We quantified changes that occurred in proportional stock density (PSD), relative stock density ... was 9l-cm diameter, about 3.66-m length, and 25-cm bar web.
Changesin PopulationIndicesof a DiminishingBurbot Populationin the KootenaiRiver,ldaho,USA and BritishColumbia,Ganada VaughnL. Paragamian, JoshuaMcCormick, andCorieLaude ldaho Depaftment of Fish and Game 2885 W. Kathleen Avenue Coeurd'Alene. ldaho 83815 USA E-mail: vparagamian @idfg.idaho.gov
ABSTRACT We quantified changesthat occurredin proportionalstock density (PSD), relative stockdensity(RSD), relativeweight (l/r), and catchper unit effort (CPUE) of a diminishing burbot (Lota lota) populationin the Kootenai River, Idaho, USA and British Columbia,Canada.Samplingwith hoop netstook placeperiodicallyover 46 years. After completionof the Libby Dam in Montanain 1913,the burbotpopulationdeclined. The from 459 mm total length(TL) in 1957-1958 meanlengthof burbotincreased to 615 mm TL in 2002-2004.PSD did not increaseappreciably,rising only from 92 in 1957-1958to 98 by 2002-2004;but, RSD (preferred)increasedfrom 17 in the early samplesto 86 by the last samplinginterval,which suggested recruitmentfailures. All otherRSD values increasedalthoughnot incrementally,which also suggestedrecruitmentfailures. As the burbotpopulationdeclined,Wy increasedfrom 75 to 98 from the early 1980sto early 2000s,suggestingthat Wr may have a length bias in burbot. We found RSD was more usefulwhen combinedwith CPUE, which also decreasedfrom I 993 through 2004. Togethertheseindicesprovide good indicationof burbotpopulationchange. Hoop nets are the most important sampling gear for burbot but are length selective;thus, PSD (quality)alonewill not detectrecruitmentissues. Managersand researchers of other burbotpopulationsshould designsamplingprogramsthat considerthe value in measuringRSD and CPUE when hoop netsare used. INTRODUCTION In Idaho (ID), the burbot (Lota lota) is endemiconly to the Kootenai River (spelledKootenayfor Canadianwaters)(Simpsonand Wallace 1982). Burbot in the KootenaiRiver,ID, USA and KootenayLake,British Columbia(BC), Canadawere once very abundantand may haveprovidedone of the most robustfisheriesin North America (Paragamian and Hoyle 2005). However,the burbotin the KootenaiRiver is now expectedto reachextirpationwithin the next l0 yearsunlesseffectiveremedialmeasures aretaken(Paragamian et al. 2008);this estimatecameaboutthroughthe analysesof 12 yearsof capture-recapture data ( 1993-2004). Burbot has beenthe subjectof periodic study by the Idaho Departmentof Fish and Gamebeginningin winter 1957-1958(PaulJeppson,PanhandleRegionArchives, IDFG) and continuingin 1979-1982(Partridge1983)and 1993-2004(Paragamianet al. 2008). Betweenthe 1957-1958samplingand the 1979collections,the Libby Dam was completedon the Kootenai River near Jennings,Montana. After the dam became operationalin 1973,the burbot fishery in Idaho rapidly declinedand was closedin 1992 (Paragamianet al. 2000). Concomitantwith the collapsein Idaho was the collapseof the burbot fishery in KootenayLake, BC (Andrusak 1976, Andrusakand Crowley 1978, Paragamian et al. 2000,RedfishConsulting1998). In additionto the Libby Dam, other ecosystemchangesoccurredsuch as dyking, disconnectionfrom the floodplain,poor forestpractices,and mining; and, the Libby Dam has createda nutrient sink by impoundingLake Koocanusa(Paragtimianet aL.2000,Anders et aL.2002). Our objectivewas to examinepopulationindicesthat would identiff changesin burbot stock structurethat had occurredin the Kootenai River. We examinedthe 553 Journalof FreshwaterEcology,Volume23, Number4 - December2008
proportionalstock density(PSD), relative stock density(RSD), and relative weight (Wr) of burbot in the Kootenai River, ID and BC, as it transitionedfrom a robust fishery in the 1960sthrough 2004 when it approachedextirpation,and evaluatedpreviously published catchper unit effort (CPUE) data as an index for populationassessment. METHODS AND MATERIALS Study area The Kootenai River is one of the largesttributariesto the Columbia River (Bonde and Bush 1975). Originating in KootenayNational Park, BC, the river dischargessouth into Montana,where Libby Dam impoundswater into Canadaand forms Lake Koocanusa. From Libby Dam, the river dischargeswest and then northwestinto Idaho and then north into BC and KootenayLake. The river