Abundance of woody debris was manipulated in a small Illinois stream to determine the ... adaptive significance of associations between fish and woody debris ...
Transactions of theAmerican Fisheries Socie•113:716-726, 1984 ¸ Copyrightby the AmericanFisheries Society1984
RelationshipsbetweenWoody Debris and Fish Habitat
in a Small
Warmwater
Stream
PAULL. ANGERMEIER ANDJAMESR. KARR1 Department ofEcology, Etholog3', and Evolution,University oflllinois 606 EastHealeyStreet,Champaign, Illinois61820 Abstract
Abundanceof woodydebriswasmanipulatedin a small Illinois streamto determinethe importanceof this materialto fish.When a streamreachwasdividedalongmidchannel,and debris was added to one side, but removed from the other, fish and benthic invertebrates were
usuallymore abundanton the sidewith woodydebristhan on the clearedside. In further experimentsduring a low-flowyear (1980), debrisremovalwasfollowedby rapid decreases in water depth and occurrenceof benthicorganiclitter, and increasesin current velocityand proportionof sandbottom.Thesechanges werelessapparentin unalteredreachesduring 1980, and in all reachesduring 1981, whichwasa high-flowyear.Artificialdebriswascolonizedby manyinvertebrates, includingchironomids, trichopterans, andephemeropterans. Mostlargefish (age 2+) avoidedreacheswithout debris,whereassomesmallerfish (suchasjohnny darter) preferred them; preferencesfor reach treatmentswere strongerin 1980 than in 1981. The adaptivesignificance of associations betweenfishandwoodydebrisappeared moreclosely related to the advantages of camouflage than thoseof increasedfood availabilityor protectionfrom strongcurrents.Extensiveremovalof woodydebrismaydisruptstructureand functionin small streams,especially low-gradientones.If the biologicalintegrityof streamresources is to be maintaineddespiteagriculturaland urbanuses,lessdisruptivemanagementprotocolswill need to be employed. Received July 27, 1983
AcceptedAugust26, 1984
Woody debrisis a ubiquitouscomponentof forestedstreamsand itsrole in physical,chemical,and biologicalprocesses in streamsiscomplex.For example,largepiecesof woodydebris are important determinantsof channel morphometryin mountainstreams(Heede 1972;
food and habitat resources available to fish,
changesin the abundanceof woodydebrisin streamscan be expectedto inducechangesin fish communities.
Definitive informationon the importanceof woodydebristo streamfishesisscant,especially Keller and Swanson1979), and debrisremoval for warmwatersystems (Marzolf 1978). Historcan causedramatic changesin channel form ically,woodydebrishasbeenactivelyremoved (Beschta1979). Large piecesof woodydebris from manyNorth Americanstreamsbecauseit alsotrap smallerorganicparticles(Naimanand wasbelievedto inhibit logging,navigation,and Sedell 1979), and often form debris accumu- drainage(Sedellet al. 1982). Althoughstream lationsthat regulateexport (Bilbyand Likens managementpracticesstill commonlyinclude 1980) and decomposition (Reice1974) ratesof debrisremoval (McConnell et al. 1980; but see organicmaterial.Woodydebrisprovidesstable Bryant 1983), effectsof removal on fish comsubstratesfor aquaticorganismssuchas bac- munity dynamicsremain poorly understood. teria (Triska et al. 1984),fungi (Shearer1972), However, habitatimprovementproceduresfor and invertebrates(Andersonet al. 1978; Benke salmonidsoften includeinstallationof logs,tree et al. 1984), all of which decomposewood and branches, deflector structures, and small dams, representmajorcomponents of trophicwebsin which mimic effects of naturally occurring stream ecosystems. Becauseall of these func- woodydebris.Suchmanipulations canresultin tionsof woodydebrisinfluencethe qualityof increased growth (Tarzwell 1938), survival (Gard 1961; Hunt 1971), and abundance(Boussu1954;SaundersandSmith1962;Burgess and • Presentaddress:SmithsonianTropical Research Bider 1980) of fish.Althoughlittle experimenInstitute, PostOffice Box 2072, Balboa,Republicof tal work hasbeen done regardingthe imporPanama. tanceof woodydebristo warmwaterfishes,many 716
CONSEQUENCES OFDEBRIS REMOVAL FORFISHHABITAT
717
