Physical Factors Influencing Fish Populations
In
Pools
of a Trout
Stream •
STEPHEN L. LEWIS 2
Department of Zoology and Entomology, Montana State University, Bozeman, Montana
ABSTRACT
The relationship between fish populations and physical parameters of pools was studied
in Little Prickly Pear Creek, Montana, during the summers of 1965 and 1966. The pools
were mapped and their fish populations sampled. Surface area, volume, depth, current
velocity, and cover accountedfor 70 to 77% of the variation in numbers of trout over 6.9
inches total length. Most of the variation was the result of differencesin current velocity
and cover. Cover was the most important factor for brown trout, and current velocity for
rainbow trout. The density of all trout per unit area of pool surface and cover increased
significantly as current velocity became greater. Deep-slowpools with extensivecover had
the most stable trout populationswith brown trout showing greater stability than rainbow
trout. The importanceof cover to trout is discussedin terms of securityand photonegative
responseand current velocity in terms of space-foodrelationships.
INTRODUCTION
Stream trout populationsare determined
largely by the quality of the habitat. The
streamimprovementwork of Tarzwell (1937,
1933); Shetter,Clark, and Hazzard (1946);
and Saundersand Smith (1962) showedthat
populationsrespondto an increasein shelter
and food. Gunderson(1966) found significantdifferences
in the browntroutpopulations
of two adjacentstreamsectionsin relation to
streammorphologyandfloodplainuse.Boussu
(1954) demonstratedthat removal of under-
objectiveswere to determinethe important
physicalfactorsinfluencingthestandingcrops
of fish and to measurepopulationstability.
The studywasconducted
from July, 1965, to
September,
1966,on Little PricklyPearCreek,
Lewis and Clark County,Montana.
DESCRIPTION
OF
THE
AREA
Little Prickly Pear Creek is a small trout
stream located 30 miles northwestof Helena,
Montana. It ariseson the east slopeof the
continentaldivide and flowsnortheasterlyfor
cut banks and brush from a section of stream
about 35 miles where it enters the Missouri
causeda decreasein the numberand weight
of residenttrout,with decreases
beinggreatest
for large fish. Shuck (1945) reportedthat
volumeand depth of water were significant
factors determiningpopulation density of
River 6 milesupstreamfrom Craig,Montana.
The drainagebasin encompasses
an area of
394 square miles consistingprimarily of
grass]and
slopeswith openstandsof conifers.
Elevation of the flood plain varies from
3,300 to 4,600 ft (meansealevel). Analyses
conductedby the Soil ConservationService
reveal that the soils of the drainagebasin
are weakly calcareous,derived mainly from
weatheredargillite rock with smallercontributionsfrom quartzite,limestoneand igneous
brown trout in a section of stream.
The social behavior of fish in relation
to
habitat conditionsis an important considera-
tion. The investigations
of Kalleberg(1958)
and Newman (1956) revealedthat salmonids
are territorial and establisha socialhierarchy.
Thusthereis competition
for thelimitednum- rocks.
Data collectedat the SiebenGage Station
ber of favorablepositionswithin a stream.
This indicatespopulationlevelsmay be self- located on the middle reaches of the stream
limiting basedon the quality of the habitat. indicatethe peak runoff occursin May and
The presentstudywas an evaluationof the June, with low flows in August. The mean
for July,August,and Sepphysicalhabitatqualityof streampools.The monthlydischarges
tember,1966 were 33, 14, and 20 cfs, respec1 Contribution from the Montana Cooperative Fish- tively. During periodsof low flow, stream
cry Unit.
widthsaveragedfrom 20 to 30 ft with depths
2 Presentaddress:OregonState GameCommission,
in most pools not exceeding 5 ft. Mean
ResearchDivision, Corvallis, Oregon 97331.
14
PHYSICAL FACTORS AND STREAM FISH POPULATIONS
monthlywatertemperatures
fromJulythrough
September,1962-65, ranged from 41.0 to
64.7 F (Swedberg,1965). Alkalinity ranged
from 4.5 to 4.9 milliequivalents
per liter, pH
from 8.0 to 8.7, conductivity(K2a) from 410
15
USGS
Gage Sta •qon
(Sieben)
to 445 micromhos, and total hardness from
215 to 235 ppm.
