappies and bluegills were taken in much
greater numbers than the other species and allow more thorough analysis
of catch fluctuation than is possible for the other species. A summary
of the findings for each of these three species \-n.ll conclude the dis-
cussion of each. A general summary and discussion of catch fluctuations
for all species will be found at the end of the paper.

BLACK CRAPPIE CATCH TRENDS

During the nine netting periods U,10ii black crappies weighing 1,85U
pounds were taken in the trapnets. Fluctuations and trends in the catch
of the black crappie are shown in Table 1 in terms of mean number and
mean weight per net lift for each netting period. The mean catch pei
net lift for each period is also expressed as a percentage of the mean
catch per net lift for the entire netting season - all nine periods.
This latter method of presentation places all catches whether expressed
as numbers or pounds on a comparative scale.

Table 1 shows that the catch of black crappies declined during the
first part of Junej remained at a low catch level for a few weeks, and
then began to rise. This rise was interrupted during the first part of
August (Period VI) by a slight drop in the catch at all but one station.
After August l8 the catch increased in a more or less rapid and regular
fashion and reached a high point for the season in the last fishing
period (September l6 to 25) .

The effect of four main factors on the catch of black crappies will
be considered. They are (l) recruitment to the catch; (2) losses from
the fish population; (3) activity and movement of the fish; and (h)
spairming activities.

1. Recruitment to the Black Crappie Catch.

Recruitment to the catch of the black crappie was studied by (1)
analysis of the c hanges in length-frequency of the catch during the
netting, and (2) age and growth determinations from scales collected
during the netting.

Length-frequency distributions of the catch during each of the nine
netting periods are shown in Figure 3. It will be noted that all black
crappies caught during Period I (May l5 to 2h) were 7 inches long or
longer. Age determinations from scales collected indicate that these
fish were of the 19l;5, 19l;U and 19U3 year classes. These 3, h and 5-
year old fish may be termed the "residual" fish for they were both (1)
large enough to be taken in the nets during Period I, and (2) present
in the area being netted during Period I,

Table 1. Seasonal fluctuation in mean trap net catches of black crappies.

^um'bers Pounds

Period

I

II
III

IV

y

VI

VII

VIII

IX

Dates Inclusive

May 15 _ Zk
May 31 - June 9
June 17 - 26
July 1-10
July 18 - 27
August 3-12
Au^st 18 - 27
September 1-10
September I6 - 25

Percent of

Percent of

season's

season's

Per

mean catch

Per

mean catch

lift

per lift

lift

per lift

16.8

121.8

11.7

186.0

2.7

19.6

1.4

22.0

2.5»

18. !•

1.2*

19.2*

5.3

38.i^

2.3

36.5

8.2

59.5

2.8

44.5

6.6

47.9

2.3

36.7

12.9

93.5

6.8

108.0

16.0*

116.0*

6.4*

101.7*

53.6

388.4

21.5

3^1.9

Mean catch for season

13.8

100.0

6.3 100.0

Starred (*) figures represent catches considered take not significantly different from
catch of the preceding period. See text for explanation.

TOTAL LENGTH IN INCHES

Fig. 3 — Length- frequency distribution of black crappies for the nine netting periods.
Approximate distribution of year-classes was determined from scales.

It will also be noted from Figure 3 that smaller black crappies were
caught in increasing numbers during later periods. Age determinations
from scales of these fish demonstrated that they were of the 19i|6 and
19it7 year classes. These 1- and 2-year old black crappies may be termed
"recruitment" for they were (l) the younger fish not large enough to be
held in the nets during Period I, and/or (2) not present in the area
being netted during Period I.

Table 2 lists the mean catches of "residual" and "recruited" black
crappies for each netting period. The first recruitment to the catch
began in Period II (May 31-«June 9) when a few individuals of the 19ii6
year class were taken. A month later (Period IV) these 2-year old fish
made up 33 percent of the catch in numbers of fish, A few of the 19U7
year class entered the catch during the following netting period (V)
when "recruited" fish made up $9 percent of the total black crappie catch.
During August and September, the 19U7 year classes entered the catch in
increasing numbers. The total "recruited" fish made up from 39 to 65
percent of the mean black crappie catch of the last four netting periods.

The appearance of the 19U6 year class during June and July did not
coincide with the growth of these fish to catchable size. The nets
readily retained black crappies as small as S inches and growth analysis
of the I9U6 year class demonstrated that most of these fish were larger
than that when the netting began in May, Their absence at that time and
their sudden appearance in quantity during July suggests that they were
not associated with older fish during the early part of the season but
joined them during July.

Figure k shows graphically the effects of recruitment ipon the catch.
The rise in catch during July probably reflects the effects of recruitment
alone; that is more fish large enough to be held by the nets were available
to be caught. The still greater catch rate in August and September is
probably the result of (1) augumentation of the population by previous re-
cruitment and (2) an increase in the rate of activity of all crappies in
the population during late summer. Catch of the nets can be regarded as
an arithmetical product of abundance times activity and large catch
fluctuations may be expected when there is a change in the activity rate
of large populations,

2^ Losses from the Black Crappie Population.