warmwaterspeciesare apparentlymore abun- throughNovember1979to examinethe influof fishand dantin the presence of woodydebristhanin its enceof woodydebrison abundances absence(Hickman 1975; Paragamian 1980). benthic invertebrates. Thus, understanding the basisfor relationships Fishwere sampledin July, September,and betweenfishand woodydebrismaybe impor- November 1979. Both endsof the studyreach tant for effective management of warmwater were blockedwith 0.48-cm-meshnetsprior to fisheries. sampling.Both sidesof the studyreach were In thisstudywe experimentallymanipulated electric-seined twice, and all stunned fish were woodydebristo determineitsinfluenceon hab- collected withdipnetsor in theblocknets.Fish itat configuration, invertebrateavailability,and wereidentifiedto species, measured to the nearfish distribution in a small warmwater stream est1.0mmtotallength(TL), andreleased.The in Illinois.We arguethat woodydebrisplaysa electricseinewas7 m long and poweredby a complex role in warmwater stream processes gasolinegeneratorwith 20-amperemaximum and suggestthat woodydebrismay be an im- capacity (120 volts,alternatingcurrent).Effecportant environmentalcomponentfor many tivenessof this techniqueis generallyhigh for stream fishes. thepredominantfishtaxainJordanCreek(Larimore 1961; Schlosser1982a). Protracted sam-
Study Stream
Jordan Creek is a second-ordertributary of the Salt Fork of the Vermilion
River
in east
pling(5 seinehauls)of a samplereachin August 1980 indicatedthat 86-89% of individualcyprinids and centrarchids, and of all fish, even-
central Illinois. Channelmorphometry,ripari- tuallycapturedwere capturedin the first two an characteristics, and land usechangesubstan- seinehauls.No newspecies werecapturedafter
tiallyalongits 17-kmcourse;the degreeof dis- the first two hauls. Thus, we believe that two increases from mouth to headwaters seinehaulsadequatelyrepresentthe fishinhab(Schlosserand Karr 1981). In the upstream iting our samplereaches. Benthic invertebratesalso were sampledin reachstudiedhere, gradientis slight(0.7 m/ km), substrates are 85% siltand sand,and pool- July, September,and November1979, when riffle configurations are poorlydeveloped(An- oneSurbersamplewascollectedfrom eachside germeier1983).The channelis3-5 m wideand of the studyreach.Samples werepreservedimbordered on both sidesby 5-10 m of woody mediatelyin 10% formalin,and subsequently vegetationincludingmultiflora rose Rosamul- dividedinto eight subsamples with a devicesimtiflora,OsageorangeMaclurapomifera,willows ilar to that of Waters (1969). One subsample Salixspp.,hackberryCeltisoccidentalis, andblack wasthensortedto familybeneatha microscope. cherryPrunusserotina, beyondwhichis intenMultiple-Reach Experiments siverow-cropagriculture(corn and soybeans). The studyreachhasnot been channelizedfor More experimentswere conductedin 1980 at least40 years.JordanCreek supportsa di- and 1981 to substantiate and extend results from versefish fauna (25-30 species) predominated the split-stream experiment.All woodydebris by cyprinidsand centrarchidsLarimore 1961; was removed from 10 35-m-long "altered" Angermeier 1983). reachesof JordanCreekin late April 1980;any newdebriswasclearedweeklythereafter.Consecutivereacheswereseparated by lessthan 20 Methods turbance
Split-Stream Experiment In June 1979, a 30.5-m-longreachof Jordan Creek with symmetricalchannelshapewasdividedalongmidchannelby 0.64-cm-meshhardware cloth supportedby steelfence posts.All woodydebriswasremovedfrom one("cleared") sideanda continuous arrayof stumps,logs,and brancheswas securedon the other ("debris")
m. Three additional 35-m reaches, all within 200 m downstream of the last altered reach, were
used
as unaltered
"control"
reaches.
Depth, current, substrate,and cover did not differ notably between altered and control reacheswhenexperiments began. Artificial-debris structures, made of pine boards, were installed in four altered reaches
to providecoverfor fishand colonizable sursidealong the bank and along the hardware facesfor invertebrates(Fig. 1). Two reaches cloth divider. This design was maintained receivedeight structuresand two other reaches
718
ANGERMEIER AND KARR Invertebrates
WATER
I
SIDE
m
"1
U
•""STEEL RODS "•
•
were
collected
from
artificial-
debris structuresin July and November 1981 ---• SURFACE to determinethe predominanttaxa colonizing
9Ocm
wood
surfaces.
