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<br />
<br />ENDANGERED SPECIES
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<br />and life stages affected by nonnative predation, and this
<br />pattern is probably similar for the humpback chub,
<br />although there is little documentation.
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<br />Control
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<br />Development of control strategies was facilitated by
<br />placing nonnative predators (i,e" small cyprinids and
<br />carp, centrarchids, channel catfish, and northern pike) into
<br />groups that reflected their source and the habitats where
<br />they affect endangered fishes, This facilitated evaluation of
<br />the potential efficacy of chemical, mechanical, and other
<br />control measures,
<br />Small nonnative cyprinids (fathead minnow, red shiner,
<br />sand shiner, redside shiner) are ubiquitous and reproduce
<br />in shoreline habitat Chemical control measures (e,g" poi-
<br />sons) would rot be feasible because native fishes occupy
<br />the same or aajacent habitat Mechanical control (e,g"
<br />active and passive netting) is feasible from a technical
<br />standpoint, but total removal is presumed impossible,
<br />Small cyprinids are chiefly a threat to native fish larvae,
<br />and control measures would target nursery habitat used
<br />by endangered fishes, Intensive use of seines in nursery
<br />habitat (Figure 3) could remove a large proportion of small
<br />cyprinids before the endangered fish spawn each year,
<br />thus possibly removing an impediment to recruitment
<br />Native fishes (such as juvenile suckers, chubs, and pike-
<br />minnow) are large enough to be identified and released,
<br />Seining also would remove some other nonnative preda-
<br />tors (e.g., centrarchids or small channel catfish).
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<br />20 Fisheries
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<br />The use of high water discharge also has been suggest-
<br />ed as a control option for small nonnative cyprinids
<br />(McAda and Kaeding 1991; Muth and Nesler 1993) based
<br />on observations of some nonnatives being displaced by
<br />flood flows in constrained channels (Minckley and Meffe
<br />1987), However, red shiner populations in the Colorado
<br />River system have rebounded within a few years follow-
<br />ing depression by high flow events (McAda et aL 1994).
<br />Unfortunately, high flows can adversely affect native fish-
<br />es by increasing mortality and/ or reducing growth rates
<br />(i,e" Tyus and Haines 1991), Lack of selectivity and the
<br />need for frequent high flow events diminishes the attrac-
<br />tiveness of flow manipulation as a mechanism for control-
<br />ling the abundance of nonnative fishes, even in regulated
<br />portions of the river system, In addition, manipulating
<br />dam operations to increase flooding could violate provi-
<br />sions of the ESA if it increased the "take" (e,g" mortality
<br />or reduced growth) of a listed species,
<br />Most centrarchids (e,g" smallmouth bass, largemouth
<br />bass, white crappie Poxolllis aml/llaris, black crappie P.
<br />nigromaculatus, bluegill Lepomis lIlacrochirus) reproduce
<br />mainly in ponds or reservoirs in the upper Colorado River
<br />basin, and can prey on almost any stage of the listed spe-
<br />cies, Control of centrarchids, with exception of the green
<br />sunfish (L cyanellus), involves preventing entra~ce into the
<br />river by structural controls, Chemical control measures
<br />would be appropriate where there is no risk to the listed
<br />fishes, but may not be politically acceptable due to pollu-
<br />tion, health risks, and sportfishing interests, Structures
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<br />Vol. 25, No.9
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