Fisheries survey of the lower Virgin River, Beaver Dam Wash, Arizona to Lake Mead, Nevada July 1993-December 1997

Holden, Paul; Abate, Paul

This report details results of studies conducted from 1993 to 1997 that were done to determine the status of the woundfin as well as other important fish species in the Virgin River below Riverside, Nevada. The results of the studies show that reproducing populations were found in the lower Virgin River and that habitats important during clear flow periods were small pools and runs below riffles.... EXECUTIVE SUMMARY In 1993 the Southern Nevada Water Authority and the Nevada Division of Wildlife initiated a cooperative study of the fish fauna of the Virgin River from Riverside, Nevada, to Lake Mead, Nevada. Emphasis was placed on woundfin (Plagopterus argentissimus), an endangered fish once common in this portion of the river. In 1996 the study was expanded to include all of the Virgin River below the Virgin Gorge in Arizona. The primary objectives of the study were to detem1ine woundfin presence and abundance in the river and which reaches were most impmiant to this species and other native fishes. Sampling with seines was initiated in July 1993 and concluded in October 1997. A total of28 sampling trips (three to eight trips per year) were made during the 5 years of the study. Flows during the 5 study years varied from very low to very high, with 1994 and 1996 being low flow years and 1993 and 1995 being high flow years. Portions of the lower Virgin River were dry in summer 1994 and 1996 (flow less than 5 cfs) due to inigation diversions, whereas during the other years summer flows were generally greater than 50 cfs. Woundfin were found during the second trip in August 1993 and during every trip thereafter. The greatest numbers of woundfin were captured during low flow summers when they were concentrated in small pools and runs below riffles. Young-of-the-year were found in 1994 and 1995. The expansion of the study area in 1996 and 1997 resulted in woundfin captures near Mesquite, Nevada, but only very few were found near the mouth of Beaver Dam Wash, Arizona. The data collected during the 5-year study indicate that a population ofwoundfin persisted in the lower Virgin River in spite of very low flows during some summers and large numbers of nonnative red shiner ( Cyprinella lutrensis) which compete and/or prey on woundfin. Sufficient reproduction occuned in the woundfin population to make it self-sustaining, although some downstream movement ofwoundfin from upstream Utah populations may also have occuned. The population below Riverside was the largest in the lower Virgin River and was likely comprised of a few hundred adults. The population near Mesquite was smaller. No population appeared to occur near the mouth of Beaver Dam Wash, an area that contained thousands of woundfin up until the early 1980s. Substrate in this area had changed from a dominant sand type in the 1970s to a cobble type, favoring native speckled dace (Rhinichthys osculus), deseJi sucker (Pantosteus clarki), flannelmouth sucker (Catostomus latipinnis), and the endangered Virgin chub (Gila seminuda). Habitat use by woundfin during low, clear flow periods was different than during more normal flow periods. Sand substrate runs are typically used, but pools and runs below riffles were selected during low, clear flow periods.

Fisheries survey of the lower Virgin River, Beaver Dam Wash, Arizona to Lake Mead, Nevada July 1993-December 1997

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