Two
workers
removed
struc-
tures from the stream, and examined them for
invertebrates;all those observedwere preserved immediately,then processedas describedfor the split-streamexperiment.De-
•
tailed
evaluations
of
habitat
features
were
conductedin June, August, October, and December1980,April,June,August,andOctober 1981, and June 1982. Evaluation techniques were modified from Gorman and Karr (1978) andSchlosser (1982a).Evaluations usuallywere completedfor all samplereacheson the same day.
\
\ \ /
\
Transects
/
\ \
\
/
were laid across the stream at 3-m
intervalsthrough each samplereach. Depth, current, substrate, and cover were measured 10
/
+
+
x+/
+
+
+
+
+
+
+
cm from the startingbank and at 1-m intervals thereafteralongeachtransect.Depth wasmeasured to the nearest Surface
current
1.0 cm with a meter
was estimated
from
the
stick. dis-
tance that water was driven up the upstream edge of a meter stick held vertically in the stream.This verticaldisplacementof water is exponentially related to current velocity (Schlosser1982a). Substratecompositionwas received sixteen structures; thus, artificial-decategorizedas silt, sand,gravel, or cobble;the brisreachescontainedmorewoodydebristhan presenceor absenceof benthic organic litter control reaches. Structures were held 1 to 12 (partially decomposedleaves,twigs, etc.) also cm below the water surfaceby steelrods (Fig. was noted. Featuressuch as woody debris or 1), and were usuallynot in contactwith the undercutbanksthat might provide cover for streambottom. Pairsof structureswere spaced fish also were recorded. This habitat evaluation evenlythroughsamplereaches;membersof a techniqueyielded50 to 80 measurementlocaof pairwere0.5-1.5 m apart.Thisdesignwasused tionsfor eachsamplereach. Measurements fromJune 1980 throughApril 1981. reacheswithin eachtreatmentwere pooledto In May 1981,sevenadditionalsamplereaches constructfrequencydistributions,which were (three control and four altered) were estab- usedto characterizechannelmorphometryand lished downstream from the 1980 reaches, which to identify changesin habitat structureresultretained their original treatment assignment. ing from the manipulations. FIGUREl.--Schematicdrawingof an artificial-debris structure(above),' 8 to 16 structures wereinstalledin a samplereach(below).
Two of the new altered reaches received arti-
ficial structures, while the other two remained
Data Analysis
as no-debris reaches. All artificial-debris reaches in 1981 received twelve structures. This ex-
Frequencydistributions of depth,current,and
perimentendedin October1981.
substrate measurements
from
altered
and con-
trol siteswere comparedwith the G-testfor independence withWilliams'correction(Sokaland Sampling Rohlf 1981). Depthswere aggregatedin four Fishwere sampledwith the electricseinein categories:1-9 cm, 10-19 cm, 20-29 cm, and for current June,August,andOctober1980,andin April, > 30 cm.There weretwocategories June,August,and October 1981. Two to five velocity:0-0.5 m/secondand >0.5 m/second. reacheswere sampledon a given day, and all Individualfishof the 10 mostcommonspecies reacheswere sampledwithin 5 days. were assignedto one of three ageclasses on the
CONSEQUENCES OFDEBRIS REMOVAL FORFISHHABITAT
719
T^BI•. 1.--Fish abundances in debris andcleared sides ofJordanCreekduringthesplit-stream experiment of 1979. July Measure
Debris
Number of species
12
Total individuals
46
Age-2+ individuals Centrarchidae
individuals
Cyprinidaeindividuals
September Cleared
8
Debris
November
Cleared
Debris
Cleared
10
4
8
24
73
4
126
38
8
28
2
7
8
1
12
1
2
0
24
13
54
1
115
314
basisof total length:age-O,age-1,age-2+. Age-0
fishwereyoungof year;age-1fishhadsurvived onewinter;age-2+ fishincludedall olderfish. Age classes were determinedfrom length-frequencydistributions of fishpreviously captured in Jordan Creek. Analysesof fishdistributions amongreach treatmentswere conductedseparatelyfor eachsizeclass.If a sizeclassdid not
8 319
26
55), and made up 58% of the age-2+ fish on the cleared side (Table 1). Differences
in invertebrate
abundance
be-
tween the two sides paralleled those of fish abundance.
