Table Of ContentC;rcMt Jiasiii Naturalist53i2>.pp. ltM-19S
USE OF BOULDER POCKET HABITAT BY RAINBOWTROUT
{ONCORHYNCHUS AHK7SS) IN FALL RIVER, IDAHO
"
Daniel N. Stifuhel' uiitlJ. S. (wilTith''
Abstract. .Vljunclancf of rainbow trout {Oncorhi/urhits mi/kiss) in relation to characteristics ofpockets created by
boulderswasstudiedin Fall River,southeastern Idaho.Todeterminedepthandsurfaceareaofpocketsmostselectedby
rainbowtrout,fisii\serecountedb\ snorkeliny;,andpocketphysicaldimensionsweremeasured.Anelectivityindexdefined
hal)itat selection in the followinsjterms: the most suitable habitatwas >{).7m nia,xininm depth, >0.5m minimum depth,
and>.3 m~surface area. Some stnd\ reaches of Kali River had more suitable pockets available lortrout than werebeing
utilized.
Kciiwords: nihihoirtrout^ Oucorhvuchus mxkiss.luihildt use. Idaho,stream nhahilitation.
Boulderscreateamajor.sourceoftrouthabi- cial outwash thn)ugh which it flows. Basalt and
tatilluian\higher-<j;ra(lieutwestern ri\ers.They ash flow tuff bedrock define the channel fonn.
create pools or pockets with increa.sed depth Sinuosityis low, approaching LO, antl there are
andprovidesurlace turbulence that niavbethe no meanderpools. 0\erall gradientinthestudy
onl\' co\er a\ailable to trout. Water depth and reach is 0.64%.
boulder coxer wen- important in deteniiining Within-channel habitat was homogeneous
den.sitv oftrout in a (.'olorado stream (Stewart and consisted predominantly of nni habitat, as
1970). Boulder placement is a connnonly u.sed defined bv Helm (1985). Little woody debris
teclmi(jue in stream rehabilitation (Ro.sgen and hadbeen retainedin the channel. Atthe 14-16
Fittante lySft) and ma\'provide eflectiw,dura- niVsec low flows of late summer 1991, the
ble trout habitat (Lt^-e 1982). streammarginhadpulledawa\ fromam \ertical
This stud\- exaluated age-1 and older wild banks fonued by high flows, lea\ing no bank
rainbow trout {Oiicorht/ticJiits ini/kiss) use of habitat to pro\ide coxer for larger trout. The
boulderpocket habitatin Fall Ki\'er, Idaho. Ob- stud\' reach contiiined Paiute sculpin {Cotttis
jecti\es were to determine the proportion of hi'ldiiit^i), longnose dace {Rhitiiclithi/s cafarac-
troutusingboukk'rj-jockethabitat,andtoassess t(ic), and a few Utali suckers {Catosfoimis
the extent to which fish selected pockets of (ifdciis) and mountain whitefish {Pi'osopiimi
specificsurface areaand depth. williainsoiii)inaddition tothewildrainbowand
occasional cutthroat {Oiicoflii/iicluis chirki)
Mi'.TIIons trout.
In August of 1990 and 1991 snorkel sune\s
The Fall River originates in th(> soulliwe.st were conductc^d to estimate trout density"
portion ofYellow.stone National Park, it flows throughoutthestudyarea.Theseindicatedthat
east into Targhee National Poorest, Idaho, and density'oftroutlargerthan age-Oa\'eraged0.35
then through agricultural lands to join Ileniys fisli/100nr,orapproximatcK-136fisli/km(Grif-
Fork of tlie Snake Hi\er appro.\imatel\- 10 km fith unpublished data). Three sites, repre-
.south ofAshlon in Fremont Countx. The stud\' senting a range of boulder pocket densities,
area, at anelexation ofabout 1740 m,extends7" were selected for the present studv. Sites were