eventuallyjoins the Columbia River near Castlegar,BC. The burbot is found throughoutthe river and lake; our study areawas from aboutKootenav Lake. BC to BonnersFerrv. ID. Burbol sampling There were three principal study intervals,with most collectionsof burbot taking placefrom OctoberthroughApril of eachinterval. All study periodsusedhoop netsto captureburbot, and only data from hoop net-capturedburbot were used in our PSD, RSD, and Wr analyses.In the first interval,Jeppson(pers.comm., [DFG, retired) sampled burbotfrom earlyNovember1957throughJanuaryof 1958andJuly 1958. Hoop net size was 9l-cm diameter,about3.66-mlength,and 25-cmbar web. Netswere setat the mouth of BoundaryCreek at the ID/BC border and Deep Creek. Only burbot total lengths(TL) were measured.In the secondinterval,Partridge(1983)collectedburbot from December 1979throughJuly 1982,usinghoop netswith a maximumdiameterof 9l cm, 3.66-m length,and 25-cm bar web. He fished BoundaryCreek and Deep Creek and the Kootenai River in the vicinity of Copeland,ID. Capturesof burbot from early March 1993through March 2004were with baitedhoop nets. The 9l-cm diameterby 3.66-mlong netswere usedbriefly from 1993through1995but were replacedwith smallerdiameterhoopsof 6l -cm becausethereis no differencein burbotvulnerabilityandthe smallerdiameter hoopswere lighterand easierto handle(Bernardet al. 1991). Nets were deployedin deepareasof the Kootenai River betweenAmbush Rock near BonnersFerry, ID, and Nicks Island,BC. Burbot were alsosampledat threetributaries- Deep Creekand BoundaryCreek,ID, and the Goat River, BC. Lengthfrequencies Length frequencieswere pooled into three samplingyear class intervalswith one exception.Length frequenciesfrom 1957-1958were the summaryof the data for fish that Jeppson(unpublished)capturedfrom November1957to January1958and July 1958. We also used length frequenciesfor burbot capturedin hoop nets from 1979-1982 (Partridge1983),1993-1995,1996-1998, (Paragamian 1999-2001,and2002-2004 et al. 2008). We pooled our sampleinto three-yearintervalsfrom 1993to 2004 to match the three studyyearsof Partridge(1983) and becausewe neededsamplesizesto correspondto Paragamianet al. (2008) so that the sampleswere large enoughto computedensityplots. Also, in the later years of sampling,burbot numberswere in rapid decline and captures in singleseasonswere low. While this violatesthe strict assumptionof independence on the results betweenyearsor between3-yr periods,it had little consequence (Paragamianet al. 2008). Only 30 of the 333 lengthswere for recaptures.In the previous study, a regressionanalysisand analysisof variance(ANOVA) were run using only "newly found" fish, both for the annualand 3-yr periods;the resultswere very similar to thoseobtainedwhen the data also included recaptures.In theory, excluding recaptures for the sakeof independencemay also bias results. Becausewe were interestedin 554
documentingchangesin mean body length within the populationover time, a sample that included between-yearrecapturesshould be random and representativeas long as all fish have the sameprobability of being capturedin a given year (Paragamianet al. resultsand conclusionsare essentiallythe samewith or without 2008). Regardless, recaptures.Burbot were not sacrificedfor age analysisof otoliths becausethe populationwas in decline. We used ANOVA and Tukey's studentizedtest to comparedifferencesbetween meansof total lengthof burbotfor all datasets(a:0.05; SYSTAT version I I, SYSTAT 2004). Proportional stock densities PSD is the percentageof the sampledstockthat is of quality lengthor longer. We usedvaluesfor stocklength(S) and qualitylength(Q) of 200 mm TL and 380 mm TL, respectively, as calculatedby Fisheret al. (1996). PSD was furtherdefinedin termsof RSD usingthe preferred(P), memorable(M), and trophy (T) lengthsrecommended by (198a)andcalculated for burbotby Fisheret al. (1996),whereP:530 mm, M Gablehouse : 670 mm, andT : 820 mm TL. We calculatedPSDsandtraditionalRSDs(Willis et al. 1993)for both of the unpublisheddatasetsfrom 1957-1958(JeppsonIDFG) aswell as for the datafrom 1979-1982and 1993-2004(Partridge1983,Paragamian andLaude2008). We calculated95oh confidenceintervalsfor PSDsand RSDs with the method of Gustafson ( 1988). Relative weight We calculatedrelativeweight (ITy;Wege and Anderson1978)of burbotbasedon burbotstandardweight(25; Fisheret