Historical Changes in Large River Fish Assemblages of the Americas
Historical Changes in Fishes of the Virgin-Moapa River System: Continuing Decline of a Unique Native Fauna
Paul B. Holden, James E. Deacon, and Michael E. Golden
Abstract.—The Virgin–Moapa River system supports nine native fish species or subspecies, of which five are endemic. Woundfin Plagopterus argentissimus and Virgin River chub Gila seminuda are endemic to the main-stem Virgin River, whereas cooler and clearer tributaries are home to the Virgin spinedace Lepidomeda mollispinis. Moapa dace Moapa coriacea and Moapa White River springfish Crenichthys baileyi moapae are found in thermal springs that form the Moapa River, and Moapa speckled dace Rhinichthys osculus moapae is generally found below the springs in cooler waters. The agricultural heritage of the Virgin–Moapa River system resulted in numerous diversions that increased as municipal demands rose in recent years. In the early 1900s, trout were introduced into some of the cooler tributary streams, adversely affecting Virgin spinedace and other native species. The creation of Lake Mead in 1935 inundated the lower 80 km of the Virgin River and the lower 8 km of the Moapa River. Shortly thereafter, nonnative fishes invaded upstream from Lake Mead, and these species have continued to proliferate. Growing communities continue to compete for Virgin River water. These anthropogenic changes have reduced distribution and abundance of the native Virgin–Moapa River system fish fauna. The woundfin, Virgin River chub, and Moapa dace are listed as endangered, and the Virgin spinedace has been proposed for listing. In this paper we document how the abundance of these species has declined since the Endangered Species Act of 1973. Currently, there is no strong main-stem refugium for the Virgin River native fishes, tributary refugia continue to be shortened, and the Moapa River native fishes continue to be jeopardized. Recovery efforts for the listed and other native fishes, especially in the Virgin River, have monitored the declines, but have not implemented recovery actions effective in reversing them.