8163 - Laserfiche WebLink

Home Browse Search

STATUS OF ENDANGERED COLORADO SQUAWFISH 967 of larvae drifted to the lower reach, and (2) re- production or hatching success in the upper reach was formerly much greater than that today, and substantial numbers of larvae were retained in the upper reach even though proportions drifting to the lower reach might have been similar to those in recent years. Status and Future Prospects This work and that of others strongly suggest that the population of Colorado squawfish in the Colorado River is small but actively recruiting new individuals to the adult population. Whether pop- ulation size is relatively constant over time is dif- ficult to ascertain, but abundance is apparently much less than that reported earlier in this century. Strong year-classes augment the population enough to cause observable changes in population structure and abundance. The significance of these large year-classes in maintaining the adult popu- lation is also difficult to ascertain, but the apparent increase in numbers of adults observed during this study suggests that the infrequency of recruitment pulses currently limits the size of the adult pop- ulation. Metapopulation theory argues that stochastic fluctuations cause 10cal extinctions, and the prob- ability of extinction decreases with increased pop- ulation size. Thus, to minimize the probability of being driven to extinction, the goal is to achieve a population size large enough to withstand sto- chastic fluctuations and also to maintain sufficient genetic diversity (see Soule 1986; Simberloff 1988). Crossing over from the Green River does occur at some unknown rate: the first documented case was an individual caught at Green River rkm 52.5 in 1994 and recaptured in the lower Gunnison River (rkm 3.5) in 1996 (B. Burdick, USFWS, per- sonal communication; T. Chart, Utah Division of Wildlife Resources, personal communication). Such movement, even at a low rate (I-tO/gener- ation), should alleviate genetic problems associ- ated with small population size (see Franklin 1980; Gilpin 1993; Mills and Allendorf 1996); therefore, stochastic demographic fluctuations are probably a greater threat to this small population. The abundance of age-O fish and subsequent re- cruitment is probably the greatest source of de- mographic fluctuation in the population of Colo- rado squawfish in the Colorado River. If popula- tion size and viability is limited by this recruit- ment, then strong year-classes are needed more frequently. Presently, variables controlling the rel- I ative success of annual reproduction and first-year survival are not clearly understood. However, if spawning and hatching success are linked to con- ditions created by spring runoff, changes in runoff patterns in the Colorado River during the past half century resulting from water development may in part explain the decline of this population. A greatly reduced frequency of high spring run- off flows (Osmundson and Kaeding 1991; Van Steeter 1996) influences four factors that might have negatively affected this population. First, high flows during spring may be required to create fresh cobble bars for spawning (Harvey et al. 1993) and adequately cleanse fines from existing bars (Haynes et al. 1984; Reiser et al. 1989), con- ditions apparently necessary for spawning site se- lection by both the northern squawfish P. orego- nensis (now northern pikeminnow; Nelson et al. 1998) and Colorado squawfish (Beamesderfer and Congleton 1981; Lamarra et al. 1985). The exis- tence of interstitial voids for protection of depos- ited eggs and creation of microcurrents among voids for successful egg incubation may contribute to high egg-hatching success. Second, high flows may serve to dilute waterborne contaminants from agricultural and urban sources that may interfere with reproductive behavior, reduce egg viability, or reduce larval survival (Woodward et al. 1985; Hamilton and Waddell 1994). Third, river bottom- lands require periodic high flows to maintain chan- nel (and thus habitat) diversity and biological pro- ductivity (Junk et al. 1989; Bayley 1991; Ras- mussen 1996) important to young Colorado squawfish. Finally, high, sustained spring flows serve to reduce numbers of nonnative minnows that now dominate backwater nursery habitats (McAda and Kaeding 1989; Osmundson and Kaed- ing 1991; Muth and Nesler 1993; Gido et al. 1997). Although a food source for subadult Colorado squawfish, the prolific red shiner Cyprinella lu- trensis preys on Colorado squawfish larvae (Rup- pert et al. 1993), and larvae of fathead minnow Pimephales promelas compete with Colorado squawfish larvae for food (Beyers et al. 1994). Low adult numbers and infrequent, pulsed re- cruitment make this population vulnerable to ex- tirpation over time. Natural variation in demo- graphics alone makes viability of this population tenuous, even if stability of habitat conditions is assumed. However, for many imperiled species, the threat of deterministic habitat change is prob- ably greater than stochastic demographic variation (Caughley 1994; Harcourt 1995). This may be es- pecially true for this population: more water de- pletions are planned, more nonnative species may