km,halfofwhich iswithintheTargheeNational 160-170 m longandaxeraged26-46 mwide.A
Forest and halfinnnecbatelybelow.Thestream boulder was defined as >().4 m diameter, situ-
channel hasbeen sliapedbycoarse-grainedgla- ated so that its toy) was at or aboxe the water
",.lV)i<i-l|l)iuiirrliIndi-iwi(hoofmHicoolroi^ixc-.siplnuSlciii.iiuK'css.liIodu.lidluilieSU;iihj-(lrUirs
194
1993] Thout Usk of B()i'Li)i;i{ Pockkts 195
TMilI 1.(Iiaractcristifsofhoiildi-rpocketsusedIn Sorainhow trout intlirecstiidx'sectionsofFallI^ivcr.Idalio.snnnnc
uwi
Numberof
('liaractcristic
Maximum depth (m)
<().46
0.46-0.55
0.56-0.65
0.66-0.75
0.76-0.85
0.86-0.95
0.96-1.05
1.06-1.15
Minimmn depth (m)
<0.26
0.26-0.35
0.36-0.45
0.46-0.55
0.56-0.65
0.66-0.75
Surfacearea (m")
<().46
().46_ 0.7,5
0.7f^ 1.25
1.26- 1.75
1.76- 2.25
2.26- 2.75
2.76- 3.25
3.26- 3.75
3.76- 4.50
4.51- 5.50
5.51- 6.50
6.51- 8.50
8.51-11.00
11.01-13.50
13.51-16.00
16.01-20.50
20.51-26.00
26.01-30.00
Great Basin Naturalist [X'olnnie 53
196
0.8
J O -1--1- -1--I-
Maximum Depth
+-1-
0.6 '- +
EH Minimum Depth + + -t-
0.4 -
0.2
0.1 0.2 0.6 0.7 0.8 0.9 1.0 1.1
-0.2
-0.4 -
-0.6
^
-0.8
Depth (m)
Fig. 1. Miniininii dcptli [m) and iiKLxiniuin deptli (ni) oflioiiklerpockt-ts u.st-dIn-rainbowtront in Falls Ri\er, Idaho.
Electi\itie.s are indicated: ++ (>().5, .strongselection), +(>0.25 l)nt <().5, moderate selection), (±0.25, no selection),
- (>-0.5but<-0.025,moderatea\oidance),and = {<-0.5,strongavoidance).
HBD
To e\altiate the selection by trout of the reach,andsmallerpocketswerepiimatily
pocket paraiiiet(M-s, an electivity index (D) wius found inthe LBD and IBD reaches.
calculated: Pocket surface area was partiallv a function
D = r - n ofboulder diameter, with pocket area = 1.881
(r + p) - 2pr + 4.5572 X boulder diameter {R- = .57, N =
182) forall sites combined. The coirelationwas
wIkmc r i.s the proportion ot the resource u.sed higheratlowerboulderdensit\-sites; but at the
In rainbow trout and p is the proportion a\ail- IIBD site, areaofan indixidual pocketwas also
able in the en\ironment (Baltz and Mo)'Ie affectedbv thepresence of adjacent boulders.
1985). Following Halt/, and Movie (1985), we All trout obseived in the stud}' siteswere in
iDnt>eq0o.5r,etmeoddestrraotnegsseelleeccttiioonn>t0o.b2e5biundti<c0a.t5e,dnboy wl)iotuhld0,er17p,ocaknedts6.6Eaitgshitt)e's-LthBrDee,fIiBshD.wearnedfHoBunDd,,
.selection ± 0.25, moderate avoidance >-{).5 respectixek'. Thetotal numberofboulderpock-
but <-0.25,andstrongaxoidance<-0.5. Elec- ets holding trout was 10 (17% ofpockets pre-
ti\it)' N-alues were calculated for maximum and sent)at IBD and27(32%ofpocketspresent) at
minimumdepthandsurfaceareaofdieboulder IIBD. A coiupaiison of utiliz^ed pocket meiis-
pockets.
urements showed no significant difference be-
tween the two sites (F < .05) and the datawere
Rksiuts pooled foranalvsis.
As water depth and surface areaofa pocket
There wus a wide range of maxinnun and increased, the numberoffish present generallv
miniimim depths and suHace area of boulder increased (Table 1). No trout used pockets in
pocketsa\ailableon Fa\\River. Maximumdepth which minimumdepthwaslessthan0.26mand
among the three study sites ranged from 0.3 to luaxinunu depthwas less than 0.36 m.
