Capture-recapture sampling of 819 km of streams in the Green River subbasin, Utah and Colorado, of the Colorado River basin was conducted in 2000-2003 to estimate population demographic parameters and recovery status for endangered Colorado pikeminnow Ptychocheilus lucius. Parameter estimates from a Huggins robust-design, multistratum model suggested a basinwide decline in the abundance of adult (total length, >= 450 mm) Colorado pikeminnow. Reductions were most severe in the middle Green River (59%) and White River (63%), the two largest population segments. Reductions were less severe in the Yampa River (29%), Desolation-Gray Canyon (11%), and lower Green River reaches (36%). In 2001, the first year the entire subbasin was sampled, adult Colorado pikeminnow abundance was estimated at 3,304 fish (95% confidence interval, 2,900-3,707) but declined to 2,142 fish (1,686-2,598) by 2003, a 35% reduction. However, accounting for a reach not sampled in 2000 makes it likely that the reduction was 48% over the 2000-2003 period. The abundance of recruits (400-449 mm) in the study area averaged 8.9% of adult abundance. The annual survival rate for 2000-2003 was 0.65 (0.59-0.71), lower than the 0.82 rate estimated for 1991-1999; mortality exceeded recruitment. The few captures and lack of recaptures of fish of at least 800 Tom suggest that apparent survival was low for large Colorado pikeminnow. The reduced abundance of adult Colorado pikeminnow was due, in pan, to the weak year-classes of age-0 fish produced in the 1990s and the reduced survival of recruits and adults in 2000-2003. Based on these results, downlisting of this species is not warranted at this time. Better understanding of the factors that influence adult survival rates, abundance dynamics, and the recruitment of early life stages as well as the effects of flow recommendations and normative predaceous fishes would assist the conservation of Colorado pikeminnow.