Invertebrates
were 3.9 times more
abundanton the debrisside(4,366 versus1,118/
m2)inJulyand 5.7 timesmoreabundanton the debris side (2,194 versus387/m 2) in Septemoccur in at least one third of the reaches samber. Each major taxon (family) exhibitedthis pled during a samplingperiod, that period was samepattern. Although mostinvertebratetaxa omittedfrom the analysis of that sizeclass.Data were also more abundant on the debris side in from remainingsamplingperiodswere pooled November, chironomid larvae were 3.1 times becauseof the smallnumberof samplereaches. more abundanton the clearedside(17,505 verFish abundancesamongtreatmentswere com- sus5,577/m2). If this taxon is excluded, inverparedwith a Kruskal-Wallisnonparametricone- tebrates were 2.0 times more abundant on the wayanalysisof variance(Conover1980).When debrisside(6,842 versus3,440/m " in Novemthisanalysis indicatedsignificant(P < 0.05) dif- ber. Fishpreferencesfor the debrisor clearedsides ferencesamongtreatments,pair-wisecomparisonsof treatmentswere performed according seemed to be based on food or cover associated to proceduresoutlined in Conover (1980) to with woodydebris.A habitatevaluationin Nodetermine which treatments differed. vember 1979 indicatedthat depth profiles of the two sideswere not significantlydifferent Results
(G-test; P • 0.05). The cleared side featured
Split-Stream Experiment
slightlygreatercurrentvelocityandproportion
Striking differencesin fish abundancewere
of sand bottom, but these differences were not
judged to be great enoughto accountfor difthe 1979 studyreach.More species,more in- ferencesin fish abundance.The experiments dividuals,andmore largefishwerecapturedon conductedin 1980 and 1981 were designedto the debrissidethan on the clearedsidein July more rigorouslytestthis hypothesis. and September(Table 1). All speciesfoundon Multiple-Reach Experiments the cleared side also occurred on the debris side. observed between
Differences
debris and cleared
sides of
in fish abundance between the two
Effects on Habitat
Features
sideswereespecially markedin September after The most striking indirect effectsof debris an extendedperiod (approximately1 week)of removal were changesin the stream'sdepth elevateddischarge,which scouredaway most profile,whichwasnearlyidenticalfor control particulateorganicmaterialonthe clearedside, and altered reacheswhen experimentsbegan but not on the debrisside.Mostfishspecieswere in June 1980 (? y 0.9; Fig. 2). Water levelsdelessabundantin the studyreachin November clinedsteadilythroughthe summer,andby Octhan they had been in July and September. tober 1980, both groupsof reacheswere shalBluntnoseminnowwasthe onlyspecies that ex- lower than they had been in June (P • 0.001). hibited a preferencefor one side;they were However, altered reaches had become much more abundant on the cleared side (249 versus shallowerthan control reaches(P • 0.001),
720
ANGERMEIERAND KARR CONTROL ALTERED JUN
80
2O
JUN
81
OCT
81
JUN
82
20
O
0 OCT
80
z 4o
40
20
0 o
•"
JUN81
40
40
20
o
1-
9
lO-2O-23o
1-
0
lO-2O-2'3o
1-
19 •
10- 20-
>30
1- 10- 20- •30 9
DEPTH
19 29
(½m)
FIgUR•2.--Depthprofiles ofcontrol andalteredreaches ofJordanCreekinJune1980, 1981,and 1982,andOctober 1980 and 1981. Eachprofilecomprises 128-716 measurements.
whichsuggests that theyhadsiltedin morerap- paralleledchangesin streamdepth.The proin the no-curidly than control reaches.In June 1981, the portion of current measurements depthprofile of control reacheswassimilarto rent (-• 0.5 m/second)categorywassimilar(P y that observedin June 1980 (P > 0.2), but al- 0.05) in control and altered reachesin June tered reacheswere significantly shallowerthan 1980 (77 versus83%), but by October 1980 controlreaches(P < 0.001), and shallowerthan currentvelocities weresignificantly greater(P • theywerein June 1980 (P < 0.001). Thus, de- 0.05) in altered reaches.Nevertheless,current positionthat occurredduring the summerof wasslight,and only 6% of the current mea1980 apparentlywas scouredfrom control surementsin alteredreacheswere greaterthan reachesduring the spring of 1981, but re- 0.5 m/second. Current velocities remained mainedin alteredreaches.This experimentwas higherin alteredreachesthan in controlreachrepeatedin 1981with the samecontrolreaches es(P < 0.001)through June 1981. Current veplusthree additionalcontrolreaches,and four locitieswere greaterin controlreachesthan in newlyalteredreaches.In June 1981,however, alteredreaches(P • 0.001) when the 1981 exnew alteredreacheswere slightlydeeperthan perimentsbegan,but byJune 1982 currentwas controlreaches(P < 0.05; Fig. 2). Again, water similarin the two groupsof reaches(P y 0.50). depthdeclinedmarkedlyfromJuneto October Similarly,proportionsof silt, sand,and gravel in both control (P • 0.001) and altered (P • in controlandalteredreacheswerecomparable 0.01) reaches,but by October altered reaches in June 1980 (P y 0.05), but in June 1981 alwereno longerdeeperthancontrolreaches(P > tered reachescontainedgreaterproportionsof 0.05). In June 1982,whichfolloweda winterof sandbottom than control reaches(41 versus exceptionallyhigh precipitation(46% aboveav- 25%;P • 0.001). This pattern wasrepeatedin erage),alteredreacheswereagaindeeperthan the experimentsbetweenJune 1981 andJune control reaches(P • 0.01).
