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a <br />information on production of larvae (Bestgen et al. 1998), abundance of juveniles in backwaters <br />(Haines and Tyus 1990; Tyus and Haines 1991; Haines et al. 1998; McAda 2002), and biotic and <br />abiotic factors that may influence year-class strength and recruitment to the adult size class <br />(Bestgen 1996; Bestgen et al. 1997; Bestgen and Bundy 1998; Osmundson and Burnham 1998) <br />Probabilities of capture.--Relationships of capture probabilities and fish length suggested <br />small and very large Colorado pikeminnow had very low probabilities of capture, which can, in <br />part, be independent of abundance of that life stage. The linear term for capture probability as a <br />function of TL indicated fish up to about 580-mm TL were progressively easier to capture, but the <br />negative coefficient for the quadratic term forced the relationship to decline for large fish. We <br />hypothesized that relatively small or very large fish may be more difficult to capture because of <br />habitat use or behavioral differences. Small fish may occupy relatively shallow water or small <br />backwaters, where accessibility by electrofishing boats is limited (Tyus and Haines 1991). Large <br />fish may occupy particularly deep water where electrofishing is simply inefficient, or may be <br />powerful enough to evade the electrofishing field when it is detected. <br />The only three Colorado pikeminnow captured during this study that were > 900-mm TL <br />had not been previously tagged. A general expectation is that an adult Colorado pikeminnow <br />captured in the Green River Basin has about a 50% probability of being tagged. Based on their <br />size and presumed slow growth rates, those fish have likely been in the system long prior to 1991 <br />(Osmundson et al. 1997). Sample size not withstanding, the first capture of those large fish <br />during this study (one died after release) was surprising given the level of sampling effort <br />expended in previous sampling efforts (e.g., ISMP). Further, those and other large fish were never <br />seen again in this study, despite relatively high levels of sampling effort, supported the notion that <br />very large fish may be more elusive to capture techniques used here. However, we also believe <br />56