It is likely that some of the increase in size of the catchable
black crappie population during the fowr and a half months was offset by
various types of losses such as natural mortality, fishing mortality and
perhaps migration from the area. Such losses would influence the catch
at the end of the season. From the present data there is no means of
estimating the total loss to the population. However there is some
evidence of a greater loss among older age classes than among 3-year-old
fish. During Period I, h and 5-year old black crappies made up about 35
percent of the catch in numbers of fish; the remaining 65 percent con-

10

Tatle 2. Mean catch of "recruited" and "resid'ual" tlack crappies expressed as numbers
per net lift and percentage of the total catch for each netting period.

Period I II III IV 7 VI VII VIII IX

May May Jl June July July Aug. Aug. Sept. Sept.

15-24 June 9 17-26 1-10 18-2? >12 18-2? l-lO l6-25

Recruited

Number per lift - 0.2 0.4 1.8 4.8 4.3 5.4 6.2 22.5

Percent of total - ? l6 33 59 65 42 39 42

He si dual

Number per lift I6.8 2.5 2.1 3.5 3.4 2.3 7'5 9.8 31. 1

Percent of total 100 93 84 6? 4l 35 58 6I 58

Total number
per lift 16.8 2.7 2.5 5.3 8.2 6.6 12.9 16.O 53.6

11

50

I-

-.40

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L. 20 -

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lioH

IZeSIDUAL"
CATCH

MAY

JUNE

JULY

AUSUST

SEPT

'''' '" \lT\TCir ''"' ''''''''' °^^<=^ ^^""^'^^ «^^-^ °^ ".ecruit„.„t... Data

12

sisting of 3-year old fish. During Period IX only about 11 percent of
the catch of these three age groups consisted of Ii and 5-year old fish.

As a check on the effects of removing fish from the area by netting,
a trap net was set in Broken Arrow Slough during Period IX. This area
had not been netted before. This net took black crappies of approximately
the same length-distribution during Period IX as did the other nets which
had been fished all summer at the permanent netting stations. This obser-
vation indicates that removal of the fish from the waters around the
netting stations probably did not significantly affect changes in length-
frequency of the catcho

It is also possible that migration influenced the catches during the
season. However, buffalo nets set in the main channel caught only an
occasional crappie during the study. Their mesh was small enough to take
black crappies over 9oO inches long. It appears that any losses from the
population due to migration are small and that the principal cause of
losses was natural mortality.

3. Activity of the Black Crappie.

Two catch fluctuations noted during the season .can best be attributed
to changes in rate of activity of the fish. They are (1) the marked drop
in catch rate that occurred between Periods I and II when only the
"residual" population was present and (2) the great increase in catch
rate observed in late August and September after most of the recruitment
to the popiolation had occiirred, A small drop in catch rate was also
noted between Periods V and VI , that may also be related to a change in
rate of movement.

Some changes in activity rate are probably related to changes in
environmental conditions. It will be noted from Figure 5 which shows
water levels and water temperatures, that the decrease in catch in June
was coincidental with the stablization of water levels. There was a
drop in water levels of 1.6 feet during Period I and by the beginriing of
Period II, the water levels had dropped another 0,6 foot. After the
beginning of Period 11^ the water level remained within 0.2 feet of nor-
mal pool stage for the rest of the season. It is generally believed by
eomiriercial fishermen who fish t he area that water level fluctuations affect
fish movement o

Water temperature is also known to affect fish movement and behavior
patterns (Fry and Hartj I9I43) . Means of temperatures taken one foot below
the surface of the water each day at each netting station are shown in
Figure L.. Catch of black crappies was highest when the water temperature
was between 6o* and 70* F.| that is during the spring and fall. Fluctua-
tions of water temperatures above 70° F., seemed to have no consistent
effect on the catch.

13

MEAN WATER TEMPEISATUJIC-S

or 80 -

S ■'0

a

o

S 60 -

+ 3-1
+ 2 -

+ 1

^ >S

^

W^

^^

1 -J

WATC-R LCYELS

-v^

-^-z*

^=^-., ^^^V-

MAY JUNE JULY AUGUST SEPT
1 I I I

Fig. 5—Fluctuation in water levels and mean water temperature in Pool S of the
Mississippi River durint^ the suiimer of 194?.. "0" water level in normal
pool staf^.

u

Turbidity readings were also made daily at each station. These
readings ranged from 22 to llU parts per million. There is no evidence
that turbidity within this range had any effect upon the movement and
catch of black crappies or any other species.

U. Effect of Spawning Activities on the Catch.

Direct observation of spawning activities was not possible because
of the turbidity of the water, but gonads of 3_^311 black crappies were
examined grossly and classified as to stage of development. The classi-
fication consisted of six categories: (1) undeveloped, (2) developing,
(3) ripe, (It) ripe and discharging when pressed, (5) partly spent and (6)
spent. A summary of the data on gonad examination i s shown in Table 3.