Changesin currentvelocityandbottomtype
1982.
The occurrenceof organiclitter on the stream
CONSEQUENCESOF DEBRIS REMOVAL FOR FISH HABITAT
w
ß --•
Control
o---o • •
Altered Altered
(1980) (1981)
50
amongtreatmentsfor at leastoneageclass(Table 2). In all thesecasesexceptage-2+ grass pickerel,abundances werenot significantly different
_c
o o o
30•'1
'
10 --"O-•//
•'•'
721
between
control
and
artificial-debris
reaches.All threecentrarchidspecies, but only one cyprinid species,showedpreferencefor control reachesover no-debrisreaches;age-1 and age-2+ bluegillsalso tended to be more abundant in control reaches(P < 0.07; Table
2). Age-2+ hornyheadchubs,longearsunfish, and rock basspreferred control reachesover no-debris reaches, but smaller fish of these
species did not. Only age-1johnnydartersand age-0grasspickerelspreferred no-debrisreachesover controlreaches.Preferencesby fishfor NION TH treatment typeswere more obviousin August FIGURE 3.--Seas0nal changes in occurrence oforganic lit- and October1980 than in Augustand October teronthestream bottom. Eachplotted pointrepresents a 1981, and appearedto be related to differences mean _ 1 standard error. in flow conditionsbetweenyears.Only two speciesexhibiting treatment preferenceswere commonin all four of thesesamplingperiods, and neither speciesexhibiteda preferencein bottomalsochangedmarkedlysubsequent to 1981 (Table 3). Fishwere alsolessabundantin Ju Au Oc De 1980
AP 3u Au ¸c 1981
3u 1982
debris removal. Bottom litter was observed at
1981 than in 1980.
similarfrequenciesin controlandalteredreachWoodydebrisnotonlyinfluencedfishhabitat esduringJune 1980 (Fig. 3), but declinedmore characteristics, but alsoprovidedsubstratefor rapidlythroughthe summerin alteredreaches manyaquaticinvertebrates,the major food rethan in controlreaches.In April 1981, litter sourceof mostJordan Creek fishes.Predomi-
frequencyin controlreacheswasagainsimilar to that in June 1980, but litter frequencyin alteredreaches remainedmuchreduced(Fig. 3). In June 1981,litter frequencyin newlyal-
nantinvertebratetaxaincludeddipteranlarvae, andtrichopteran andephemeropteran nymphs (Table4), all of whichare commonlyeatenby Jordan Creek fishes(Angermeier 1982). Altered reaches was intermediate between old althoughchangesin invertebrateabundanceand teredreachesand controlreaches,but by Oc- compositionon woodydebris probablyoctober 1981 litter frequencywassimilarin all curred between July and November 1981, alteredreaches(Fig. 3). The greaterdeclineof patchydistributions of invertebrates amongarlitter frequencyin controlreachesduringthe tificial structurespreventedus from clearly summerof 1981 comparedto the summerof identifyingchangesin overall patterns.How1980 wasassociated with severalperiodsof el- ever, comparisonsbetweenartificial debris and evated dischargethat occurred in 1981. Pre- benthic substrates in November 1981 indicate cipitationfromJunethroughOctober1981was that althoughtotal invertebratedensitieson the 45% aboveaverage.Similarly,high discharge twosubstrates weresimilar(3,867 versus4,076/ during winter and springof 1982 resultedin m•),trichopteran andephemeropteran nymphs low litter frequencies for all reachesin June were 17.5 and 3.7 times more abundant on wood 1982(Fig.3). In summary,woodydebrisinflu- surfaces. Becauseof their relativelylargesize, encesmanyhabitatfeaturesthatarepotentially thesetaxamaybe preferredpreyitemsfor inimportant to streamfishes,and the extent and sectivorous fish. durationof its effectsare closelytied to flow Discussion
