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<br />predation by nonnative fishes because food organism availability is low in <br />the main channel and backwaters of Upper Basin rivers. <br /> <br />Nonnative red shiners and fathead minnows are often the most abundant fish <br />species in backwaters of the Upper Basin rivers. For example, Cranney <br />(1994) reported that red shiners constituted 66.5% and fathead minnows <br />31.9% of 3,599 fish collected in 13 seine hauls in the lower Duchesne <br />River, a tributary to the middle Green River. Only one native fish (a <br />bluehead sucker) was captured during that sampling effort. Red shiners <br />and fathead minnows constituted 90.4% of 149,489 fish collected between <br />1986 and 1994 from primary backwaters of the Colorado and Green rivers <br />during the Interagency Standardized Monitoring Program (McAda et al. <br />1994a, 1994b, 1995). In 1996, nonnative minnows (combined species) <br />comprised 92-99% of the total number of fish seined from backwaters of <br />Upper Basin rivers during standard monitoring (McAda et al. 1997). <br /> <br />Adult red shiners were found to be predators on fish larvae in the Yampa <br />and Green rivers (Ruppert et al. 1993). Adult red shiners (36.1 mm TL) <br />consumed all 100 razorback sucker larvae and terminated a cage experiment <br />in a wetland to determine competition between the two species (Modde and <br />Wick 1997). Fathead minnows have also been documented to be predators on <br />catostomid larvae (Dunsmoor 1993). Therefore, it is reasonable to assume <br />that nonnative minnows such as the red shiner and fathead minnow are <br />important predators on razorback sucker larvae. The razorback sucker <br />spawns on the ascending limb of the hydrograph and their larvae drift <br />downstream during May and June when zooplankton and benthic invertebrate <br />numbers are low in backwaters in the turbid waters of Upper Basin rivers. <br />Razorback sucker larvae would be highly susceptible to predation at that <br />time by abundant nonnative minnows. <br /> <br />The composition of fish species in existing backwaters along six reaches <br />in the middle Green River was made up primarily of nonnative minnows. <br />Fathead minnows composed between 37.7 and 88.1% of fyke net catches in <br />1997 and red shiners composed 6 to 48.5% of the catches at these six sites <br />(G. Birchell, 1998, personal communication). In backwaters that were <br />reconnected with the main channel along the same six river reaches, <br />fathead minnows composed over half (50.4 - 72.4%) of fyke net catches and <br />red shiners comprised between 3.1 and 18.3% of the catches in 1997. <br /> <br />Although successful natural spawning of razorback suckers occurs on wave- <br />swept rubble along the shoreline of Lake Mohave, survival of larvae only <br />occurs in habitats such as predator-free isolated coves (Minckley et al. <br />1991). However, in the absence of predaceous fish, large numbers of <br />odonate nymphs were produced in these coves and they replaced fish <br />predators (Mueller et al. 1993). Horn et al. (1994) reported that <br />odonate nymphs were very effective in capturing and consuming razorback <br />sucker larvae in the laboratory. Larger juvenile razorback suckers are <br />now stocked into the isolated coves to reduce or eliminate predation by <br />odonate nymphs. <br /> <br />Nonnative fishes quickly occupy floodplain habitats that are reconnected <br />to the main channels of Upper Basin rivers (G. Birchell, 1998, personal <br />communication; Burdick et al. 1997). For example, nonnative .fishes <br />invaded and colonized a gravel-pit pond in the floodplain of the upper <br />Colorado River within four months after the pond was drained, all <br />nonnative fishes removed, and the pond reconnected with the river (Burdick <br />et al. 1997) and, within eight months, five species of nonnative fishes <br />successfully reproduced in the pond. This pond had a irregular bottom <br />that was below the streambed of the main channel and did not drain as <br />streamflows from the spring runoff subsided. Burdick et. al. recommended <br />that deep gravel-pit ponds be rehabilitated by reshaping and sloping the <br /> <br />24 <br /> <br />I <br />I <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />J <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />I <br />