al. 1996),wherelogrcWs: -4.868+ 2.898log1sTL. Relativeweightswere calculatedfor the 1979-1982dataset and the 1993-2004dataset. We usedANOVA and Tukey's studentizedtest to comparedifferencesbetween meansfor I(y of burbotfor the 1979-1982 and 1993-2004 datasets(a:0.05; SYSTAT version11,SYSTAT 2004). To determineif pre-spawnand post-spawnburbotweights could be combined,we useda two-tailedt-testto comparethe meansof 14, for prespawnand post-spawnburbot. The mean Wy's for pre-spawnand post-spawnburbot includedpooleddatafrom the 1979-1982and 1993-2004datasets(cr: 0.05;SYSTAT version11,SYSTAT 2004). Fish collectedbetween I Octoberand l0 Februarywere consideredpre-spawn,and fish collectedbetweenl l Februaryand 30 Novemberwere 2000). consideredpost-spawnfish (Paragamian Weights and analyseswere not separatedby seasonbut combinedbecausewe foundno differencein IVyat the 0.01 level;p : 0.158. Pre-spawnburbothad a mean147 of 92, and post-spawnburbothad a mean Wyof 89. Catch per unit effort (CPUE) We usedCPUE datain Paragamian et al. (2008),whereone unit of effort wasa24h set,as a referenceto populationchangefor comparisonto that of PSDsand RSDs. RESULTS Lengthsof burbot in the samplesfrom 1957through 2004 increasedsignificantly (Fs,+za:21.49, p < 0.001)from a meanof 459mm TL in 1957-1958 to 615mm TL by 20022004(Table I and Fig. I ). Mean lenglhsof burbot in all classintervalsfrom 1979to 2004 weresignificantlylargerthanthe meanof the 1951-1958sample(ANOVA p < 0.01). Mean TLs of burbot coilectedin 1996-1998and2002-2004were longerthan the meanTL of thosecollectedin 1993-1995,while the 1996-1998burbotwere longerthanthoseof 1999-200t. The greatestchangesin PSD and RSD occurredbetweenthe 1957-1958samples 555
25 20 li
(t)
t5
z
10
I 958 1957-
5 U 6 5 le79- I 982
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4 3
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=
z
6 4
0
10 8 L q,i
€- 6
z
+ 2 0 1',70 220 270 3ZO370 420 470 520 570 620 670 720 770 820 870 920 970 ClassInterval(nnn)
Figure l. Length frequencydistributionsfor burbot in the Kootenai River, Idaho and British Columbiafor intervalsbetween1957 and2004.
l0 9 8 7 3o trt
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2002-2004
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2.5 2 1.5 I 0.5 0 t70 220 ?70 320 370 420 470 520 570 620 670 720 770 820 870 920 970 Classhterval(mm)
Table 1. Total length(TL) summarystatistics fbrburbot sampledin the KootenaiRiverat intervals from l95Tthroush2004 Statistic n Mean'fL (mm) SD Range
r957-l958
t979-1982
tw
v
459
s94 (r l 5) 358-8r3
(6e) 3t7-7U
1993-1995 1996-1998 t999-2991
6l 533 (121) 349-958
135 596 (r34) 300-9r5
ill 557 (82) 332-770
2002-2W 50 615 (e7) 352-U1
and the more recentsamples(Table 2). PSD was similar throughoutthe intervalsfrom l95l to 2004but increasedfrom l7 to 86 by the last samplinginterval. All otherRSD valuesalso increasedalthoughnot incrementally.Sincesamplesize was sufficientto approximatea normaldistribution,we were ableto calculatethe 95ohconfidenceintervals for 15 of the 24 PSDsand RSDs(Gustafson1988). Weightswere not collectedduring the 1957-1958burbotsampling,but mean relativeweights in the five post-Libby Dam samplingintervalswere significantly different(F : 13.40;p < 0.001)and increasedoverthis period(Table3). Catchper unit effort varied from 1993through 1998(range: 0.029to 0.054 and declinedsteadilythereafter(Fig. 2). burbot/net-duy) DISCUSSION that increasesin PSD could be indicatorsof low Willis et al. (1993) suggested recruitment andlorenvironmentaldisturbances.Changesin the Kootenai River occurred soonafter closing of Libby Dam in 1973 andsuggestthat therewas an almost immediate 557
Table2. Populationsizestructureindices(PSDsand/orRSDs)for burbotfrom the KootenaiRiver,Idahoand BritishColumbiafor 1957-2004.Approximate95% confidenceintervals(Gustafson1968)arein parentheses arestock(S), quality(Q), prefened(P), memorable(M), andtrophy(T). Lengthcategories t979-1982
1957-1958 n S
a P M T ulikely
199 184 JJ
2 0
PSD/RSD
n
PSD/RSD
u 92(fi) tf la -0
33 23 1l
97u 68(+23) 324
n
6r s9
PSD/RSD
n
PSD/RSD
r33 t27 94(+s)
9f
32 s3(+22) t2a 7 35a864
% 4
69(+10) 33(+17)
2W2-20M
1999-2001
r99Gl998
t993-1995
n
PSD/RSD
ill 109 98(+5) 68 6r (+r3) 8/ 0-
n
PSD/RSD
50 gga 49 43 86(+r3) 14 zga 124
representsa sample size insufficient to approximate a normal distribution.
