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techniques were quite low in Black Rocks and they were used sparingly in this study. Stress and injury are also associated with these sampling techniques (Reynolds 1996). Hoop nets were effective in the confined reaches of the Little Colorado River in Grand Canyon (Gorman and Stone 1999), but were not very useful in the Yampa River (Haines and Modde 2002). In addition to negative impacts to existing populations, delayed mortality or behavioral problems associated with stressed fish that subsequently recover could also mean that numbers reflect overestimates because tagged fish are not captured at the same rate as untagged fish. Equal catchability is an important assumption associated with all population estimation models (White et al. 1982). Sampling was terminated early in 2000 because of the concerns discussed above. However, the reduced catch rate in 2000 may be unrelated to stress or mortality associated with handling. Catch per effort is quite variable and may be affected by environmental conditions. Water levels were much lower in 2000 than in 1998 and 1999 and water temperatures were warmer. It is possible that the different water conditions affected distribution or movement of humpback chub within Black Rocks. As an example of how environmental conditions may affect catch rate, low water flows in spring 2002 are suspected to be a cause of reduced catch rates of Colorado pikeminnow (Ptychocheilus lucius) in the Green River compared with similar efforts in previous years (T. Modde, personal communication). However, the reduced catch rate and capture of the single humpback chub with fungus problems suggested that sampling be terminated until an evaluation could be done. Trammel nets have been used to sample humpback chub populations for at least 20 years (Valdez 1990; Kaeding et al. 1990; McAda et al. 1994; Douglas and Marsh 1996; Chart and Lentsch 1999a,b) and can continue to be used. Sampling to produce population estimates of humpback chub in Black Rocks and other locations should continue. However, sampling should be done later in the fall after water temperatures decline further and use of less effective, but potentially less stressful, techniques should be explored. 15