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<br />UOZ5~.3 <br /> <br />There is some evidence that the abundance of these species is reduced by high river <br />discharges and lower water.temperatures (reviewed by Lentsch et al. 1995). <br />Management of flow regimes to approximate natural hydrographs could suppress the <br />abundance of these cyprinids. Muth and Nesler (1993) found that moderately high <br />daily mean discharges were associated with later initiation of spawning and shorter <br />spawning season for the red shiner, sand shiner, and fathead minnow, Higher <br />discharges resulted in an earlier initiation of spawning for the redside shiner, probably <br />due to the preference of this species for cooler water. Smaller life history stages of <br />cyprinids and centrarchids may be especially susceptible to flow changes, and Harvey <br />(1987) found that fishes 10mm or less in length were very susceptible to downstream <br />displacement by flooding. However, displacement of the small life stages may depend <br />on "small differences in the timing of reproduction and of flooding" (Harvey 1987). <br />Cause and effect relationships between discharge and the abundance of cyprinid <br />populations in the UCRB appear to be weak at this time. <br /> <br />For some of the nonnative species, the principal source of recruitment is not the <br />riverine environment. Nonnatives, especially most of the centrarchids, are reproducing <br />in impoundments or floodplain ponds. Escapement from these areas provide a steady <br />source of individuals into the river system. Control of these nonnative species would <br />be more effective if the sources were eliminated. Chemical removal techniques could <br />be applied very effectively in isolated water bodies, but may not be palatable to the <br />public where recreational opportunities would be lost. Escapement controls might be <br />an acceptable alternative. Many traditional devices exist for preventing escapement. <br />In addition, new technologies have recently been developed for filtering and/or <br />destroying small organisms from discharges and intakes. These new technologies are <br />the result of recent invasion of the zebra mussel into the Laurentian Great Lakes, and <br />the zebra mussel veliger, which is smaller than ichthyoplankton, is now being <br />completely removed (e.g., see Nalepa and Schlosser 1993 and references therein). <br />The technology is available for preventing the escapement of even the smallest fish. <br /> <br />SECTION V. DEVELOPING SOLUTIONS <br /> <br />Background <br /> <br />Considerable scientific effort has been devoted to understanding how nonnative <br />fishes have affected the endangered big river fishes of the upper Colorado River <br />basin. The scope of the problem is well known, albeit complex: predation and <br />competition by nonnative fishes have contributed to the decline of endangered native <br />fishes. Resolution of the problem, in a broad sense, will require removal or reduction of <br />nonnative fish populations that threaten listed fishes. The problem and the general <br />solution were clearly defined in element 4.4 of the Recovery Implementation Program <br />(Program; USFWS 1987). <br /> <br />24 <br />