impactof the dam on recruitmentandwell-beingof the KootenaiRiver burbot, (2000),Paragamian et al. (2005),and Paragamian supportingthe findingsof Paragamian and higher and Wakkinen(2008). Thesestudiesimplicatedwarmertemperatures dischargesduring the winter migrationand spawningperiod as impactson recruitment of burbot. Increasesin meanlengthmay havebeena resultof the combinedeffectsof in the averageageand sizeof fish, failing recruitment,and a declining increases et al. (2008). For population.Failingrecruitmentwas confirmedby Paragamian comparison,T. Horton (pers.comm.,MontanaFish Wildlife and Parks)found an average RSD-Pof 3l in the more stableburbotpopulationin a reachof the MissouriRiver in Montana. Relativeweights of burbot during the yearsfollowing closureof the Libby Dam suggestingthat Wy may have progressivelyincreasedas populationdensitydecreased, beendensitydependent.However,the increasern [4y occuffeddespitedecreased productivity in the Kootenai River systembelow the dam becauseof nutrient trapping within KoocanusaReservoir.Woods(1982)and Snyderand Minshall (1996) found that the KootenaiRiver below Libby Dam was phosphorus-and nitrogen-limited,inhibiting primary and secondaryproduction. KootenayLake, BC, farther downstreamof our studyreach,was alsonutrient-limited(Daily I 981). Fisheret al. ( 1996)recommended establishmentof different IYy valuesbecausethey found differencesin lotic vs lentic populationsof burbot. The changein mean Wylikely could be relatedto abiasedW7. equation(Gerowet al. 2004and 2005). If smallerburbottendto be naturally"skinnier,"a biasedequationwould indicatethat the fish are in poor conditionwhen they are not. If there is in fact a biasedequation,this could provide an explanationfor the increasein the abundanceof the size classesof fish mean Wr. A lack of recruitmentwould decrease that a biasedequationwould inaccuratelylabel as being in poor conditionand would resultin a greatermean Wr. M. Abrahamse(pers.comm.,Universityof Wyoming) found in his preliminary data analysisof the burbot W5 equationthat stock-qualitylength burbot have the lowest W, values. that therewere four samplingconsiderations in the Willis et al. (1993) suggested quantificationof length frequencysampleswith PSD and RSD - samplesize, selectionof samplingsites,gearrelatedbias,and seasonalinfluences.In our study,samplesize was most likely affectedby populationdensitybecausein most yearswe expendedto the maximum our allottedlogistic effort while burbot numberswere diminishing(Paragamian et al. 2008). For example,from 1993through2004,we averaged1,292net-daysof effort in capturingburbot (Paragamianand Laude 2008). But despiteour effort to captureas many burbotas possiblefiom 1993through2004,somesamplesfor calculationsof PSD and RSD did not meet the requirementsof size sufficient for approximationof a normal distribution(Gustafson1988). Samplingsitesand effort were relatively uniform from 1996through2004,while 1993-1995were yearswe spentdefining samplelocations. The 558
numberof burbot capturelocationsand effort in 1957-1958were limited, effort was not tabulated,and samplingwas conductedonly in ID as it was from 1979to 1982. Sampling during the pre-spawnand spawning seasoncould have biased length and age of the catch of burbot. Sampling efforts by seasonwere most consistentduring 1993-2004and 1957-1958becausethey includedthe spawnmigration, spawning,and the immediatepost-spawnperiod. The burbot in the Kootenai River is believedto spawn during the last week in Januaryand the first two weeksin February(Paragamian2000). Becausethe burbot is less active during the spring and summermonths becausewater arewarffler(Nikdevicet al. 2000,Piiiikkrinenet al. 2000),we restricted lunperraturcs its samplingto mostvulnerabletime of the year. In the 1957-1958sample,it is likely that many of the smallerstock length fish were males,sincemalesmatureearlierthan females and at much shorterlengths(Evenson2000). Arndt and Hutchinson(2000) found that the smallestmalesrangedfrom 