The speciesof fish collectedin order of
decreasing abundance were: brown trout
(Salmotrutta), longnosesucker(Catostomus
catostomus),rainbow trout (Salmo gairdnerii), mountain whitefish (Prosopiumwilliamsoni), brooktrout (Salvelinus]ontinalis)
16
10-15
and white sucker (Catostomuscommersoni).
Mottledsculpin(Coitusbairdii), although
numerous,were not consideredin this study.
There has been no stockingof fish in the
Miles
8-9
stream since 1954.
Nineteenpools were studiedand located
within a 6.2 mile length of streamfrom 0.7
mile abovethe mouthof Trinity Creekto 0.2
mile below the mouth of Big SheepCreek
(Figure 1).
DEFINITIONS
LITTLE
AND SAMPLING TECHNIQUES
F•gva•. 1.--Study area of Little Prickly Pear Creek
A poolwasdefinedas any relativelylarge showing
the approximatelocation of pools.
streamarea capableof providingshelterfor
largefish. The poolsrangedfrom deep-slow
portionsof the streamto areasof shallow-fast greaterthan1.00fps), deep-fast
(greaterthan
water associated
with cover. Physicalcharac- 1.5 ft andgreaterthan1.00 fps). Any future
teristicsusedto selectpoolsweresize,depth, reference
to depthor currentvelocitywill folcurrent velocity, and cover. Pools selected low these criteria.
were distinct units, limited above and below
by the presenceof shallow water without
cover. This minimizedthe error of overlapping home territoriesof fish from adjacent
pools.
All poolswere mappedbetween9 August
and 12 September1966, duringlow stabilized
flow. Transects were established at 10-foot in-
The cover associated
with a pool was
mappedandthis includedbrush,overhanging
vegetation,undercutbanks,andmiscellaneous.
The term brush describeddead submerged
woodyportionsof bank vegetationoccasionally strengthened
by live growth. Overhanging vegetation
waslive growththat provided
an overheadcanopylessthan one foot above
tervalsanddepthsweretakeneveryfoot along the water's surface. Miscellaneous cover in-
each transect.
Current velocities were mea-
suredwith a Gurley currentmeter at 0.4 of
the observeddepth every 2 ft along each
transect.The water comprisinga pool was
classifiedinto typesbasedon depthand current velocity (Burkhard, 1964). The water
typeswere: shallow-slow(lessthan 1.5 ft in
cludedunderwater
shelves
providedby clay
and rock, tree roots, and debris.
Pool boundaries
weredelineated
by depth
and cover. The surface water area included
withinthepoolwasdeeperthan1.5ft or areas
associated
withcoverregardless
of depth.Surfacearea,water-typecomposition,
and extent
depthandlessthan1.00fpscurrentvelocity), of coverweredetermined
from mapswith a
deep-slow(greaterthan 1.5 ft and lessthan planimeter.Meandepth,meancurrentveloc1.00 fps), shallow-fast(lessthan 1.5 ft and ity, andvolumewerecalculated
for eachpool.
16
STEPHEN L. LEWIS
TABLE
1.--Physicalparameters
of poolsat low stabi-
4.O
lized/low in the summero/1966
Mean
Pool
Surface
area
Volume
number (ft 2)
1
585
2
3
4
5
6
1,454
497
838
889
1,613
11
12
1,849
1,630
14
15
16
17
'667
665
700
456
7
8
9
10
13
18
19
487
367
577
563
i 070
433
274
Mean
depth
current
velocity
(ft a)
(ft)
1,054
1.8
2,471
1,092
1,676
1,866
3,386
Total
cover
Per
cenP
(fps)
(ft 2)
cover
0.78
254
43
1.67
0.89
0.81
0.77
144
79
159
30
201
159
30
22
28
5
731
808
1,153
1,069
4,068
3 748
1.7
2.2
2.0
2.1
2.1
1.5
2.2
2.0
1.9
2.2
2.3
0.30
0.32
0.42
0.48
0.63
1,201
1,442
1,330
593
693
685
1.8
2.2
1.9
1.3
1.6
2.5
1.00
0.94
1.02
1.22
1.32
0.85
2',675
2.5
0.52
0.67
0.69
294
46
47
55
497
20
9
6
6
31
11
10
290
27
79
0
18
0
186
66
134
236
28
10
19
52
FIGURE2.--Relationship of total trout density per
50 ft2 of pool surface area and cover to current velocity showingfitted regressionlines.
