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effective for capturing juvenile Gila and may be a useful technique for monitoring humpback chub reproductive success. Chart and Lentsch (1999) also had good success seining juveniles in Westwater Canyon. One drawback was the difficulty in field identifying humpback and roundtail chub juveniles and, recently, bonytail, which were reintroduced to the Yampa River in 2000. However, using the same field criteria in the laboratory on collections from Island Park confirmed the field identifications (as per Darrel Snyder, personal communication), suggesting that with training good field identification is possible. Population estimate. We were unable to make a satisfactory population estimate. Our study design met most of the assumptions for capture-recapture studies (Otis et al. 1978). The population was geographically closed because we sampled most of the occupied range and because humpback chub movement is primarily local during summer (Douglas and Marsh 1996, Valdez and Ryel 1995). It was demographically closed because we were able to make three passes within 15-20 d, during which time mortality and recruitment was assumed to be minimal. We assumed tagged fish behaved similar to untagged fish because PIT tags are small and unobtrusive, although stress due to electroshock and handling is a potential problem. Nevertheless, we were unable to recapture enough tagged fish for a reliable population estimate. We tagged and released 83 fish and recaptured only three, resulting in a probability of capture per pass of about 0.03 Simulations showed that for a three pass estimate we would need to get this probability up to near 0.10 for a satisfactory (bias < 10%, CV < 0.02) estimate. This seems infeasible considering we already were using two electrofishing rafts, and additional electro fishing would probably stress fish with little chance to improve the value of the estimate, 14