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7/14/2009 5:02:36 PM
Creation date
5/22/2009 12:27:16 PM
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UCREFRP
UCREFRP Catalog Number
9565
Author
Bestgen, K. R. and e. al.
Title
Population Status of Colorado Pikeminnow in the Green River Basin, Utah and Colorado
USFW Year
2007
USFW - Doc Type
Transactions of the American Fisheries Society
Copyright Material
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<br />1372 <br /> <br />BESTGEN ET AL <br /> <br />animals mixed freely between concentration habitat <br />(backwaters, shorelines, eddies, and the main channel) <br />and adjoining areas between sampling passes and that <br />sampling was spatially extensive such that all animals <br />in the population were available for capture. Evidence <br />for such mixing was provided by Osmundson and <br />Burnham (1998), who found high probabilities of <br />capture within concentration backwater habitat in a <br />single sampling pass but relatively low probabilities of <br />recapture in those same locations between passes (i.e., <br />many fish moved into and out of their concentration <br />habitat between sampling passes). In this study, we <br />demonstrated similar mixing, as initial capture proba- <br />bilities were equal to recapture probabilities among the <br />short-term sampling passes. If fish were not mixing, we <br />would expect recapture probabilities to be much higher <br />than initial capture probabilities because the same <br />shoreline habitat was sampled during each pass. Our <br />sampling was spatially extensive, although we assumed <br />lower sampling efficiency in deep pools (>2 m deep). <br />However, the amount of that habitat type relative to <br />shallower, easier-to-sample areas where Colorado <br />pikeminnow typically reside is generally small in this <br />system (Tyus and McAda 1984). We also assumed that <br />recognition of marked Colorado pikeminnow was high <br />(all were scanned) and that loss of internally placed PIT <br />tags was low (Burdick and Hamman 1993). <br /> <br />Abundance Estimates <br /> <br />Given the absence of obvious violations of the <br />assumptions underlying abundance estimation models <br />and that riverwide estimates were relatively precise, <br />conclusions regarding a decline in the abundance of <br />Colorado pikeminnow in the Green River basin over <br />2000-2003 seem reliable. We also conclude that the <br />abundance estimates presented here (and reported as <br />preliminary estimates by Valdez and Muth 2005) are <br />representative of the entire Green River basin popula- <br />tion of Colorado pikeminnow because of the high <br />fidelity of these fish to their home ranges and the low <br />and only seasonal use of reaches outside of the main <br />study area (e.g., Irving and Modde 2000; Kitcheyan <br />and Montagne 2005). <br />The abundance estimates for the Yampa River <br />suggest that the number of adult Colorado pikeminnow <br />was similar in 2000 and 2001 but that the abundance <br />declined in 2002 and 2003. Given the large differences <br />in the number of individuals captured in 2000 and 2001 <br />compared with 2002 and 2003, one might expect lower <br />abundance estimates in the latter years, particularly <br />considering the presence of large, predaceous northern <br />pike Esox lucius, which prey on Colorado pikeminnow <br />(J.A.H., personal observation). However, in each of <br />those latter 2 years, not a single fish was recaptured <br /> <br />from samples taken that year, so p* was relatively low <br />and abundance estimates remained relatively high. <br />With the exception of the 2001 estimate, definitive <br />statements about abundance levels and status trends for <br />Colorado pikeminnow in the Yampa River are difficult <br />to make. This is due to relatively imprecise estimates <br />(high CVs, wide CIs) caused by the limited number of <br />fish captured and recaptured in the Yampa River. <br />The rates of decline of Colorado pikeminnow were <br />variable among other reaches in the Green River basin. <br />The declines of adults in the White and middle Green <br />River reaches, the largest population segments, were <br />particularly severe and suggest that the factor( s) <br />influencing the apparent basinwide declines were more <br />acute there. The apparent declines in the Desolation- <br />Gray Canyon and lower Green River reaches were less <br />severe. This may be due to the larger number of new <br />recruits in those reaches relative to other reaches or to <br />the upstream movement of recruits or adults. Trends in <br />point estimates during the study period also suggest a <br />decline in the abundance of recruits in most of the <br />reaches where they occurred; the exception is the lower <br />Green River, where the number of recruits apparently <br />increased slightly in 2003. <br />We do not know how our estimates of Colorado <br />pikeminnow abundance relate to historical levels. <br />Descriptive historical accounts from the 1930s and <br />before certainly suggest higher Colorado pikeminnow <br />abundance in the past (Jordan 1891; Quartarone 1995), <br />but perhaps reductions are not surprising given the <br />degree of habitat and biotic change that has occurred in <br />the Colorado River basin. However, more recent <br />estimates (e.g., Tyus 1991), which put the abundance <br />of Colorado pikeminnow in the Green River (about <br />8,000 fish during the 1980s) well above our highest <br />estimates are probably biased upward because of <br />assumptions that were either unmet (e.g., spatial and <br />demographic closure over periods of several years) or <br />unreasonable. Also, Green River basin ISMP catch rate <br />data from the 1980s that are similar to or (often) lower <br />than data from the 1990s through 2000, particularly for <br />the large White and Green River population segments <br />(McAda 2002; Figure 8), further support the notion that <br />the abundance estimates for the 1980s are too high. <br />The population rate-of-change analysis and ISMP <br />CPUE data are useful in giving a longer temporal <br />perspective on Colorado pikeminnow population trends <br />in the Green River basin. Estimates of ^ greater than 1 <br />for most years from 1991 to 2000 and increasing CPUE <br />values suggest an increasing population. After 2000, ^ <br />was consistently much less than I and CPUE values <br />were relatively low, which suggest a declining <br />population. <br />The ISMP CPUE indices reflected the trends in <br />
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