to 1.67fps. Coverrangedas highas497 fta
andmadeup asmuchas52%of thepoolsurThefishpopulations
of thestudypoolswere face area. Brushcomprised77%, undercut
banks11%, overhanging
vegetation7% and
sampled
between
3 August
and1 September
miscellaneous
types
5%
of
all poolcover.The
1965,andbetween11 Julyand 27 July 1966
Percentages
are per cent of surfacearea.
constituting
coverwere
by electrofishing
usinga 300volt,850 watt importantplantspecies
(Cornussp.).
direct current unit. Individual pools were willow(Salixsp.) anddogwood
isolatedwithblockingnetsandat leastthree
passes
weremadethrough
eacharea.Cap-
Fish Populations
A total of 247 trout over 6.9 inches was
turedfish wereanesthetized
with MS-222 (TricaineMethanesulfonate)
and measuredto the collectedin the 19 studypoolsduring 1966
60%,rainnearest0.1 inchtotallength. Onlyfishlarger (Table2). Browntroutcomprised
than 6.9 incheswere includedin this study. bow trout 36%, and brook trout 4% of the
Troutweretaggedwithplasticbandjaw tags, troutsampled.The numberof troutin indiwhitefishwithmetalopercletags,andsuckers vidualpoolsrangedfrom2 to 34 fish. Trout
with 1966
withplastic
darttagsinserted
at thebaseof numbersin 1965werecomparable
the mid-dorsal fin.
figures.The meanlengthof browntrout
was11.5inches
compared
with10.8
The efficiency
of fish sampling(ratio of sampled
markedto unmarked
in a second
sample)was inches for rainbow trout.
Twenty-five
whitefish
and121suckers
were
determined.Samplingefficiencyfor troutwas
84 to 100%,andfor suckers
56%. Therewere collected in 1966. Whitefish occurred in 9
werefound
too few whitefishpresentfor efficiencyde- poolswithI to 13 fish. Suckers
termination.
in 13 poolswith 1 to 22 fish per pool. In
werepredominant
Thedatawereanalyzed
in a multiplelinear severalof thepoolssuckers
regression
andanalysis
of variance
accordingby numberand/orweight.Whitefishand
variedgreatlybetween
years;
toBailey(1959)andSuedecor
(1956)to de- suckernumbers
terminetherelationships
between
fishpopula- however,the fluctuationsdid not appearto
tionsandphysical
parameters
of pools.
RESULTS
PhysicalParameters
affect trout populations.
Relationship
o] Trout Populations
to
Pool PhysicalParameters
A multiple
linearregression
wassetupwith
Poolsrangedin surfaceareafrom 274 to
pool
surface
area,
volume,
mean
depth,
mean
1,849ft• withvolumes
from593to 4,068fta
current
velocity,
percent
cover,
and
total
cover
(Table1). Meandepths
variedfrom1.3 to
variables
andnumberof
2.5 ft and mean current velocitiesfrom 0.30 as the independent
PHYSICAL
FACTORS AND STREAM
TABLE2.--Fish populationso/pools in the summero/
1966
FISH
POPULATIONS
TABLE &--Results o] multiple regression analyses
with current velocityand total coveras the independentvariables
Standing crops of fish over 6.9 inches
Partial
Trout
Pool
number
1
2
3
4
5
6
7
8
9
I0
11
12
13
14
15
16
17
18
19
Totals
White-
Total
14
17
2
4
2
31
34
16
20
4
15
7
5
11
7
21
19
13
5
247
Brown
6
12
1
3
0
24
19
7
13
2
10
5
4
7
3
11
9
10
1
147
17
Rainbow
8
2
1
0
1
6
14
7
7
2
5
2
i
4
3
10
9
3
4
89
Brook
0
3
0
1
1
1
i
2
0
0
0
0
0
0
i
0
i
0
0
11
fish
13
0
0
0
1
2
1
0
0
0
1
2
0
3
1
1
0
0
0
25
Suckers
0
10
2
1
0
20
1
3
0
20
4
10
11
Anal-
Trout
1
Total
121
rag.