242 to 297 mm TL and the smallestfemalesrangedfrom 295 to 365 mm TL in resultsof weir trappingduring a spawningrun on a tributary to ColumbiaLake, BC. We suspectthat the most seriousbiaswould have beenduringthe 1919-1982samplingbecausesamplesincludedthe post-spawn(springand summer) periodwhen the burbot is lessactive. PSD andRSD as calculatedby Fisheret al. (1996)are likely accurate,but some bias in our estimatestowardshigher and stablePSDsoccurredbecauseof gearbias (Willis et al. 1993). Unlike many otherspeciesthat are vulnerableto otherstandard samplingtechniques(e.9.,gill nets,trawling,electrofishing), the burbot is difficult to capturewith most sampling methodsbecauseit is a benthic predatorand often occupies large deeplakesand rivers (McPhail and Paragamian2000). Although cod traps have beeneffective for capturingburbot in lakes(Spence2000), the hoop net is the primary methodby which the burbot is sampledin rivers in North America (Parkeret al. 1988, Bernardet al. 1991,Paragamian 2000,Spence2000,Hortonand Strainer2008). Studiesby Bernardet al. (1991) indicatedthat the burbot can be caughtin hoop nets at about350 mm TL but is not fully recruiteduntil 450 mm TL. Hoop netsusedin its captureare selectivefor fish much largerthan 200 mm TL, the initial length of stock length fish (Fisheret al. 1996),and will samplea higherproportionof quality length fish at 380 mm TL. As a consequence, our datashowedlittle changein PSD despitediminished recruitment(Paragamianet al. 2008), but RSDs increasedthrough the study intervalsand indicatedrecruitmentfailures. Willis et al. (1993)suggested that PSD and RSD alonewere not alwayssufficient to describepopulationchangesbut were more usefulwhen combinedwith fish condition (Wy) andCPUE. Our findingssupportthis idea. CPUE hasbeenusedto compareburbot stockdensitiesin the KootenaiRiver and otherwaters(Parkeret al. l988,Paragamianet al. 2008,Hardy et al. 2008)and was a suitableindicatorof the burbotpopulationchange (Paragamian et al. 2008). Had thesechangesin RSD and CPUE beennotedin the late 1970s,conservation measures may havebeenimplementedsooner(KVzu 2005). Table 3. Relativeweight summarystatisticsfor burbot sampledfrom the Kootenai River for intervalsfrom 1979throueh2004. No weishtswere collected during1957-1958. Statistic
n Mean
SE SD
19791982
19931995
r9961998
19992001
20022004
34 74.9 2.4 13.9
54 88.4
135 93.9 1.2 t4.l
110 89.3 1.5 16.2
50 98.2 2.9 20.7
r.9 14.0 559
2500 2000 z l 5 o oP
50 840
fr30
1000E
920 510 z
500
0
0 0.06 0.05 o
644
004
30 "E 3zo
0.03
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810
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Figure2. Kootenai River burbot catch(histograms)and samplingeffort (A) and CPUE (B) from 1993to 2004. Datashownincludewinter hoopnet effort only. ACKNOWLEDGMENTS We acknowledgethe dedicatedfield work of Vint Whitman and GretchenKruse, formerlyof the IDFG, and the many bio-aideswho helpedin this study. We thank Brian Pyperof CramerFish Sciencesfor importantstatisticalwork and Martin Stapanianof the US GeologicalSurvey,Dan Schill of the IDFG, Matt Abrahamseof the Universityof Wyoming, an two anonymousreviewersfor technicalreview of our manuscript. BonnevillePowerAdministrationprovidedthe funding for this study. LITERATI.IRECITED white sturgeon Anders,P.J.,D.L. Richards,and M.S. Powell. 2002. The first endangered population:repercussions in an alteredlargeriver-floodplainecosystem. AmericanFisheriesSocietySymposium28:67-82. Andrusak,H. 1976. KootenayLake sportfishery 1974-1976.British ColumbiaMinistry of Environment,Fish and Wildlife Report,Nelson. Andrusak,H. andM.A. Crowley. 1978. KootenayLake sportfishery 1974-1976.British ColumbiaMinistry of Environment,Fish and Wildlife Report,Nelson. Argent,D.G., R.F. Carline,and J.R.Staufer.2000. A methodto identi$zand conserve rare fishesin Pennsylvania.Journalof the PennsylvaniaAcademy of Science74:
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Received:2 June 2008
Accepted:17 July 2008