It will be noted that spawning was in progress at the beginning of
netting and continued until late in July. During the last half of
August all the crappies examined had undeveloped gonads. Development of
the gonads for spawning the next year began in some fish in August and
was observed in more and more fish as the season progressed. The appear-
ance of the I9U6 year class and general mixing of the crappie population
was coincidental with the end of the spawning season. Although most of
the 2-year-old fish were apparently not with the older fish during the
spawning season, gonad examinations show that at least some of these
younger fish were capable of spawning and may have spawned elsewhere.
Gonad examination£ of 10 black crappies all under 7.6 inches long, which
were taken during Period III, showed six undeveloped, three partly spent
and one ripe.

The present findings on age at sexual maturity of the black crappie
agree with the statement of Smith and Moe (19hh) that black crappies are
sexually mature at the age of two years.

Except for the apparent separation of the older and younger crappies
during the spawning and the subsequent remixing of the population, spawn-
ing activities had no apparent effect on the catch.

SUMIiiRY OF BLACK CRAPPIE CATCH TRENDS

1. The means of black crappie trap net catches for each of the nine
netting periods fluctuated between 2,5 and 53.6 fish per net lift and be-
tween 1,2 and 21,5 pounds of fish per lift. There was? (1) a sharp drop
in catch during the first part of Jxine; (2) a gradual rise during which
catch was more than tripled during July; (3) a slight but general drop
during the first part of August; (h) a rise during which catch was about
doubled during the last half of August; and ($) an increase in the latter
half of September during which catch again was more than tripled.

15

Tacle 3. Gonad development of black crappies, showing percentage of gonads examined that

were assigned to each developnientaJ. stage.

Period

1 II III IV V VI VII VIII IX

May May 31 June July July Aug. Aug. Sept. Sept.

15-24 June 9 17-26 1-10 18-2? 3-12 18-2? 1-10 l6-25

Nuicber examined ^4

117

15

176 315

271 611

553

709

Undeveloped

0.2

9.^

48.0

63.1

9^.6

99.3

69.2

73.2

61.9

Developing

-

-

-

-

-

-

10.3

26.8

38.1

Eipe

46.7

29.9

8.0

0.6

-

-

-

-

-

Ripe ♦

J^5.2

6.8

1.3

1.7

1.3

-

w^

-

-

Partly spent

6.8

53.0

42.7

3^.6

2.5

-

-

I—

-

Spent

1.0

0.8

""

••

1.6

0.7

•*

""

••

16

2, Recruitment to the catch consisted of the sudden appearance of
the I9U6 year-class during July after the spawning season and the gradual
appearance of the 19Jj-7 year-class during and after the last part of July.
The recruitment affected catch fluctuations in two ways. First it caused
the increase in catch as the recruitment occurred, and secondj, it increased
the magnitude of subsequent catches and c atch fluctuations resulting from
changes in the rate of activity.

3o The drop in catch during the first part of June is thought to have
been caused by a decrease in the rate of activity of the black crappie.
This may have been caused either by (1) stabilization of water level during
Period II; (2) water temperatures rising above 70 degrees F, during Period
II or (3) a combination of these two changes in the environment.

U. The increases in catch during Augiist and September are attributed
to the combined effect of previous recruitment and increased rate of activity.

WHITE CRAPPIE CATCH TRENDS

During the nine periods of netting, 929 white crappies, weighing
U56 pounds were taken in the trap nets. Seasonal fluctuation of this
catch is shown in Table k which lists the mean catch per lift for each
netting period. Seasonal fluctuation in the catch of white crappies was
less than that already noted for black crappies.

The white crappie catch dropped during the first part of June. After
this period no significant change occurred in the catch until the latter
part of July, when the catch almost doubled. By the last of August the
catch had declined to a level somewhat above the June catch rate. During
the last half of September, a small rise in catch was observed.

1. Recruitment to the White Crappie Catch.

Length-frequency of the white crappie catch during each netting period
is shown in Figure 6. All white crappies taken in trapnets during Period I
Here 7.0 inches long or longer. Age determinations from white crappie
scales collected during the netting indicate that mjst of these fish were
of the 19li5, I9I4.U and 19ii3 year classes - the "residual" population.

Table 5 shows the relative numbers of "recruited" (I9I16 or younger
year classes) and "residual" (19U5 or older year classes) white crappies
taken during each netting period. These data have been used in Figure 7
to show the effect of recruitment upon catch.

Recruitment to the white crappie catch did not begin until Period IV
(July 1-10) when the I9U6 year class made up 19 percent of the total catch
in numbers of fish. During Period V, (July 18-27) the I9U6 year class,

17

Table ^. Seasonal flucttrntions in mean trap net catches of white crappies.