ression
coeffi-
ysis group variables cient
velocity
Multiple
T•
15.12
3.85**
0.05
4.28**
velocity
7.60
2.85*
Totalcover
0.04
4.96**
7.86
4.59**
0.01
1.83 u.s.
7.57
4.16'*
Totalcover
F2
1R
a R2•
15.25'*
0.81 0.66
15.26'*
0.81 0.66
11.60'*
0.77 0.59
17.38'*
0.71 0.51
Current
2
Brown
IRain-
3
bo•v
Current
velocity
Totalcover
15
22
0
2
Independent
IRaln-
4
bow
Current
velocity
z T refers to sOtdent-type test to determine if the partial
regression coefficient is significantly greater than zero.
aF tests significance of overall regression.
a Multiple correlation coefficient.
• R2 is amount of variance explained by the independent
variables.
** Significant at 0.01 level.
* Significant at 0.05 level
trout per pool as the dependent
variable. Foln.s. Not significant.
lowing the initial regressionanalysisthe independentvariablesthat added little to the
There were differences in the relative imoverallrelationshipwere removedand the regressionrecalculatedto find the factorsthat portance of current velocity and cover to
significantlyinfluencedtrout numbers. Re- brown trout and rainbow trout as shownby
coefficients
and the siggressionlines were computedfor total trout, the partial regression
rainbow trout, and brown trout. Trout data nificance of their correspondingT values
from 1966 wereusedin the statisticalanalyses (Table 3). Cover was the most important
sincepool physicalparametersweremeasured factor for brown trout indicatingutilization
of currentvelocity.Howin that year and low water levels were be- of coverirrespective
lieved to have increasedsamplingefficiency. ever, faster current velocitiescontributedto
The six physicalparametersaccounted
for an increase in brown trout numbers above that
75, 77, and 70% of the variation in the num- expectedon the basisof coveralone. Current
bers of total trout, brown trout, and rainbow velocity was the only statisticallysignificant
trout, respectively.The multiple regressions factor for rainbow trout and in analysis4
involvingall physicalfactorsweresignificant (Table 3) accountedfor 51% of the variance
at the 0.05 level,and the multiplecorrelation in rainbow trout numbers. The fast-water
coefficientsranged from 0.84 to 0.88. Pool poolsutilizedby rainbowtrout alsocontained
area, volume,and depthaccounted
for very extensive cover.
little of the variation in trout numbers. PerThe densityof trout per unit area of pool
centcoveraddedlittle to the relationshipsince surfaceand cover increasedsignificantlyas
it was closelycorrelatedwith total cover and currentvelocitybecamegreater.This is shown
its effecthad alreadybeenaccounted
for by by the significant(0.01) regressionanalyses
the latter. Current velocity and total cover with numberof trout per 50 ft2 of coverand
were the importantfactors. Togetherthey per 50 ft2 of poolsurfaceareaagainstcurrent
accountedfor 66, 66, and 59% of the varia- velocity (Figure 2).
tion in numbersof total trout, brown trout,
Other Species
and rainbowtrout, respectively.Regressions
with thesevariableswere highly significant Suckers were excluded from the statistical
(0.01 level), and the multiplecorrelationco- analyses
because
of poorsamplingefficiency
efficientsrangedfrom 0.77 to 0.81 (Table 3).
