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electrofishing fields normally used for sampling, on sub-adult Colorado pikeminnow (mean = <br />337-mm TL). An X-ray analysis of Colorado pikeminnow captured by electrofishing also <br />suggested low incidence of damage to vertebrae (Hawkins 2002). Further, recapture rates of <br />Colorado pikeminnow tagged and released throughout the Upper Colorado River Basin were not <br />different for groups of fish first captured by electrofishing or nets (trammel or fyke nets, Hawkins <br />2003). A final and compelling reason that electrofishing may not be a main reason for increased <br />mortality of Colorado pikeminnow from 2000-2003 was that the relatively high survival rate <br />(0.82) estimated for Colorado pikeminnow in the Green River Basin in the period 1991 to 1999 <br />was from fish that were captured and recaptured exclusively with electrofishing gear similar to <br />that used during this study. Collectively, information presented above does not suggest that <br />increased mortality due to electrofishing effects was the likely main cause of increased mortality <br />of Colorado pikeminnow in the Green River Basin in the period 2000-2003. Additional years of <br />sampling may yield information on whether large fish were in fact present in the area during <br />sampling and were simply difficult to capture, or if they were not present. <br />The relatively low mortality rates estimated for Colorado pikeminnow in the Colorado <br />River from 1991-1994 (Osmundson et al. 1997; Osmundson and Burnham 1998) were derived <br />from fish captured exclusively by trammel nets set in backwaters. Estimates of survival during <br />more recent capture-recapture sampling that used both electrofishing and trammel nets as capture <br />techniques are not yet available. Comparison of survival rates of Colorado pikeminnow in the <br />two periods when different mixes of gear types were used may shed additional light on wether <br />electrofishing may increase mortality rates. Estimation of survival rates of Colorado pikeminnow <br />first captured with either electrofishing or trammel nets may also be a useful technique to assess <br />potential effects of electrofishing (Hawkins 2003). <br />49