I.l m and axeraged 0.7 m. Minimum depth Assurfaceareaincreased,thenumberoffish
ranged from 0.2 to 0.7 m, averaging 0.45 m. perpocketgenerallyinci"eiisedtoamaximumof
P(K-ket surface area nuiged from 6.25 to 28 nr 5 (Table 1). Average numberoffish perpocket
andax'eraged2.4 nr. Thelargerandintenuedi- was 1.4 inpocketswith surface areas <2.25 m",
ate-sized pockets were prinuuih found in the 2.2 in surface areas of 2.26-4.50 m~, 2.2 in
—
1993] Trout Ush of Bouldi.k Pockkts 197
0.8 T
++
++
0.6
++ ++ ++ ++ ++ ++ ++ ++ ++ ++
0.4 --
-
0.2
fJ 4 ^ 4^
OJ I lC~) r-~) lO uO uO U3 r-- OO C3 Csl lO . OJ Csj
-0.2 +
-0.4
-0.6 - =
-0.8 ^
Surface area (m^)
Fit;;. 2. Surfacearea(nr)ofhoukierpocketsused l)\ niinhowtrout in Falls Hi\cr klalio. I'll('cti\itiesareindicated: ++
(>0.5,strongselection), + (>0.25but<0.5,moderateselection),()(±0.2.5.noselection). ( >-()..5l)nt<-0.02.5,moderate
avoidance),and = (<-0.5,strongavoidance).
surfaceareasbetween4.51 and8.50in", and3.5 liow trout in the Fiill Rixer Bait/, and Moxle
insurface areas >8.5 ni" (1985) exaluated rainbow trout habitat in a
Tlieelectivit)'indexdemonstratedthat trout tiibutan ol the Sacramento RixcM; California,
were selecti\'e in the uiicroliabitat thev occu- and found strong selection for depths greater
pied. Electi\it\'viiluesformaxinunndepthindi- than0.6m,similartothethreshold\alueforour
cated moderate selection at depths equal to or stud\. The Habitat Suitabibt)' Index (HSI) for
greaterthan0.7mandstrongselectionatdepths rainbowtrout(Raleighetal. 1984)indicatesthat
greater than 0.9 m (Fig. 1). Mininnun pocket depths greater than 0.46 m hax'e a suitabilit\'
Adtepmtihnwiansumniotdeapstehnssitoif\'0e.6inmdexanodfdtreoeuptedrentshietr\e. uinntdielxFvaalllueRio\fer1,ptohcekheitghdeesptt\hasluoef p>o0s.s7ibml(\we\roet
was moderate selection and o\er 0.7 m, strong reaclu^dwastheremockM'atetostrong.selection,
selection (Fig. 1). Pockets with sm-face areas andtrout moderateK"a\()i(k'dpocketsatdepths
equiil to orexceeding 3 nr were moderateK-or of0.5 m; thus, the IISl did not accurateK'pre-
stronglyselected (Fig. 2). dict depth selection on Fall Ri\er Minimum
Habitat for which rainbow trout showtnl a pocket depth appeared to be a less u.seful indi-
"strong"or"moderate" selectionwasviewedbv catorofhabitat selection (or f^ill l-lixcrrainbow-
usasthemostsuitablehabitatforthestucK'sites.
trout.
Fifty of the 178 pockets in the three sit(\s fell Pocket surface areawas also a factoraffect-
within tliose limits. Thirteen optimal pockets ing trout densih. Onl\ four fish were found in
wwietrheinlotchaeteLdBwDithrienacthh.eTIhiBiDty-rseeavcehnawnedrenolnoe- pleocctkeedt.sl.<(n1\.is5(1m9'6,9)anfdoutnhdotsheat>s3urfnarcewaerreeaa.nsed-
cated within the HB13 reach, and more fish depth along with noIuiuc, cun'ent velocitv, and
were found in that reach. Atotal of23 ofthe 50 vovcv accounted for70-77^ ofthe xariation in
optimal pocketswere not occupied In trout.
lunnbers of trout in pools of Little Prickly
Creek, Montana.
Discussion If surface area "requirements" reflect the
size of tcMiitorics defendedby individual trout,
Maxinnmi water depth in boukk'r pockets in o})timal habitatagonisticbehaviorbyindivid-
strong!)-influenced selection ofhabitat byrain- ual trout mi<iht serve to establish maximum
C;hkat Basin Natuhalis' [Volume 53
198
deiisit)-. Allen ( 1969) and Grant and Kramer LiTERATUHK ClTED
(1990) rexiewed the literature for flu\ial sal-
liinmointiedds,; sttlrioDnugghsidmaitlaariftoiresnwieirnebofwoutnrodutamwoenrge Al.i.i'p.NopnKl.atIIi.onItsJfii^nJ.sEtirieiaimitsa.tiPoangseosn3p-r1oSduinctTio(nJ.iXnosratlihiciooitiei,d
the se\en siilmonid speeies they reviewed. For ed., Svnipdsiunion salmonandtroutinstreams. Insti-
Hsh 15-20 em long, a\erage territoiy size in tuteofFisheries, Uni\ersit\ ofBritishColumbia.Wiu-
ptoheolesstwiiLmsataepdpraorxeiamoaeteeuKp-ie1d-5bynri.ndiIn\idFualall tRrio\u'te,r B\i;rzea.onnDva.esrsM,e.mC,baalnaangddea.Po.fBCaMloifyolrnkia1s9t8.r5e.amMifeirsohleisa:bidteavteluospeiin)gy
basedonourobsenationsoffishabundaneeper criteriaforinstreamflowdeterminations.Transactions
I)oeket, ranged from0.5 to6.0nrandaveraged oi theAmeric;ui Fisheries Society114: 69.5-704.
