LAUSR

Colorado pikeminnow Ptychocheilus lucius, a large, endangered, piscivorous cypri nid once abundant throughout warm-water reaches of North America’s Colorado River system, has been reduced to 2 wild populations inhabiting the Colorado and Green rivers. Status and trends of these remaining populations were unknown when a recovery program was initiated in 1987. During 1991 to 2013, we used mark-recapture to monitor the smaller Colorado River population. Adult abundance was estimated and patterns of recruitment and dispersal assessed to determine if recovery actions produced a population response. In 1992, adults were rare (N̂ = 345; 95% CI = 216 to 583) in the 288 km study area, but recruitment of a strong 1986 year class began a positive trend, and adult estimates reached 674 (95% CI = 517 to 897) by 2008. A significant decline then ensued, and by 2013, an estimated 282 adults remained (95% CI = 204 to 407). Annual adult survival was relatively high and stable. Juvenile survival was variable, making catch rates of young-of-the-year unreliable predictors of later recruitment strength. An estimated average of 6.5 fish immigrated annually to the Colorado River from the larger Green River population, and movements appeared balanced in direction. Though self-sustaining over the 23 yr study, low abundance and a recent rapid decline suggest long-term population persistence is tenuous. Population increase is impeded by a high frequency of weak recruiting cohorts. Results suggest 25 yr of recovery efforts have not sufficiently addressed ongoing threats affecting recruitment, including river regulation, non-native fish invasions, and other potential threats yet to be evaluated.