regime. Fish and Invertebrate
Abundance
Woodydebrisplaysa multidimensional role in the structureandfunctionof streamecosys-
Six of the ten analyzedfishspeciesexhibited tems.Studiesconductedin Vermont (Zimmersignificant differences in their distributions man et al. 1967) and Oregon (Beschta1979;
722
ANGERMEIER AND KARR
TABLE2.--Fish species andageclasses analyzed for theirdistributions amongexperimental reaches ofJordanCreekin 1980 and 1981. Forfishgroupsexhibiting significant (P < 0.05) differences in abundance amongtreatments, descending orderoffish abundance alsoisgiven. Samplingperiodsentering analysis 1980
Species Blackstripetopminnow Fundulusnotatus Grasspickerel Esoxamericanus Bluntnoseminnow Pimephalesnotatus
debris(A)
0 1 0 1 2+ 0 1 2+ 0
0 (0-14) 1.5(0-9) 1 (0-9) 1 (0-3) 1 (0-10) 1 (0-533) 4.5(0-424) 2 (o-33) 3 (0-148)
1 2 0 1 0 1 2 1 2
1.5(0-76)
X X
X
X X X X
X
X X
X
X
X
X
x
x
x
Notropis ch•ysocephalus1
X
X
X
X
X
2+ Creek chub 0 Semotilusatromaculatus 1 2+ Hornyheadchub 0 Nocomisbiguttatus 1 2+
X
X X X X X X X
X
X
X X X X
X
X
X
X
X
X X X X
0
Lepomismacrochirus
Longearsunfish Lepomismegalotis Rock bass
Arabloplitesrupestris
Johnnydarter
1 2+ 0 1 2+ 0 1 2+
1
Etheostomanigrum
2+
X
X X X
X X X X X X
X X X
X
X
X
X
2.5(0-32)
X
X
X X X
X X X
2 0 0 0 8 1 1
X X X
X X
X
X X
X
X
X X
X
X X
X
X X
X X
X X X
X X X
X
X
X
X
Significant
KruskalWallis tests
Artificial
Age Jun Aug Oct Apr Jun Aug Oct Control (C)
X X x X
Bluegill
Alteredreaches
1981
X X x X
Stripedshiner
Median numberof fishper reach(range)
X
(0-23) (0-0) (0-2) (0-6) (0-81) (0-57) (0-15)
(0-23) (0-11) (0-7) (0-3) (0-3) (0-89) (0-137) (0-21) (0-224)
No debris(N) (P < 0.05)
2 (0-15) 2 (0-24) 1.5(0-11) 0 (0-1) 0 (0-2) 3 (0-44) 3 (0-236) 1 (o-3o) 5 (0-93)
N > C, A C, A > N C> A,N
1 (0-96)
1 (0-19) 0 (0-19) 0 (0-18) 2 (0-6) 1 (0-9) 0 (0-5) 1 (0-15) 0 (0-4) 17.5(0-144) 18 (0-197) 4 (0-242) 3 (0-162) 5 (0-29) 0 (0-16)
3 (0-67)
0.5(0-4)
0 (0-2)
2 (0-5) 2 (0-11) 2 (1-3) 3.5(0-19) 2 (0-7) 1 (0-3) 1 (0-2) 3 (0-9)
0 (0-11) 2 (0-12) 0.5(0-3) 1 (0-8) 0.5(0-4) 0.5(0-4) 0 (0-2) 2.5(0-14)
1 (0-4) 0 (0-7) 0.5(0-2) 1 (0-10) 0 (0-5) 0 (0-5) 0 (0-1) 1 (0-13)
1 (0-9)
1 (0-6)
2.5(1-12)
0 (0-6)
2 (0-2)
2 (0-11)
A,C > N
C> N
C> N
A, C > N
N > A, C
Keller and Swanson1979) indicate that large tebrates,and that productionof invertebrates piecesof debrisoften alter width and depth on wood is particularlyimportant in habitats dimensionsof streamchannels,and thuschange with unstable bottom substrates. Our results,
habitatsuitabilityfor residentfishes.Studiesin
from a small Illinois stream, are consistent with
rivers of Illinois (Nilsen and Larimore 1973)
thesefindings,and indicatethat manyfishare and Georgia(Benkeet al. 1984) indicatethat sensitiveto effectsof woodydebrison availwoodydebrisisan importanthabitatfor inver- abilityof their habitatand foodresources.
TABLE3.--Between-year comparisons offish abundances in experimental reaches ofJordanCreek.
