longer furnishes optimal conditions for different in-
tervals of thelife cycle. Insomeareas recruitment is
high butrearinghabitat islimited.Elsewhererear-
ing habitat is abundant butspawning habitat isnot.
The net result is that biomass and potential yield are
less inimpoundedstocks than in theunimpounded
stock (Figure 2).
Productivity ofsome impoundedstocks is espe-
cially limited bypoorrecruitment resultingfrom
lack ofsuitable spawninghabitat.Columbia River
white sturgeon spawn in areas ofhigh velocity
(> 0.8 ms–1) overlargerockysubstrate nowavaila-
ble only inriverineareasdownstreamfromdarns
(Parsley et al.1993). Availability of usablehabitat
increaseswithriver discharge and recruitment is
correlatedwithhabitatavailability(Figure 3). Dis-
chargeeffectsvaryamongareas as aresult of differ-
ences inchannel morphology, andsome areas pro-
vide littlehabitatexcept atvery high flows. Dis-
charge regulation in isolatedareas of theupstream
Kootenai andSnakerivers hasresulted in complete
reproductive failure(Apperson &Wakkinen1992,
Marcuson^15 ).
Harvestlevelswhichcould besupported by the
productiveunimpoundedstockcannot be sustained
by the impounded stocks (Beamesderfer et al.
1995). As aresult, fisheries for severalimpounded
stockscollapsed in thelate1980safter aperiod of
intense exploitation assport andcommercialfisher-
ies switched tosturgeonfollowing declines ofsalm-
on fisheries.Hydropowersystemmanagers are now
cooperatingwith governmentagencies andIndian
tribes responsible for managingthesefish toprotect
and enhancetheseimpoundedstocks.
One element ofthisprogram isintensiveharvest
management. Before 1988 sturgeonstocksthrough-
out the lowerColumbiaRiverwere managedwith
similarregulations andonly a few keystockswere
monitored.Fisheries are now being regulatedwith
stock-specificregulations tailored to the unique at-
tributes ofeachstock in anattempt tooptimizefish-
ery benefits. Amoreintensive monitoringprogram
has also beenundertaken toregulate harvest at op-
timumlevels.
A secondprogram element isevaluating trans-
plants of juvenilesfrom thelarge andproductive
unimpoundedstock into reservoirs where poor
recruitment appears to have understocked the
availablerearing habitat. Webelievethatproduc-
tion ofsturgeon by thesystemwillultimately be
limited by the carrying capacity of therearing hab-
itat andthat peak productionwillresult fromfull
stocking of allareas.Survival,growth, and condi-
tion of transplantedfishwill bemonitored todeter-
mine thecosts and benefits ofthisalternative.
Transplantsalsoprovide a low-costmeans ofeval-
uating the potential forenhancement ofreservoir
stocks without capitalcosts,geneticrisks, ordisease
problems of a hatcheryoperation. Hatcherysupple-
mentationwill b econsidered inmoredetail if trans-
plants are notfeasible or effective.
A third programelement isdeveloping and sup-
porting recommendations for hydropowersystem
operation tooptimizeriverdischarge andvelocity
during spring periodswhenwatertemperature is
suitable forspawning.However, thelargesocial
and economiccosts of modifications inhydropower
systemoperation arclikely topreclude changes in
waterallocation for the solebenefit ofsturgeon.
Programcooperators are thereforeimplementing
intensive samplingdesigns for eggs,embryos and
larvae in anattempt toidentify effects of within-
year differences inflow and todeveloprecommen-
dations for using available water to optimize
spawningconditionswithineach year.
Flowmanagement is theonlyelement in the low-
e r ColumbiaRiverprogramwhichattempts to en-
hancesturgeon bydirectly modifyinghabitat. In-
tensiveharvestmanagement for eachstock and
supplementationrecognizehabitatlimitations but
maximize productivity of the existinghabitatrather
thanproducinghabitatimprovements.ConclusionsThe flexible andopportunisticlifehistory style of
sturgeons mayhelpexplain their persistence and
successover thelast 100 millionyears.However,
system-widechanges in the largeriver systemsthey
inhabit now pose seriousrisks tothese remarkable
creatures. Thelargescale ofdetrimental habitat
changesmake themextremelydifficult tocontrol
for the solebenefit ofsturgeon and so sturgeon