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EXECUTIVE SUMMARY Adults and larvae of razorback sucker were sampled in the Green River Basin, Utah and 1 1 ii Colorado, from 1996 to 1999 to assess their status and monitor population trends. Data from other studies and years were added to enhance the strength of the relatively sparse data set. The wild adult population in the middle Green River, Utah, from 1985 to 1992 was small at about 300 to 600 adults but recruitment was thought sufficient to replace annual mortality. Sampling since 1992 indicated that wild razorback suckers remained in a large portion of the Green River, including the lower Yampa River and the middle and lower Green River. Negligible change in total length (TL) of razorback suckers in the middle Green River, based on recaptures of PIT- tagged fish since 1990, mirrors the findings of other investigators for this and other populations of this species. Abundance estimates conducted with data collected since 1992 suggested a substantial decline in the number of wild adult razorback suckers in the middle Green River when compared to earlier estimates. On average, the most recent and reliable estimates from 1998 and 1999 data suggested that only about 100 (95% CI, 80 to 180) wild fish remained. Survival rate estimates calculated from 1990 to 1999 data were similar to the earlier period and suggested that recruitment failure was the reason for population decline. Larvae were captured in each year from 1996 to 1999 in the lower and middle Green River and sporadically in the lower Yampa River. Concurrent with declines in adult abundance, captures of larvae have also declined throughout the Green River Basin since about 1994. Wild razorback suckers may soon disappear from the Green River system entirely. Reasons for decline and suggestions for future monitoring of stocked razorback suckers are discussed. iv