Median number offishperreach (range) Artificial
Significant
Kruskal-Wallis tests
Species
Age
Sampledates
Control(C)
debris(A)
No debris(N)
(P < 0.05)
Hornyheadchub Hornyheadchub Johnnydarter
2+ 2+ 1
Aug-Oct 1980 Aug-Oct 1981 Aug-Oct 1980
1 (1-9) 0.5 (0-15) 1 (0-3)
11.5(1-21) 1 (0-19) 1 (0-6)
0 (0-16) 0 (0-2) 3.5 (1-12)
A, C > N
Johnnydarter
1
Aug-Oct 1981
0
(0-9)
1 (0-5)
1 (1-6)
N > A, C
CONSEQUENCES OFDEBRIS REMOVAL FORFISHHABITAT
723
T ^BLE4.--Numericaldensities (perm2)ofmajorinvertebrate taxaonartificial-debris structures inJordanCreekin 1981.
Ephemerop•
Month
Total
Diptera
Trichoptera
Isopoda
tera
Copepoda Coleoptera
3,302
1,079
432
524
469
79
283
453
184
481
501
167
12
100
3,867
1,553
999
173
341
392
348
738
266
287
154
56
203
90
July (N = 4) Mean SE
November (N = 3) Mean SE
Woody debrisinfluencesdepth, current, and material,which is scouredawaylessoften than substratecharacteristics through its interaction in high-gradientstreams.The tendencyfor fine with hydraulic processes.The importance of particlesto distributeevenlyresultsin low-grathese habitat features to distribution and abundance of stream fishes is well documented. Fish
dient streamshavingrelativelyfiat, homogeneousbottoms.Flowconstrictions due to woody speciessegregatealong gradients of depth debriscan mitigatethis tendencyby promoting (Mendelson 1975; Smart and Gee 1979; An- particle-sortingand scour,thereby increasing germeierandKarr 1983),current(Gibbonsand depth, current, and substratediversity. WoodydebrisenhancesinvertebrateproducGee 1972;Mendelson1975),and bottomtype for (Finger 1982; Paine et al. 1982). At the com- tion directlyby providingsuitablesubstrates munity level, thesepatternsare manifestedin colonization,and indirectlyby promoting acpositivecorrelationsbetweenhabitat diversity cumulation of detritus, which often is correlatand fish speciesdiversity(Gorman and Karr ed with invertebrate abundanceand diversity 1978;Schlosser1982a).Large piecesof woody (Egglishaw1969; Malmquistet al. 1978). Redebriscanconstrictthe streamchannel,thereby movalof woodydebrisfromJordanCreekwas with relativelylow invertebratedenincreasing theerosionpotentialof flowingwater, associated and enhancingpool formation. Poolstypically sities on benthic substrates in 1979 and with fill with fine particulatesduring low-flowpe- low frequenciesof bottom litter in 1980 and riods and are scouredduring highly elevated 1981. Although invertebratedensitiesin Jorflows(Nunnallyand Keller 1979). Pool-filling dan Creek may sometimesbuild up in the abis accentuatedin the presenceof unstablesub- senceof woodydebris(for example,clearedside, stratessuchas silt and sandthrough the for- November 1981), they are unlikely to persist mationof tiny migratingdunes.Scouringflows, beyondthe next high-flowperiod. In higherwhichoccurmostfrequentlyduringwinterand gradient streams,coarser substratessuch as springinJordanCreek,are necessary to remove graveland cobblemay retain detritus(Rabeni thesefine particlesfrom pools.In the absence and Minshall 1977) and provide invertebrates of channelconstrictions providedby woodyde- with refugia from predatorsor scouringcurbris,poolsof alteredreacheswerenotre-scoured rents. Thus, the role of woodydebrisin proin the winter of 1980. Debris removal had less duction of stream invertebratesprobablydeimpact on channelmorphologyin summerand pends on numerous variables, including autumn 1981, whenrecurrenthigh flowsmin- hydrology,substratecomposition,and detritus imizedsiltationof pools.Thus, the importance availability. The adaptivesignificance of fishassociations of woodydebristo pool formation and maintenancedependson flow conditionsand sub- with woodydebrismaybe relatedto protection strate composition.This suggests that woody from current, food (invertebrate)availability, debrismay be more importantin determining or camouflage'from predators or prey. Furchannel morphometryof low-gradient,fine- thermore, the relative magnitudeof theseadsubstratestreamsthan of high-gradient,coarse- vantagesprobablyvaries with environmental substratestreamsfor the followingreason.Low- conditions.For example,protectionfrom curgradient streamstypicallycontain more fine rent wasprobablyof little importanceduring
724
ANGERMEIERAND KARR