(56%) and becausea large numberof those
18
STEPHEN L. LEWIS
sampledmay not have been residentin the
poolsbut were spawningmigrants.Although
little emphasiscould be placed on habitat
preferenceby suckers,they were most common in large poolswith a predominance
of
deep-slowwater and extensivecover. The
smallnumberof whitefishcollectedprecluded
any attemptto determinethe habitat preference of this species.
seek out areas with overheadcover (Gibson
and Keenleyside,1966; McCrimmon and
Kwain, 1966). Populationincreases
associated
with fast streamvelocitymay be rheotactic
responses,
but most probably result from
space-foodrelationships(Chapman, 1966).
Miiller (1953) and Nilsson (1957) found that
food organismdrift wasthe major sourceof
food in streams,and in areas of fast current
velocitythe supplyof drift would be greater.
PopulationStability
Thus, in swift portionsof the stream,fish reAnothermeasureof pool quality is popula- quire lessspaceto obtainneededfood, territionstability.Annualpopulation
stabilitywas tory size is reduced,and populationdensities
determinedfor all poolsfrom August,1965, can be greater (Chapman,1966). Kalleberg
to July, 1966, on the basisof electrofishing (1958) foundsmallerterritoriesfor juvenile
Atlantic salmon and brown trout in areas with
tag recoveries.The numberof taggedfish
(231) was adjustedfor thoseknownto have high current velocityrelating this to visual
been caughtby anglers (20). Of 211 avail- isolation. These conceptsmay explain the
of trout per unit areaof pool
able taggedtrout, 47 fish or 22% were found greaterdensities
surface
and
cover
foundin thefast-waterpools
in their home pools one year later. Brown
studied.
trout populations
were moststablewith 30%
The importanceof currentvelocityto rainrecoveredin homepoolscomparedwith 13%
for rainbowtrout. Deep-slowpoolswith ex- bow trout and coverto browntrout may inor mayreflect
tensivecoverhad the moststablepopulations. dicateactualhabitatpreference
species
segregation
caused
by
inter-specific
Fast-waterpoolsand thosewith limited cover
competition.
Newman
(1956)
states
that domihad the greatestpopulationturnover. The
losses observed from one summer to the next
nance is based on size and since brown trout
were from naturalmortality,unknownangler were larger than rainbow trout the habitat
mortality or movement.The extent of move- preferenceof rainbowtrout may have been
obscured if behavioral interaction was a facment was unknownsinceno systematic
effort
tor. Rainbow trout may have utilized cover
was made to recover fish above and below the
in slow-water
poolsto a greaterdegreein the
pools.
The annualpool stabilityof whitefishand
suckerpopulations
was very low. Only 2 of
46 whitefish(4%) and 3 of 152 suckers(2%)
wererecovered
in the poolwheretagged.The
low stability of suckersappearedto be due
to movement.Tag returnsindicateda general
upstreammovementin the springand a gradual downstreamdrift throughoutthe summer.
A similar patternwas observedby Stefanich
(1951) on lower portionsof the samestream.
DISCUSSION
The populationdenskyof trout in pools
appearsto be determinedby the physicalenvironment,especiallyavailablecoverand current velocity.The valueof coveris probably
related to a fish's security and the photonegativeresponseof trout causingthem to
absence of brown trout.
The relationships
found apply only to low
summer water
conditions
since the relative
importanceof physicalfactors may change
seasonally.Chapman(1966) speculates
that
in spring, summer,and early fall the spacefood conventionmay be mostimportant,but
in winter space alone may govern density.
Thus,duringthe winterotherpooltypesmay
assumegreater importance.
There were other factors not measured that
could accountfor some of the unexplained
variationin trout numbers.Foremostamong
theseis foodwhichmaybe considerably
more
abundantin one pool than another,partially
dependingon the extentof riffle immediately
abovethe pool. Anotherfactor may be light
intensity under the cover. In view of the
photonegativeresponseof trout, densecover
PHYSICAL
FACTORS AND STREAiVf FISH POPULATIONS
].9
tinalis).
J. Fish. Res. Bd. Can. 23: 1007-1024.
that permitslittle light penetrationmay be
GUNDmaSON,
D. R. 1966. Stream morphologyand
most attractiveto trout. Angler mortality of
fish populationsin relation to floodplain use.
fish was alsoa possiblesourceof error since
M.S. Montana State Univ., Bozeman. 21 p.
