WSP04079 - Laserfiche WebLink

Home Browse Search

.. I '00 1981 '00 wae '00 '00 ~.; J 1 , Fig. 14. Relation between discharge and spawning period for razorback sucker, Yampa River, 1975, 1981, 1988, and 1989 (discharge data incomplete for 1989). Vertical bars delineate spawning period. (One collection of ripe fish in 1988.) and the absence of juvenile and subadult life history stages (McCarthy and Minckley 1987; Tyus 1987). The apparent decline of the razorback sucker toward extinction throughout the Colorado River basin emphasizes the need for more immediate measures toward recovery of this species. II Limiting Factors \ Aa evaluation of limiting factors is difficult because ~ of complex relations between environmental and biological variables that limit the distribution and abundance of organisms. A limiting factor is simply one component of a multidimensional system. Thus, single factor studies (e.g., determination of temperature threshold for successful reproduction) should be evaluated from a holistic perspective for application in natural systems and not in isolation. We stress the need for system-level cognizance and interpretation in evaluating factors that are potentially limiting to species in decline. Studies of factors limiting the distribution and abundance of rare fishes in the Yampa River are complicated by the variability of the environment (e.g., seasonal fluctuations in discharge, temperature, food base, and species abundance) and by logistical problems associated with studying fishes in large, turbid rivers. The determination of limiting factors for rare fishes is further complicated because of limited life history information. Habitat use by the rare fishes may only reflect temporary, seasonal, or marginal habitat availability. Thus, caution must be exercised when determining habitat needs and limiting factors for fishes in decline, and professional biological judgment must be recognized as critical in data interpretation. Some factors that may be limiting the distribution and abundance of rare fishes in the Yampa River are summarized in Table 6. This information is presented for consideration in future updates of the Sensitive Areas report (Upper Colorado River Basin Coordinating Committee 1984) and for aiding the Recovery Implementation Program (U.S. Fish and Wildlife Service 1987) for the Yampa River. Each species is discussed separately in the following sections. Although more information is available regarding habitat needs and limiting factors for the Colorado squawfish, we do not suggest that protection of a single species or a single life history stage will adequately protect all- we emphasize that each species is unique and has different requirements for survival. Colorado Squawftsh An evaluation of factors limiting the distribution and abundance of Colorado squawfish in the Green River system is complex because of the wide range of habitat and flow conditions required by the different life history stages. High spring flows, in addition to increasing water temperatures, are necessary for the initiation of the spawning migration (Figs. 7 and 8). Decreasing flows and warming river temperatures in early and mid-summer are necessary for successful spawn and downstream transport of drifting larvae (Tyus et al. 1987; Fig. 11). Low flows in late summer and fall are correlated with availability of nursery habitat and young fish abundance and growth (Tyus et al. 1987; M. Pucherelli and R. Clark, written communication). A stable winter base-flow is necessary for maintenance of winter habitats (Wick and Hawkins 1989). Adult The potamodromous migrations and homing behavior of Colorado squawfish (Wick et al. 1983; Tyus and McAda 1984; Tyus 1985) from downstream Green River and upstream Yampa River to Yampa Canyon mandates protection of known migration routes, since feasibility of fish passage facilities for this species has yet to be demonstrated. We presume that blockage of these river sections by dams or water diversions will directly result in the local extinction of Colorado squawfish (Tyus 1984), as evidenced by the recent loss of 80 km of 18