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previous trips were considered the marked portion of the population. Unique fish are individuals that are captured within a trip, but do not include subsequent captures of that same fish during the same trip. Recapture events occurred during the second spring trip (26 April to 7 May), and during the second fall trip (20 October to 31 October). The target population was all HBC ~ 150 mm. However, frequently the target and sampled population (Le., the size specific component of the population that is effectively sampled) differ, and it is only possible to estimate the abundance of the sampled population. Therefore, we first examined our data to define our sampled population. Bernard and Hansen (1992) suggest setting the lower boundary of the sampled population equal to the length of the smallest fish recaptured. However, we allowed for growth and measurement error that could have occurred between the marking and recapture events (10 mm). Provided that the smallest recaptured fish was within the expected growth rate curve for HBC in the LCR (Robinson and Childs 2001), we did not truncate our lower boundary for the estimate. We also did not truncate the upper end of our estimates, since the types of hoop nets used in our study have been shown to effectively capture large HBC in previous studies (Gorman and Stone 1999). The Chapman modified Petersen two-sample mark-recapture model (Seber 1982) was used to estimate the abundance of the sampled population. Assumptions associated with this estimator are: ~ 1. .~~c '>l\\) 2. 3. The population is closed, with no additions or losses between marking and recapture events either through recruitment, immigration, mortality, or emigration. Marking does not affect capture probability during the recapture event. All HBC in the target population have an equal probability of capture during the marking event or the recapture event; or marked fish mix completely with unmarked fish prior to the recapture event. 4. Marks (tags) are not lost between the marking and recapture events. 5. All marked fish captured can be recognized from unmarked fish. The first assumption, addressing population closure, could potentially be violated in this system since HBC in the LCR have free access to the mainstem Colorado River. We attempted to minimize potential for violation of this assumption by only allowing a short time span (less than a month) to elapse between our mark and recapture events. It was also assumed that growth related recruitment was minimized due to the short time span between marking and recapture events. The first assumption has a higher probability of being violated during spring than during fall mark-recapture events. HBC movement and migration is known to occur during the spring of the year (Kaeding and Zimmerman 1983; Douglas 15