Stocking of Endangered Razorback Suckers in the Lower Colorado River Basin over Three Decades: 1974–2004

Abstract

The razorback sucker Xyrauchen texanus was historically widespread and abundant throughout the larger streams of the Colorado River basin, ranging from Sonora, Mexico, to Wyoming. The species was federally listed as endangered in 1991 because it has been extirpated from most of its range. Its decline is attributed to habitat loss and predation by nonnative fishes. Thirty years of federal and state effort have resulted in stocking millions of razorback suckers to the lower Colorado River basin, but only a few individuals have been recaptured because the young are rapidly consumed by introduced predators, resulting in insufficient recruitment to adulthood. Elderly, wild adults of this long-lived species are vanishing, and lower Colorado River basin recovery efforts now focus on replacement of these fish with repatriated (or reintroduced) adults. Stocking success and subsequent survival increases with size at release. When estimates of size-based, first-year survival rates were applied to individual batches of repatriated fish, we observed less than 1% overall first-year survival, and most fish stocked to date are thought to have been consumed soon after release. Overall, stocking has been unsuccessful, long-term survival is unknown, and no new populations have been established.