Decline of the Razorback Sucker in Lake Mohave, Colorado River, Arizona and Nevada

Abstract

The razorback sucker Xyrauchen texanus is an endangered fish endemic to the Colorado River basin in the western United States. Once widely distributed and common throughout the basin, the species has been eliminated from most of its former range by establishment of nonnative fishes and water development, and remaining numbers have dwindled precipitously from historical levels. Although Lake Mohave, Arizona and Nevada, supports the largest and genetically most diverse remaining population, razorback sucker abundance in the lake plummeted from historical numbers in the hundreds of thousands to only 44,000 in 1991 and fewer than 3,000 in 2001. This population is limited primarily to large, old adults because predation on their larvae by nonnative fishes has precluded measurable recruitment for nearly half a century. At the current rate of decline, extirpation is anticipated within this decade because, at this time, there is no practicable method to remove the continuing threat of nonnative predators to razorback sucker larvae. As such, the persistence of razorback suckers in Lake Mohave depends on a repatriation program begun in 1991, which uses wild-produced larvae that are reared in captivity and returned to the lake at a size that is less susceptible to predation.