9539 - Laserfiche WebLink

Home Browse Search

stable backwater conditions likely favored reproduction of the NNC, as well as enhanced the growth of ~ larvae into catchable subadult fish. Any reproduction fundamentally compromises closure assumptions behind removal experiments at any spatial or temporal scale. Lentsch et al. (1996) postulated that controlling nonnative cyprinids would be difficult because of 1 their high abundance, ubiquity, adaptability, and reproductive potential. Red shiner, fathead minnow, and sand shiner are the most common species in the upper Colorado River basin, frequently comprising more than 80% of fishes captured in low velocity habitats (Valdez 1990; Muth and Nester 1993; McAda et al. 1994a and 1994b, 1995, 1996, 1997; Trammell and Chart 1999a, 1999b, and 1999c). Minckley (1973), Scott and Grossman (1973), Cross and Collins (1975), Pflieger (1975) Carlander (1977), Matthews (1987), and Sublette et al. (1990) all report the tolerance of red shiner, sand shiner, and fathead minnow ~ to various perturbations of "quality" habitats or conditions. All three of these species are classified as extremely tolerant of extremes in temperature, pollution, discharge, and siltation. Their small size and ability to persist in and re-colonize a variety of habitats makes removal difficult. Cross and Collins (1975) ~ state that it will never be possible to completely remove red shiner and suggest they "will be with us until the end, no matter how badly we treat our water". If reduction in abundance was possible, detection of the reduction may also prove difficult. For example, nonnative cyprinid data collected from 1986 to 1997 (McAda et al. 1994a, 1994b, 1995, 1996, t 1997) during annual ISMP surveys in both the Green and Golorado rivers shows nonnative cyprinid numbers vary greatly from year to year despite the use of standard sampling protocols (Figures 10, 18 and 19). Furthermore, it appears that NNC density was at historically low levels in 1997, prior to removal efforts. Some studies have shown that nonnative cyprinid abundance is reduced in high water years (McAda et al. 1994a, McAda and Ryel 1998, Trammell et al. 1999, Lentsch et al. 1996); peak flows in 1997, 1998 and 1999 were all higher than average. Thus, due to inherent NNC population variability and NNC density antecedent to this study, attributing any changes in NNC abundance to removal efforts is difficult. Fluctuating habitat availability (especially in the Colorado River) also contributed to difficulties in interpreting data. Most of the variation in area sampled was due to differences in discharge and -16-