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<br />. . <br /> <br />. " <br /> <br />Proposed Modification to Mechanical Removal of Non-native Fish <br />Bureau of Reclamation, U.S. Geological Survev, National Park Service <br /> <br />July 25,2003 <br />Page 2 <br /> <br />, <br /> <br />(primarily rainbow and brown trout), and c) reduce the abundance of non-native fishes in <br />the control reach as much as practicable. <br /> <br />The proposed action was to conduct six removal trips per year below Lees Ferry, from <br />river mile (RM) 56.2 -65.7, during 2003 and 2004. To determine if differences in fish <br />population characteristics (e.g., relative abundance and size structure) in the removal <br />reach are a function of other environmental influences and not the mechanical removal, a <br />control area was established (RM 44-52). During each trip, 24 randomly selected sites <br />within the control reach are sampled to estimate the relative abundance and size structure <br />of native and non-native fishes inhabiting this reach. <br /> <br />Mini-hoopnets are used to estimate the relative abundance (catch rate) ofHBC at <br />standardized sites downstream of the LCR confluence. Nets are deployed for three nights <br />da..--ing the remcv~l ,operations ~t times when ~!er:tr0fishjne :-1~tiviti~~.'arp. not .heing_ <br />conducted at hoopnet sites. <br /> <br />RESULTS OF THE ONGOING MECHANICAL REMOVAL <br /> <br />. Total Samoling Effort and Catch <br /> <br />A total of 21 ,304 minutes (355 hours) of electrofishing effort was expended during the <br />January, February, and March 2003 removal trips (Coggins and Yard 2003). Total <br />electro fishing catch in the removal reach was 7,573 fish. The numerically dominant <br />species was rainbow trout with a combined catch over the three trips of6,703 (89%). <br />Catches of the remaining non-native fishes represented about 4% of the total catch with <br />brown trout (130 fish) and common carp (135 fish) being the predominant species. Native <br />fishes constituted 7% of the total catch: flanne\mouth sucker (430 fish), humpback chub <br />(65 fish), bluehead sucker (37 fish) and speckled dace (17 fish. <br /> <br />Deoletion Abundance Estimates <br /> <br />Depletion abundance estimates were attempted for rainbow trout, brown trout, and <br />common carp at three geographic scales: I) upstream of the LCR confluence 2) <br />downstr~a.aYI ofth-: LCR conflueii~e, 6.ild.3) ,,;'ithin.thc'entire removal reach..The Sll..'11 of <br />the upstream and doWnstream estimates does not i1ecl~ssarily equal the total reach <br />estimate because the estimators are independent and based on stratified or pooled data. <br /> <br />Rainbow trout Rainbow trout depletion data displayed the declining and strongly linear <br />pattern expected in depletion type experiments for each of the geographic scales. Initial <br />estimated January rainbow trout abundance was 6,499 throughout the removal reach. A <br />total of 3,606 rainbow trout was removed during the January trip resulting in an ending <br />abundance of 2,893 and an overall removal efficiency of 55%. <br /> <br />Initial rainbow trout abundance during the February trip was estimated as 2,935 fish. <br />Abundance estimates in the upstream and downstream sections suggest that rainbow trout <br />had a similar geographic distribution during January and February, with approximately <br />