Period

I

II

III

IV

T

VI

VII

VIII
IX

JTumbers

Pounds

Percent of

Percent of

season's

season's

Per

mean catch

Per

mean catch

Dates inclusive

lift

per lift

lift

per lift

May 15 - 24

4.0

138.0

2.9

207.2

May 31 - June 9

1.9*

65.5

1.3*

93.0

Jime 17 - 26

2.0*

69.0

l.3»

93.0

July 1-10

2.1*

72.4

1.1*

78.5

July 18 - 27

3.9

134.4

1.^

114.2

August 3-12

3.2*

110.3

1.1*

78.5

August 18 - 27

2.7*

93.1

1.2

85.7

September 1 - 10

2.6

89.6

1.0

71.^

September 16 - 25

3.^

117.2

1.4

100.0

Mean catch for season

2.9

100*0

1,4 100.0

Starred (*) figures represent catches considered not significantly different than catches
of the period immediately preceding.

18

TOTAL LENGTH IN INCHES

Fig. 6— Length- frequency distribution of white crappies for the nine netting periods.
Approximate distribution of year-classes was determined from scales.

19

Table 5. Mean catches of "recruited" and "residual" white crappies expressed as numbers
per net lift and percentage of the total catch for each netting period.

Period I II III IV T VI VII VIII IX

May May 31 Jime .July July Aug. Aug. Sept. Sept.
15-24 June 9 17-26 1-10 18-2? 3-12 18-2? 1-10 16-25

Recmiited

liuiabers per lift - - -0.4 2.0 2.0 1.2 1.7 2,3

Percent of total - - _ 19 51 6I ^5 65 6?

Residual

Nunber per lift ^.0 1.9 2.0 I.7 1,9 1.2 I.5 0.9 1.1

Percent of total 100 100 100 81 49 39 55 35 33

Total man'oer
per lift 4.0 1.9 2.0 2.1 3.9 3.2 2.7 2.6 3.4

20

, 4 -

TOTAL CATCH

\ y

^><^

z

oi Z -

\___^^^

'2Y/y^

Hi

^^^^^^^^^^^^vy^^i

X 1 -

"eesidual"

CATC H ^"^ ^<<Z/:i^^^-

t-

' 1 1

1 1

MAY

JUNC

JULY

AUGUST

SEPT

Fip. 7"Fluctuation in white crappie catch showing the effect of "recruitment".
Data from Tsble 5.

21

plus possibly a few individuals of the 19U7 year class made up 5l percent
of the total numbers of white crappies caught in trap nets. This sudden
recruitment resulted in the increase in catch observed during July. Addi-
tional recruitment from the I9U6 year class and the 19h7 year class kept
the white crappie catch from falling below the low catch level of June in
August and September.

2, Losses from the White Crappie Population

There is evidence of considerable mortality among older white crappies
during the summer (Figure 6). During Period I, 60 percent of the catch of
white crappies, that were three years old or older ("residual" population),
consisted of h and 5-year old fish. By Period IX these older fish made up
only seven percent of the catch.

The trap net fished in previously unnetted Broken Arrow Slough during
Period IX caught too few white crappies to allow comparison of length-
frequency distribution here, with that of the catch from regular netting
stations during Period IX. However, it seems doubtful that enough white
crappies were removed from regular netting stations during the season to
effect noticeably the size structure of the white crappie population.

No migration of older white crappies was noted during the season and
the loss of older fish was probably due mostly to natural mortality.

3»_ Activity of the White Crappie and the Catch.

Two fluctuations in the catch of the white crappie can be related to
changes in the activity rate. They are (l) the decreases in catch during
the first part of June, and (2) the increase in catch during the last part
of September.

Changes in the rate of activity that apparently caused these catch
fluctuations are probably related to changes in water levels and trcn^jera-
ture. The decline in catch during the first part of June coincided with
the stabilization of the water levels after the spring high water period.
Dropping water levels may have caused the higher catch during Period I
and their stabilization may have resulted in the lower catch rate during
Period II o This drop in catch also coincided with the warming of the
water to above 70 degrees F.

Examination of gonads of white crappies throughout the season showed
that spawning activities were probably at a peak when netting began (Table
6) „ Spawning activites declined steadily until raid-June when 96 percent
of the white crappies examined were found to have undeveloped gonads.

There was no apparent change in catch rate from the "residual" popu-
lation as spawning activities declined and the increase in catch rate in
J"aly can be attributed to the entrance of 2-year-old white crappies into

22

TalJle 6. Gonad development of white crappies, showing percentage of gonads examined
that were assigned to each developmental stage.

Period

I

II

Ill

IV

V

VI

VII

VIII

IX

May

May 31

June

July

July

Aug.

Aug.

Sept^

Sept.