H. 1958. Observationsin a stream tank
someof thepoolsstudiedweremoreaccessible KALLEBmRG,
of territoriality and competitionin juvenile salto anglersthanothers.Twelvepercentof the
mon and trout (Salmo salar and S. trutta). Inst.
Freshw. Res. Drottningholm 39: 55-98.
taggedtrout in poolswereknownto havebeen
H., ANDWmN-HWAKWAIN. 1966. Use
caughtby anglersas of September,
1966. The McCR1MMON,
of overheadcoverby rainbowtrout exposedto a
percentreturn for brown trout and rainbow
seriesof light intensities. J. Fish. Res. Bd. Can.
23: 983-990.
trout wasaboutthesame.This figureis miniMOLLmR,K. 1953. Investigations on the organic
mal asno effortwasmadeto recovertags.
drift in north Swedish strean•s. Inst. Freshw.
ACKNOWLEDGMENTS
The authorwishesto thank the following
for their contributionsto this study: Dr.
RichardJ. Grahamfor directingthe studyand
aiding in preparationof the manuscript;John
Peters for technical advice; Allen Elser for
assistance
in all aspectsof the study; William
Res. Drottningholm 35: 133-148.
Nzw•aN, M. A. 1956. Social behavior and inter-
specificcompetitionin two trout species.Physiol.
Zool. 29: 64-81.
NILSSON,NtLs-Aavm. 1957. On the feeding habits
of trout in a stream of northern Sweden.
Inst.
Freshw. Res. Drottningholm 38: 154-166.
SAUNDZaS,
J. W., aNDM. W. S•ITm 1962. Physical
alteration of stream habitat to improve brook
trout production. Trans. Amer. Fish. Soc. 91:
185-188.
Gould and Melvin Kraft for field assistance; Scut•cK, H. A. 1945. Survival, populationdensity,
and David Jacobsen for aid in the statistical
growth and movementof the wild brown trout
in Crystal Creek. Trans. Amer. Fish. Soc. 73:
analyses.The investigationwas financedby
the MontanaCooperative
FisheryUnit.
SIIETTER,D. S., O. H. C•.aa•, aND A. S. HazzaaD.
209-230.
1946.
The
effects of deflectors
in a section of
a Michigan trout stream. Trans. Amer. Fish.
LITERATURE
CITED
BAILEY, N. T. J. 1959. Statistical methods in biology. John Wiley and Sons, Inc., New York.
2OOp.
Bousst•, M. F. 1954. Relationship between trout
populationsand cover on a small stream. J.
Wildl. Mgmt. 18: 229-239.
BuaKHaaD,W. T. 1964. Colorado stream fisheries
studies. D.J. CompletionReport, Project F-26R-2. ColoradoGame and Fish Dep. 103 p.
C•avMaN, D.W.
1966. Food and space as regulators of salmonid populationsin streams. Amer.
Natur.
100: 345-357.
GIllSON, R. J., AND M. H. A. KEENLEYSIDE.1966.
Responsesto light of young Atlantic salmon
(Salmo salar) and brook trout (Salvellnus/on-
Soc. 76: 248-278.
SNmDmCOa,
G. W. 1956. Statistical methods. The
Iowa State College Press,Ames, Iowa. 534 p.
STEFANlClt,
F.A. 1951. The populationand movement of fish in Prickley Pear Creek, Montana.
Trans. Amer. Fish. Soc. 81: 260-274.
SwzDamag,S. 1965. Evaluation of fish habitat destruction in Little Prickly Pear Creek due to
construction of interstate highway 15. D. J.
CompletionReport, Project F-5-R-13.Mont. Fish
and Game Dep. 14 p.
TaaZWZLL,C. M. 1937. Experimental evidenceon
the value of trout streamimprovementin Michigan. Trans. Amer. Fish. Soc. 66: 177-187.
1938.
An evaluation of the methods and re-
suitsof stream
improvement
in thesouthwest.
Trans. N. Amer. Wildl. Conf. 3: 339-364.