2.5 nr. Howexer, two-thirds of the fish were (iHANr |.W.A..AND D. L, Kkamer i990.Territon-sizeas
inhabiting areiLS <2.5 m", suggesting that apredictorol the upperlimittopopulationdensityof
juN'enile salmonids in streams. Canadian Journal of
smallertenitoriesmightIk^ recjuiredinboulder FisheriesandAquaticSciences47: 1724-1737.
poeketsthaninthepools fromwhiehthedataof IIki.m W. T, ko 19S5. Clossan-of.stream habitat terms.
Grant and Kramer(1990) were generated. Special report,W'esti'rn Di\ision, American Fisheries
Liiekofsummerholdinghal)itat in the LBD Society.34pp.
reaeh appeared tolimit trout abundanee, as the Kknnedy, C.J. A.,A\n (L D. .Sthancf,. 19S2. The distri-
bution of salmonids in upland streams in relation to
reaeheontiiined nocpialitvpocketsand notrout depth and gradient. Journal ot Fish Biology20: 579-
were present. Sunnner holding habitat did not 591.
appeiu- to limit trout numbers in the IIBD and l.KHK. M. K. 19S2. The long term elfectiwness of three
IBD reaches because there were 23 pockets Kpes ot stream impro\euient .structures installed in
Montanastreams. Unpublished nuister's thesis, Mon-
with optimal dimensions thatwere notutilized. tanaState Uniyersits', Bozeman.99pp.
Trout tlensitx' in these reaches might have been LkwIS S. I.. 1969. PliNsicalfactorsinfluencingfishpopula-
depressedb\'low^rc^cniitmentorfactoi'ssuch as tion in pools of a trout stream. Transactions of the
wintermortalit\'and fooda\ailabilit\. .American Fisheries Societ\'9S: l-f-19.
Although trout distribution is closeK tied to Halk\ikciu.s()H.\ F.1,9T84.HkH:abkimtaant,siRii.taCbi.liSt\o-lionmfoornm,atiaonnd: Pr.aiCn.-
plnsical habitat in Fall Ri\er, it is clear that bowtrout. U.S. Fish andW'ikllitc Ser\ice Publication
simpK adding boulck'rs to rixers will not auto- RVSOBS-8210.60. 64pp.
maticalK' increase trout pojMiIations. Pockets KoscKN, D—.,AND B. L. Fittanti-: 1986. Fishhabitatstruc-
createdb\' boulders nmst meet depth and sur- tures a selection guide using stream classification.
face area retjuirements before fi.sh will inhabit CPaargleisne16e;ds3.-.17F9if/t/(hJ.trCo;u.tMsitlrleeanmJ.iiAal.iiAtaitwiaiyuparnod\eRm.enFt
them,asshownonFallRixerOtlierstudieshave workshop, Lock Haven. Penns\i\ania.
found that water depth alone is not the major StkuaUT.p.a. 1970.Physicalfactorsinfluencingtroutden-
limiting factor for trout populations (Kennedy sit\' in a small stream. Unpublished master's thesis,
and .Strange 19(S2); watervelocityand available (ColoradoState Uni\ersit\, FortCoUins. 78pp.
VVu.KiiAM. C. M. 1967. Physical mieroliabitatoftrout. Un-
co\eralso influencetroutdensity(Lewis 1969). published masters thesis, (Colorado State Uni\'ersit\\
These environmental requirements as well as Fort(Collins. 42pp.
other limiting factors nmst be unck'r.stood be-
fore boulders are effectixely used for habitat Rccciicd lOJiiur 1^2
impro\-ement. Accepted 1 FchriKin/ 19!-J3