15-2^

Jttne 9

17-26

1-10

18-27

3-12

13-27

1-10

16-25

Number examined

99

83

62

63

132

130

119

87

65

Undeveloped

-

k.Q

12.9

48.5

96.2

99.2

85.9

75.9

61.5

Developing

-

-

-

-

-

-

11.8

24.1

33.5

Ripe

57.6

50.6

2if.2

2.9

-

-

-

-

-

Ripe +

38.3

21.7

6.4

-

-

-

-

-

*■

Partly spent

3.0

22.9

56.4

48.5

3.8

-

-

-

-

Spent

1.0

-

-

-

-

0.8

2.5

-

-

23

the catch. Growth analysis suggests that these 2-year-old white crappies
were^ like the 2-year-old black crappies, large enough to be taken during
tne earlier netting periods, Beca\ise they were not taken, it appears
that the 2-year-old white crappies were elsewhere during the spawning of
the older fish and joined the older fish only after spawning was completed,
Gonads of the 2-year-old white crappies that were examined were found to
be undeveloped so it is probable that most of these fish were not capable
of spawning during the I9U8 season.

SUMMARY OF WHITE GRAPPIE CATCH TRENDS

1. The means of white crappie trap net catches for the nine netting
periods fluctuated betwen 1.9 and U.O fish per net lift and b etw«en 1.0
and 2.9 pounds of fish per net lift. There was (1) a drop during the
first of June, (2) a rise during the last of Julyj (3) a gradual decline
until mid-September, ani (i^) a small rise in the catch during the last
half of September.

2. The decline in catch rate during the first of June is thought to
have been caused by a decrease in activity rate of the white crappie. This
decrease in activity rate may, in turn, have been related to (i) stabili-
zation of water levels after spring high water, (2) water temperatures
rising to above 70 degrees F,, or (3) both of these factors,

3. The rise in catch during July was caused by the entrance into
the catch of the I9U6 year class after the spawning activities of older
fish ceased. Recruitment of this I9U6 year class dioring July and of the
I9I47 year class during August, held up the catch rate during August and
September,

ho The slight increase in catch rate during the latter half of
September is thought to have been caused by the combined effects of an
increased rate of activity, and previous recruitment.

BLUEGILL CATCH TRENDS

During the nine netting periods hshhQ bluegills weighing 1,218
pounds were caught in trapnets. Seasonal catch fluctuations can be judged
from Table 7 which shows the mean catch per net lift for each of the nine
netting periods, Bluegill catch rate dropped significantly during the
first part of June, remained at a low level until J-'.ily, ar.d rose during
J-oly, August and September, During the last netting period (September
16-25) the bluegill catch was about UO times what it had been d'oring
Period II (May 31-June 9).

2u

Table ?• Seasoiial fluctuations in meaji trap net catches of bluegills.

Period

I

II

III

IV

7

VI

VII

VIII

'u.

Sates inclusive

May 15 - 2k
May 31 - June 9
June 17- 26
June 1-10
July 18 - 27
August 3-12
August 18-T, 27
September 1 - 10
September l6 - 2$

Numbers

Percent

of

Percent of

season

«s

season's

Per

laean ca

tch

Per

mean cf.tch

lift

per lift

lift

per lift

6.9

46.9

2.6

65.0

1.3

8.8

0.5

12.5

2.6»

IBA

1.0*

•25.0

3.2

21.8

1.0*

25.0

6.2*

1*2.2

1.2»

30.0

7.9

53.7

1.7

42.5

17.^

118.3

5-5

137.5

35. i

2^0.8

9.2

230.0

51.1

3^^8.0

13.5

337.2

Mean catch for season

l4.7

100.0

4.0

100.0

Starred (*) figures represent catches the.t are considered not significantly different
from catches of preceeding periods. See text for explanation.

25

1. Recruitmeat to the Blue gill Catch.

Length-frequency distributions of the bluegill catches during each
netting period are shown in Figure 8, All bluegills caught during Period
I were 5.2 inches long or longer. A^;e determinations from s cales collected
during the netting indicates that these fish were probably all of the 19U5
or older year classes ; that is of the "residual population".

Table 8 lists the relative numbers of "recruited" fish (19i;6 and
younger year class) and "residual" fish (19U5 and older year class) in the
catch during each netting period. These data are presented graphically in
Figure 9.

Recruitment began during Period II when a few fish of the 19U6 year
class appeared briefly in the catch. Dxiring Period IV, more of these 2-
year-old fish were caught, and in Period V they made up 8l percent of the
total number of fish caught. Additional recruitment from the 19U6 year
classes together with the 19ii7 year classes continued until September.
During September 92 percent of the bluegills being caught were "recruited"
fish that had not been available at the beginning of netting in May.

Like the 2-year-old black crappies and white crappies, the 2-year-old
bluegills were large enough to have been caught during May and June had
they been available, then. Their sudden appearance in quantity during
July suggests that they were not associated with the older bluegills dur-
ing the early part of the season. Since these younger fish first appeared
at stations closest to the main river channel, it may be that during early
spring they were in the deeper waters of the river. The recruitment of
the I9U7 year class seems to coincide with their growth to catchable
size.

2. Losses from the Bluegill Population.

As with the black aid the white crappie, there is evidence of a high
loss among older bluegills during the summer, (Figure 8). During Period
I, 17 percent of the catch (all 3, h and 5-year-old fish) was made up of
h and 5-year old fish. By Period IX, these two oldest age groups had
disappeared from the catch entirely.

