9565 - Laserfiche WebLink

Home Browse Search

1374 evaluations did not suggest that electrofishing was harmful to Colorado pikeminnow. Controlled labora- tory studies (Meismer 1999) found minimal soft-tissue and vertebral damage from the electro fishing gear at intensities normally used for sampling sub adult Colorado pikeminnow (mean TL = 337 mm). An X- ray analysis of Colorado pikeminnow captured by electrofishing (n = 46) also suggested a low (7%) incidence of acute damage to vertebrae related to electrofishing capture, and no mortalities were ob- served (Hawkins 2002). Further, the recapture rates of Colorado pikeminnow tagged and released throughout the upper Colorado River basin were not different for groups of fish first captured by electro fishing compared with trammel or fyke nets (Hawkins 2003). A final and compelling reason that electro fishing was not a major reason for the increased mortality of Colorado pike- minnow in 2000-2003 is that the relatively high survival rate (0.82) estimated for Colorado pike- minnow in the Green River basin in 1991-1999 pertained to fish that were captured and recaptured exclusively with electrofishing gear similar to that used during this study, albeit sampling intensity was higher in the recent period. Collectively, the information presented above suggests that the increased mortality of Colorado pikeminnow in the Green River basin in 2000-2003 was caused by factors other than electro- fishing or handling. Future sampling may yield additional information on whether very large fish were present in the area during sampling and were simply difficult to capture, learned to avoid the capture gear, used habitat where capture gear was not effective, or were not present. Drought-reduced stream flows may also be a reason for the lower survival rates of Colorado pikeminnow in the Green River basin from 2000 to 2003. This may be particularly true in the White River, which was the smallest stream sampled during this study. Base flows were relatively low in all years from 2000 to 2003, and particularly so in 2002. For example, the mean monthly flow levels of the Green River at Jensen, Utah (U.S. Geological Survey gauge 09261000) in July 2000, 2001,2002, and 2003 were the 8th, 3rd, 2nd, and 7th lowest, respectively, that have been recorded since gauge operation began in 1947; other years oflow July flows were 1961 (5th), 1963 (1st), 1989 (4th), and 1994 (6th). Because summer base flows were typically higher in the postdam era than before, the occurrence of low summer base flows in 2000-2003 is particularly significant. The mean annual flows for water years 2000, 2001, 2002, and 2003 were the 19th, 7th, 2nd, and 9th lowest, respectively, that have been recorded in the 57-year period of record. The exact mechanisms for the increased mortality BESTGEN ET AL due to drought or variable declines in abundance over reaches are not apparent but could be decreased habitat size, the density-dependent effects of crowding pisciv- orous Colorado pikeminnow and other, nonnative predators into reduced habitat and thus reducing their food supply, the increased incidence of disease, and other, unknown factors. The poorer condition of Colorado pikeminnow in 2000-2003 than in 1991- 1999 indicated by the differences in length-weight relationships in the two periods may be responsible, in part, for the reduced survival in drought-affected streams. Given the rather severe reductions in stream flow, perhaps the reduced abundance of Colorado pikeminnow in the Green River basin during the study period is not smprising. Reductions in habitat size may have also reduced survival by increasing the potential for encounters with predaceous nonnative fishes. Northern pike, which utilize the same deepwater habitat as Colorado pike- minnow and are abundant in the Yampa River and present in the middle Green River reach, have the potential to consume directly or to inflict fatal bite damage to large adult pikeminnow. Smallmouth bass Micropterus dolomieu have also increased dramatically in distribution and abundance in the upper Colorado River basin, including the Yampa and main-stem Green rivers. Smallmouth bass are documented predators on stocked endangered bony tail Gila elegans in the Green River and may be significant predators on other native fishes, particularly those smaller than about 200 mm TL (Bestgen et al. 2006b). Recruitment Rates Reduced recruitment may also be responsible, in part, for the decline of adult Colorado pikeminnow in the Green River basin. Recovery criteria define a recruit as a subadult Colorado pikeminnow 400-449 mm TL (U.S. Fish and Wildlife Service 2002). Based on expected growth of about 43 mm/year for Colorado pikeminnow in that size-class (Osmundson et al. 1997), the average recruit-size Colorado pikeminnow would become part ofthe adult population (2:450 mm TL) the following year. The expectation of this recovery goal is that recruitment will, over time, balance the mortality of adults to produce a stable adult population. The percentage of recruits in the samples collected during the period 2000-2003 (4.9-13.3%; Table 4) suggests that recruitment was not sufficient to offset the 35% apparent annual mortality rate (I - S = mortality X 100) estimated for adult Colorado pikeminnow in the Green River basin. The number of recruits in ISMP samples in 2000- 2003 was only 3.4% of the number of adults in the samples and much reduced from 1991-1999, when .