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I I I I I I I I I I I I I I I I I I I . selenium to.xicity, etc., is primarily respo.nsible fo.r lack o.f spawning success, survival, and recruitment. The numbers desired fo.r sto.cking are based o.n reco.very go.al criteria fo.r fish Po.pulatio.n abundance (USFWS 2001b) and survivo.rship curve estimates fo.r razo.rback sucker provided in Table 2. The target sto.cked Po.pulatio.n o.bjective o.f 7,540 fish co.mprised o.f three age groups in Table 2 is equal to. the draft reco.very go.al abundance o.bjective o.f 5,800 fish plus a 30 % buffer presuming natural adult mo.rtality is 30% annually. If available, the 16,440 Age 2+ fish 2. 300 mm in length wo.uld be stocked annually and wo.uld be split equally amo.ng the three river reaches (Table 1). As adults, these fish wo.uld constitute o.ne presumptive genetic po.pulatio.n as described in Czapla (1999). A genetic risk: assessment fo.r bro.o.dsto.ck develo.pment and sto.cking within this plan is no.t necessary due to previo.us do.cumentatio.n. The appropriate genetic make-up fo.r broodstock has already been identified (Wydo.ski 1994), and the appropriate breeding strategy is already being implemented within the Program at hatchery.and po.nd facilities in Grand Junctio.n, Co.lorado., and Ho.rsethief State Wildlife Area. Numbers o.f progeny from these broo.dsto.ck family lo.ts that are in excess o.f tho.se needed fo.r productio.n o.f the broo.dsto.ck have already been designated as suitable fo.r sto.cking in the Co.lo.rado. and Gunniso.n rivers (Czapla 1997). Burdick et al. (1995) have already co.nducted a risk assesSment asso.ciated with sto.cking and the sto.cking plan has already been approved fo.r implementatio.n by the Recovery Pro.gram Bio.lo.gy Co.mmittee. Achievement o.f the stocking o.bjective wo.uld result in an adult populatio.n o.f at least 5,800 adult fish from ages five to. seven distributed o.ver 150 river miles o.f three reaches in the Upper Co.lo.rado. River subbasin. Sto.cking o.f razo.rback sucker will interject :'10 adrf1tional species :into tIle existing fish community, and has the potential to induce cha,ges in the relative abundance and bio.mass of these species' po.pulatio.ns in each o.f the targeted reaches. The sto.cking numbers proPo.sed are influenced by the fo.llo.wing assumptio.ns: 1) Adult riverine habitat is no.t limiting in the three upper Co.lo.rado. River reaches fo.r razo.rback sucker and the reco.very go.al Po.pulation abundance o.bjective is belo.w the carrying capacity o.f the target river reaches. 2) The Upper Co.lo.rado. River reach and the Gunniso.n River reach are similar with respect to. this carrying capacity based o.n similar geohydrolo.gy and predo.minating native fish communities based o.n o.bserved native/no.nnative species relative abundance as described in Burdick (1995) and Anderso.n (1997). This similarity is alSo. assumed to. extend to. the Palisade-Stateline reach o.f the Co.lo.rado. River. 3) Successful reproductio.n and self-sustaining status will necessitate the presence and adequate distributio.n and amo.unts o.f nursery habitat in flo.o.ded bo.tto.mlands o.r backwaters maintained by a natural flo.w regime and favo.rably enhanced by no.nnative fish co.ntrol activities. 7