The trap net set in previously unnetted Broken Arrow Slough during
Period IX took no bluegills large enough to be considered h or 5-year-
old fish (Figure 6) . From this evidence it appears likely that the re-
moval of bluegills during the summer's netting at the regular stations
had little effect upon the length-frequency of the catch.

No migration of bluegills was noted dxiring the summer, and it is
probable that most of the losses of the older bluegills during this
period was due to natural mortality »

26

liJ

TOTAL LENGTH IN INCHES

Fig. P— Length-frequency distribution of bluegills for fie nine netting periods.
Approximate distribution of year classes was determined from scales.

27

Table 8. Mean catches of ^recroited" and "residual" bluegills, expressed as numbers per
net lift and percentage of the total catch for each netting period.

Period I II III IV V 71 VII VIII IX

Nay May ^1 June J\U.y July Aug. Aug. Sept. Sept.

15-2^1 June 9 17-26 1-10 18-2? 3-12 18-2? 1-10 16-25

Recruited

Number per lift - 0.1 - O.5 5.O 6.9 15.3 32.6 47.0

Percent of total - 8 - I6 8I 87 88 92 92

Hesidual

Number per lift 6.8 1.2 2.6 2.7 1.2 1.0 2.1 2.8 k.l

Percent of total 100 92 100 84 19 13 12 8 8

Total number
per lift 6.8 1.3 2.6 3.2 6.2 7.9 17.9 25.4 51. 1

28

50 -

^ 40

I-

z
at

O^ 50

O 20 -

ta

ID

3

Z 10

MAY JUNH

JULY

AUGUST

SEPT

Fig, 9 — Fluctuation in bluegill catch showing effect of "recruitment". Data from Table S.

29

3. Activity of the Bluegill and the Catch.

Apparently all the significant fluctuations in the catch of blue-
gills, except the rise during July due to recruitment, were influenced
by changes in the rate of activity of the fish. The drop in catch rate
during June can, like similar drops in the white and black crappie
catches, best be attributed to a decreased activity rate alone. Also
the increase in bluegill catch during August and September can largely
be attributed to an increase in the activity rate of bluee;ills. It
should be noted however that most of the fish affected by this increase
in activity rate were "recruited" bluegills that had not been available
during the early netting periods.

Changes in activity rate were probably related to environmental
changes. The drop in catch during the first part of June coincided
with the stabilization of water levels after the spring high-water
period. The drop in catch also was coincidental with water tempera-
tures rising above 70 degrees F.

It» The Effects of Spawning Activities .

A summary of data gathered from examination of the gonads of blue-
gills caught in trap nets during the season is presented in Table 9.
Gonads from the bluegills indicated that many were spawning when the
netting began in May. Spawning activities apparently declined during
July and by the first of August no fish with ripe gonads were taken.

There was no change in the catch of the "residual" bluegills which
coincided with change in the spawning activities as measured by gonad
analysis. However the 2-year-old bluegills, (like the 2-year-old black
crappie and white crappie) entered the catch with the older fish as
spawning activities declined.

Most of the 2-year-old bluegills examined had either ripe or spent
gonads. They were sexually mature and apparently spawned apart from the
older fish during the first part of the 19i|8 season.

SUMMARY OF BLUEGILL CATCH TRENDS

1. The means of bluegill trap net catches for the nine netting
periods fluctuated between 1.3 and 5l.l fish and 0.5 and 13.5 pounds
of fish per net lift. There was (1) a sharp drop during the first

part of June and (2) a continual rise during July, Auj^.ust and September.

2. The sharp drop in catch during the first of June is thought to
have been caused by a decrease in the activity of the bluegills. This
decrease may have been caused by (1) stabilization of water levels after

30

Table 9. Gonad development of 'bluegills, showing percentaee of gonad exeiained that

were assigned to each developments-J. stage.

Period 1 II III IV V VI VII VIII IX

May May 31 June July July Aug. Aug. Sept. Sept.

15-2^ June 9 17-26 l-lO 18-2? 3-12 l8-27 l-lO 16-25

Humher examined 2l6 ^k 68 105 189 298 629 ^77 33O

Undeveloped - - - 8.6 30.I 33.9 63. if 89.9 9S.8

Developing - - _____ 0.6 0.9

Ripe 33*^ 1^.8 16.2 22,8 1.1 _ _

Ripe t 66.6 3l.2 79.4 5^.3 19.5 - 0.2 -

Partly spent - 3.7 ^.^ 3-8 1-6 0.3 _

Spent - - - 10.5 ^4-7. 6 65.8 36.4 9.k O.3

31

the spring high water period, (2) water temperatures warming to above 70
Degrees F., or (3) both of these changes in environmental conditions.

