EnvironmentalBiology ofFishes48:420–421,1997.
© 1997 Kluwer Academic Publishers. Printed in the Netherlands.
Threatened fishesofthe world:Scaphirhynchus albus(Forbes&
Richardson,1905)(Acipenseridae)
Richard L. Mayden &Bernard R.Kuhajda
Department of Biological Sciences, Box 870344, University of Alabama, Tuscaloosa, AL 35487-0344, U.S.A.
Commonname:Pallidsturgeon(E).
Conservation status:ListedunderEndangeredSpecies Act as
FederallyEndangered(Federal Register 1990).Currently re-
ceivesfederal protection; detailedrecovery planprepared by
Dryer &Sandvol(1993).
Identification:One of threespecies ofScaphirhynchusdistinguished by havingpallidcream,gray, or whitishhead,body, and fin col-
oration inmatureadults;verysmallorbit diameter; elongate and pointed snout: inner barbelspositionedanterior toouterbarbels; few to
no sharp,retrosespines onsnout tip and anterodorsal to eye; andpoorlydeveloped squamationon venter.Otherfeaturesinclude
differentnumber ofdorsalplates,anus toanal finplates, platesposterior to analfin.lateralplatesanterior to dorsal finorigin,anal and
dorsal fin rays, and head, fin, and body proportions, relative to its close relatives S. platorynchus andS. suttkusi. One of the most detailed
morphological analyses currentlyavailable waspresented byBailey &Cross(1954);recentmorphologicalcomparisons andanalyses
presented byKeenlyne et al.(1994a) andMayden & Kuhajda(1996).Illustration fromForbes & Richardson(1920).
Distribution:Endemic toMississippiRiverBasin but naturally limited to the Missouri and lowerMississippiriverdrainages(Bailey &
Cross1954).Historicrangeincludes theAtchafalaya River,lowerMississippiRiverupstream toconfluencewithMissouriRiver, and
MissouriRiver (Keenlyne1995). Thespecies hasnever been found tooccur ineither the Ohioor upperMississippirivers where the
shovelnose sturgeon,S. platorynchus,typicallyabounds.Abundance:Speciesrare,abundancedeclinedmarkedlyfollowingchannel-
ization and dam constructionin the lowerMississippi andMissouririvers. Theseactivities notonlylimitmigratoryroutes of thespecies
but havelargelycurtailed naturalSpring floodingperiods that are thought totrigger spawning.Habitatalterationshave alsoimpacted the
naturallyturbidcharacteristics of theMissouri and lowerMississippirivers to theextent thatforage species are declining and thetypical
turbid,large-riverhabitat of thepallidsturgeon hasdeclined.Riverineecosystemshistoricallyoccurring inthese majorwaterways are
beingreplacedwithlentichabitatsthat arelessturbid andwithaquaticspeciesadapted totheseclear,lenticenvironments(Pflieger &
Grace 1987). Dryer & Sandvol (1993) provide detailed account of the distribution and abundance of this species. Habitat and ecology:
Life history aspects ofS. albusare relativelypoorlyknown.Species isfound inlarge riverchannelswithconsiderable diversityin micro-
habitats.They areusually associatedwithrapid current over sand,gravel or rockysubstrates(Kallemeyn1983,Carlson et al.1985.
Erickson1992).Known toprefer turbid waterconditionsthathistoricallycharacterized theMissouri and lowerMississippirivers.Diet of
adults isdominated byfishes, typicallylarge-riverminnows andshiners(Cyprinidae) (Erickson1992). Based on observedfood habits, the
pallidsturgeon depends onthe historical,naturally occurringturbid waterconditions to concealitselffrompreyitems (Keenlyne1995).
The documenteddecline of many cyprinid species known toserve as regular food items for thepallidsturgeon islikelyinvolved in its
imperilment.Relative to theshovelnosesturgeon,growth is muchmore rapidthroughoutvarious agegroups (Carlander1969,Ruelle &
Keenlyne1993).Pallidsturgeon mayweigh up to 45 kg(Brown1971);males reach 39years of age,whilefemales maylive aslong as
41 years(Ruelle & Keenlyne1993).Reproduction:Reproductivebiologypoorlyknown.Spawning believed to occurbetweenApril and
mid-June,dependinguponlatitude (Keenlyne & Jenkins1993). Malesmature at 53 to 58 cm or 5 to 7 or 9years,with 2-or 3-yearintervals
betweenspawning; size offemales at maturity unknown,but estimated tooccur by 9 to 12 or 15 to 20 years ofage,with 3-to 10-year
intervalsbetweenspawning (Kallemeyn1983,Dryer &Sandvol1993,Keenlyne & Jenkins1993). Likeshovelnosesturgeon,pallidstur-
geon havebeen observed topossessmaturegametes during periodscoincidingwith high river flowlevels, possiblyindicatingthatonset of
spawning isinitiated bytypicalSpring floodingof rivers.Spawninghabitat not known. Gross habitatmodificationsmade to thelarge river
habitats in theMississippi andMissouri river drainagesthroughchannelization anddams fornavigationalpurposes preclude an accurate
appraisal of the naturalspawninghabitats ofthisrarespecies.Undernaturalconditions itlikelyspawns infast-flowingsections of the
main-stem portions of the rivers. Because very few records exist for the species outside of main rivers, this species may not ascend smaller
tributaries tospawn asdoes theshovelnosesturgeon.Althoughunknown forpallidsturgeon,eggs ofshovelnosesturgeon areadhesive
and require current forproperdevelopment,indicatingthatboth a stable andsilt-freesubstrate isnecessary fortheirsuccessfuldevel-
opment. Hatching probably occurs in five to eight days under natural conditions (see Mayden & Kuhajda 1996). Threats: The gross
human-induced habitatmodifications in the Missouri and lowerMississippirivers are the primaryfactors involved in the decline of the
pallidsturgeon. Thesealterations, madeprimarilyunder theguise ofnavigation and floodcontrol,haveresulted inregulatedflow
patterns ofthese majorrivers andhave createdhabitatsmorelentic than lotic. Both oftheseconditionsdiffer radicallyfrom thenatural
habitats towhich thepallidsturgeon isadapted(e.g.,braidedchannels, irregularflowpatterns,flooding of terrestrialhabitats,extensive