our sampling periodsbecause currentvelocities weregenerallylow.Indeed,fishassociations with debriswere lessapparent in 1981, a year of relativelystrongcurrent, than in 1980, a lowflowyear. Protectionfrom currentmaybe more importantduring late winter and early spring, whenstrongcurrentsare commonbut fishmetabolismis slow.The importanceto fishof food organismson woodydebrisalsomay vary seasonally,andisprobablygreatestin summerwhen the abundanceof other organiclitter is relativelylow. The role of woodydebrisin providing fish with coverfrom predatorsshouldbe greatestduringlow-flowperiods,whenhabitat availabilityis reduced,and poolsbecomerelativelyaccessible to bird and mammalpredators. Our experimentaldesigndid not permit us to clearly separatethe significance of differ~ ences in food versus habitat
characteristics
for
fish distributions.Most fish exhibiting treat~ ment preferencesdid not distinguishbetween control and artificial-debrisreaches.This pattern suggests that the greaterabundance of cover in the form of artificial structurescompen-
ruptivefor fishin low-gradient systems, yetthese systemsare most likely to be cleared or channelizedbecauseof their suitabilityfor agriculture. Debrisremovalmaycausereducedhabitat heterogeneityand lossof deeppools,whichare preferredhabitatsfor mostgamefish.The longterm trend in highly modified streamsis for lowerspecies richness(Bulkleyet al. 1976;Griswold et al. 1978) and predominance by small, generalizedspecies(Karr and Schlosser1978; Schlosser1982b), which may exhibit dramatic fluctuations
in abundance
between
seasons and
years(Toth et al. 1982). Becausethe goalsof streammanagersshouldinclude maintenance of biotic integrity(Karr and Dudley 1981), alternativesto traditionalmanagementprotocols often need to be employed.More selective methodsof debrisremoval,suchas thosesuggestedby McConnellet al. (1980; 1983), serve agriculturalandurban usesof streams,but minimizedisruptionof biologicalproperties.Further developmentand implementationof such protocolswill be essentialto reversingrecent declinesin the qualityof streamresources.
satedfor the shallowerdepthsrelativeto control reaches. Smallmouth
bass in Iowa streams are Acknowledgments reported to use shallowareas when cover is This paperwasdevelopedfrom part of a Docpresent,but avoidthem otherwise(Paragamian toral dissertationby P.L.A. We thank the nu1980).Alternatively,with respectto food avail- merous laboratory and field assistantswho ability,reducedabundances of bottomlitter in helpedwith thiswork. G. O. Batzli,D. A. Braatz, artificial-debris reaches may have been com- R. W. Larimore, M. R. Lynch, S. R. Reice,M. pensatedby increasedavailabilityof woodsur- F. Willson, and an anonymousreviewer made face. Althoughavailabilityof food and cover manyusefulcommentsand criticismson earlier areboth importantto fish,wesuspect that many versions.Financialsupportwasprovidedby two fishin smallstreamsare more responsive to cov- Universityof Illinois GraduateFellowshipsto
er, whether it be in the form of deep water,
P.L.A.
and two United
States Environmental
undercutbanks,or submergedstructuressuch ProtectionAgencygrants(R806391 and CRaslargerocksor woodydebris.Deepwaterpro- 810745) to J.R.K. videsprotectionfrom bird and mammalpredators (Kramer et al. 1983; Power 1984); subReferences mergedstructures provideprotectionfrom fish N.H., J. R. SEDELL,L. M. ROBERTS, AND predators(Savinoand Stein 1982; Werner et ANDERSON, F. J. TRISKA.1978. The role of aquaticinveral. 1983), aswell asbirds and mammals.Large tebratesin processingof wood debris in coniffishshouldbe especially sensitive to declinesin erous forest streams. American Midland Naturalist 100:64-82. water depthor coveravailabilitybecausethey are especially attractivepreyitemsfor bird and ANGERMEIER, P. L. 1982. Resourceseasonalityand fish diets in an Illinois stream. Environmental Bimammalpredators.In our study,fish that exologyof Fishes7:251-264. hibited no responseto debris manipulations P. L. 1983. Effectsof depthand cover (bluntnoseminnow,stripedshiner,creekchub, ANGERMEIER, on fish distributions in selected Illinois streams. blackstripetopminnow)were predominantly Doctoral dissertation. University of Illinois, smallindividuals(