3. The rise in catch during July was caused by recruitment of the
I9U6 year class as spawning activities declined.

It, The continued rise in catch after July resulted from effects
of recruitment and increased activity of the bluegills. The increased
activity of previously recimted bluegills had the most effect on the
catch.

CATCH TRENDS FOR OTHER SPECIES

Seasonal fluctuation in the catch of eight other species is described
below. These species were not taken in quantities large enough to justify
detailed analysis of catch fluctuations but it is felt that trends in
their catch during the season are worth noting. Mean c atches of these
species for individual netting periods do not represent large numbers of
fish and differences between the catch of successive netting periods may
represent sampling errors as often as they do actual differences in
abundance or activity of these species. Trends for the entire season are
probably more significant.

Table 10 lists the mean catch for each of these eight species, for
each netting period. Each mean catch is expressed as the number and
pounds of fish per net lift.

Northern Pike

Four hundred northern pike weighing 1,279 pounds were caught in
trap nets during the season. The mean catch for the entire season was
1.2 fish and U.l pounds of fish per net lift. Mean catches for individual
periods fluctuated from 0.8 to 2,14 fish per lift and from 2.0 to 9.2
pounds of fish per lift. Northern pike catches were highest at the
beginning of netting; in Mayj declined during June and remained rather low
during the remainder of the season.

White Bass

One hundred and forty-five white bass weighing 121 poxinds were caught
in trap nets during the season. The mean catch for the entire season was
O.k fish and O.U pounds of f ish p er net lift. Mean catches for individual
netting periods fluctuated from 0.1 to 1.1 fish p er net lift and from 0.1
to 0,9 pounds of fish per net lift. A drop in catch similar to that ob-
served in the centrarchid catch during the first of June, was evident in
the catch of the white bass. The catch then increased until September.

32

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33

Channel Catfish

One hiindred and ninety-one channel catfish weighing kSS pounds were
caught. The mean catch for the entire season was 0.6 fish per net lift
and 1,1; pounds of fish per net lift. Mean catches for individual netting
periods fluctuated from no fish at all to 1.9 fish and U.6 pounds of fish
per net lift. Channel catfish were most effectively taken in trap nets
during their spawning period in June and July, Very few were caught
after the last of July. Catch fluctuations were similar in the large
mesh buffalo nets ^et in the main channel. A drop in catch during the
latter part of June coincided with a period of cool Heather, almost
daily rainfall, and a few degrees drop in water temperature. It is in-
teresting that this sudden but temporary decline in channel catfish was
predicted by the commercial fishermen who handled the nets with the
author,

Flathead Catfish

Eighty-three flathead catfish weighing 390 pounds were caught. The
mean catch for the entire netting season was 0.3 fish per net lift and
1.3 pounds of fish per net set. Mean catch for individual netting periods
fluctuated from 0,02 to 0.7 fish and from 0.05 to 3.h pounds of fish per
net lift. Like the channel catfish, most of the flathead catfish were
caught in trap nets during their June- July spawning season, A drop in
catch rate, less than that of the channel catfish, occurred with the cool
rainy weather during the latter part of June. Flathead catfish were
taken fairly reKularly during most of the season in large mesh buffalo
nets set in the main channel.

Carp

Six hundred and forty-four carp weighing 2,317 pounds were caught
in trap nets during the season. The mean catch for the entire netting
season was 2.0 fish per net lift and 7.3 pounds of fish per net lift.
Mean catches for individual netting; periods fluctuated from 1.3 to 2.6
fish and from 3.i| to 11.3 pounds of fish per net lift. There was less
fluctuation in the carp catch when expressed as numbers of fish than in
the catch of any other species. There did not seem to be any particular
trend in catch fluctuation although catch was lowest during August and
the first peirt of September.

Northern Redhorse

Two hundred and seven northern redhorse weighing i;72 pounds were
caught in trap nets during the season. The mean catch for the entire
season was 0.6 fish per net lift and l,k pounds of fish per net lift.
Mean catches for individual netting periods fluctuated from 0.1 to 1,5
fish and from 0.2 to 3.6 pounds of fish per net set. The catch rate
rose during June and Julyi declined during August; and rose
again during September,

3h

Carpsucker

One hundred and forty-six carpsucker weighing 39^4 pounds were caught
in trap nets during the season. The mean catch for the entire season
was 0,5 fish per net lift and 1.3 pounds of fish per net lift. Mean
catches for individual netting periods fluctuated between 0.2 and 1,6
fish and between 0.6 and 3.2 pounds per net lift. Catch was highest
during the June spawning period. Catch in the large mesh buffalo nets
set in the main channel was also highest at this time.

Sheepshead

One hundred and ninety-eight sheepshead weighing 2U7 pounds were
caught in trap nets during the season. The mean catch for the entire
season was 0,6 fish per net lift and 0.8 pounds per net lift. Mean
catches of individual netting periods fluctuated from 0.3 to 1.3 fish
and from 0.2 to 1.8 pounds of fish per net lift. The catch rate increased
during June| declined during July and August; and increased slightly dur-
ing September.

k

SUMMARY AND DISCUSSION

1. In order to study seasonal trends in trap net catches, five
permanent netting stations were selected in backwaters of the Upper
Mississippi River a few miles below La Crosse, Wisconsin. One trap net
was fished at each of these stations, for nine 10-day netting periods
that were evenly spaced between May l5 and September 25, 19^8.

