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from ISMP data, which was collected from about 23% of the range of Colorado pikeminnow in S the Green River basin and in one sampling pass. More precise X estimates, inclusion of reach specific survival rates or probabilities of capture, or estimates for individual river reaches would require more intensive data collection. Decisions about appropriate monitoring tools should evaluate the costs associated with a particular technique relative to the importance and precision . of the particular estimates obtained. Effects of increased sampling effort expended in capture- recapture studies relative to that for less intensive monitoring should also be considered. Even though we found no detectable effects of sampling on Colorado pikeminnow, the possibility remains that increased sampling may have undetected effects, behavioral or otherwise, on Colorado pikeminnow. Future sampling will also be instructive to determine if any of the Colorado pikeminnow > 800 mm-TL that were captured only once during this study persist. The relatively close regression relationships between adult Colorado pikeminnow abundance and years for the period 2000-2003, for both river-wide and reach estimates, was striking because they suggested a relatively consistent rate of year-to-year decline, but one that varied among reaches. This was not an artifact of the abundance estimation model because probabilities of capture were computed for each sampling occasion, river reach, and year and S was held constant. Instead, the consistent rates of decline suggested that chronic, ongoing effects caused decline of adult Colorado pikeminnow throughout the Green River Basin. However, as noted above, rates of decline varied in various river reaches. Survival rates.--Decline in abundance of adult Colorado pikeminnow over the study period in the Green River Basin may be attributable to reduced survival, reduced recruitment, or both. Apparent survival for the period 1991-1999 was higher than survival estimated for the more recent 2000-2003 period. The recent rate is also lower than survival rates of adult Colorado 46