Laserfiche WebLink
<br />l 1124 I <br /> <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 3 <br /> <br />22% of the rainbow trout found below the LCR confluence in both months. Comparison <br />of the January final abundance with the initial abundance in February at all three <br />geographic scales suggests minimal net immigration into the removal reach. In February <br />1,898 rainbow trout were removed resulting in an ending total abundance estimate of <br />1,037 fish and a removal efficiency of 65%. <br /> <br />The March estimate of initial rainbow trout abundance within the entire reach was 1,978 <br />fish, suggesting an overall net immigration of 941 rainbow trout since the end of the <br />February trip. A comparison of the final February abundance with the initial March <br />abundance shows virtually no net immigration in the downstream reach, but nearly a <br />doubling of abundance in the upstream reach. Additionally, the estimare of initial <br />rainbow trout abundance during March suggests that only 15% of the total abundance <br />was below the LCR confluence. These two observations suggest that there may have been <br />- - - ---- -lm':"'nj'-gr-afl'.n" "'O....-:.;:~"-,::;'~C"l~...,' --;.,;;,.flf.(.-.....t. -..-:..,... 1... ,~..~. '.. ., - . -.' l.o'a'I"o.r~ ..- <br />. _.1':1 ~ "I.. -. ".".." _.>.. ...e...._,,1 .:......_ a."............., V,,:,," Vala.: "'.LUUU."U~''''''''''L1Cc:UJl ....a...,u. h l J <br />1,196 rainbow trout was removed during March, which resulted in a final abundance <br />estimate of 782 fish and a removal efficiency of 60%. Approximately 8% of the rainbow <br />trout were concentrated below the LCR confluence (66 fish) following the March trip <br />and nearly 92% resided in the upstream reach (687 fish). <br /> <br />Brown trout. Brown trout depletion abundance estimates are somewhat problematic. The <br />desired negative linear relationship between cumulative catch and catch rate was not <br />always realized. However, there was a systematic and large reduction in both catch rate <br />and total removals from January to February, and to a lesser degree from February to <br />March. Removal efficiency estimates varied from a low of26% in February to a high of <br />65% in January. This variability is indicative of differences in catchability between and <br />within sampling months, and is suggestive of differences in immigration rates or <br />shoreline habitat use. <br /> <br />Common carp, Estimates of depletion for common carp, as evidenced by model fit and <br />consistency, were generally better than brown trout, but not as good as rainbow trout. For <br />data collected during January and March, abundance estimates in the upstream section <br />were not possible owing to a pattern of increasing catch through the depletion passes. <br />However, the estimators of abundance for both the total reach and the downstream reach <br />c::.r.i::;istently j::cj:.iCC~ i=~sitive c.)tirnCites. i"Ol-" the h::':al {ei~Hj-i~i ll!~4, Jlt: JaaJU~'1 firial <br />estimate was identical to the February initial estimate, 'suggesting reasonably good <br />performance'and low'nehmmigration:-Catch-rate estimates and total removals declined <br />between January and February in the downstream section and in total, but increased <br />slightly within the upstream section. Removal efficiencies for common carp, which <br />ranged from 67% to 92%, were higher than either brown trout or rainbow trout. <br /> <br />PROPOSED MODI FICA TION TO MECHANICAL REMOVAL <br /> <br />The proposed modification does not include any change in experimental releases from <br />Glen Canyon Dam. It includes only the mechanical removal of non-native fish. The <br />effectiveness of the ongoing non-native removal has exceeded expectations at the <br />initiation of this action and this realization has lead to a proposal to expand the removal <br />