2. Eleven species were caught in quantities large enough to note
catch trends during the season. Fluctuations in mean catch during this
four and a half month period were considerable. They were greatest for
the black crappie and the bluegill, and least for the carp and the white
crappie,

3. Three species, the black crappie, the white crappie, and the
bluegill were cauj^,ht in quantities large enough to allow study of causes
of catch fluctuations. These causes may be divided into twc groups, (1)
those that resulted in changes in abundance of fish, and (2) those that
resulted in changes in the rate of activity of the fish. Evidence was
gathered that both abundance and activity rate of bluegills and black
crappies changed considerably during the season.

U, About iiO percent of the black crappies, 60 percent of the white
crappies^ and 90 percent of the bluegills, caught during August and
September were fish of the I9U6 and 19ii7 year classes that had not been
available during May and June. The 19ii7 year class was not available
during the early part of the season because these fish were too small to
be retained in the nets. The 19[i6 year class was not available because
these fish were apparently not in the netting area during the spawning
seasono

35

5o Recruitment of the I9U6 and 19h7 year classes of black crappies,
bluef;ills and white crappies influenced catch rates by (1) causing a rise
during July when these year classes first appeared, and (2) magnifying
the effects of increased activity rate among the bluegills and black
crappies during August and September.

6, There was some evidence of high losses among older crappies and
bluegills during the season. Fewer members of the 19Ui| and 19U3 year
classes were caught as the season progressed. Loss of older fish from the
population is thought to have b een largely the result of natural mortality.

7. The stabilization of water levels and t he warming of waters to
above 70 degrees F., in June may have been related to a decreased rate of
activity noted among the centrarchids.

8, No relationship was noted between changes in water turbidity and
catches of trap nets.

9. Spawning activities were apparently related to the high catches
of channel and flathead catfish during June and the first part of July and
to the high catch of carpsuckers during the last half of June.

10. Species composition of the total catch ty weight changed markedly
throughout the season (Figure 10). During Period III (June 12-26) 68 per-
cent of the catch was of commercial species 5 13 percent of panfish and I8
percent of game fish. A month later in Period V (July 18-27) 60 percent
of the catch was of commercial speciesj 27 percent of pan fish and l5 per-
cent of game fish. In Period VII a month later (August 18-27) 25 percent
of the catch was of commercial species; 59 percent of panfish and 10 per-
cent of game fish. Such changes in composition of the catch probably do
not represent similar changes in the structure of the fish population of
the area fished. Ihey do demonstrate that several factors influence the
catch of stationery nets and these must be taken into account if catches
from such nets are to be used to estimate fish population, size and
structure.

ACKNOWLEDGEMENTS

The writer wishes to acknowledge aid and encouragement received from
conservation agencies of the states of Minnesota, Wisconsin and Iowa and
the U.S. Fish and Wildlife Service. Special thanks are due Mr. Joe
Martel for aid in the netting, Mrs. Wentzel Satek for assistance in the
laboratory, Dr. John T. Greenbank who supervised the field work and gave
generously of his time and advice and Dr. Raymond E. Johnson and Dr.
John B, Moyle who helped prepare and revise the manuscript. Work inthe
field was carried out from the U.S. Fish Cultural Station at La Crosse,
Wisconsin where space and facilities were made available by Mr. C. F.
Culler, Mr. Harry Canfield and Mr. Harry Carson. The photograph of the
trap net was supplied by Mr. John Dobie.

36

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AUGUST 18- Z7

Fig. 10~Precentape composition of the total catch expressed as numbers of fish in netting
periods III, V and VII, showing changes in structure of the catch as the season
progressed. Catches of less common species are omitted.

37

REFERHTCES CITED

Fry, F. W. J. and J. S. Hart

I9I48. Cruising speed of goldfish in relation to water temperature.
J. Fish. Res. Bd. Can. Vol. 7 (U) 19U8 pp. 169-175.

Hartley, P. H. T.

19li7. The natural history of some British freshwater fishes.
Proc. Zool. Soc, Vol. 117, Part I, pp. 129-206.

Hubbs, Carl L. and Alfred Perlmutter

19ii.2. Bioraetric Comparison of Several Samples with Particular

Reference to Racial Investigations. The American Naturalist,
Vol. LXXVI, pp. 582-592. Nov. -Dec. ,19l|2.

Moyle, John B.

1950. Gill nets for sampling fish populations in Minnesota waters.
Trans. Am. Fish. Soc, Vol. 79, 19ll9, PP. 195-20U

Smith, Lloyd L., Jr. and Normal L. Moe

19UIt. Minnesota fish facts, Minn. Dept. of Cons. 28 pp.

38

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