Fisheries survey of the Virgin River from Halfway Wash to Lake Mead

Golden, Michael; Holden, Paul

In order to assist in evaluating the various alternatives for water development in the lower Virgin River, SNWA contracted BIO-WEST to survey the fish community from the Halfway Wash area to Lake Mead in 2004. Additional surveys were completed in 2005 after severe flooding and high spring runoff significantly altered habitat in the Lower Virgin River. The following report presents the results of that survey and compares those results to historical information on the fish community below Halfway Wash... EXECUTIVE SUMMARY Halfway Wash (RM 41.25) is the lower boundary of critical habitat for two endangered fish species of the Virgin River, the woundfin (Plagopterus argentissimus) and the Virgin River chub (Gila seminuda). The area around Halfway Wash has often been considered one of the best places for a water diversion or fish barrier in the lower Virgin River. However, a barrier would isolate fishes in the area below it from the community above. If native fishes do not occupy the area below Halfway Wash, then installing a barrier in this area may effectively keep nonnative fishes from migrating upstream without negatively impacting native fish populations. The Southern Nevada Water Authority (SNWA) is considering several options to develop its water rights on the Virgin River at Halfway Wash. This water development will probably result in reduced flows below Halfway Wash. Additionally, water development may involve the placement of a diversion somewhere near Halfway Wash. In order to assist in evaluating the various alternatives for water development in the lower Virgin River, SNWA contracted BIO-WEST, Inc. (BIO-WEST) to survey the fish community from the Halfway Wash area to Lake Mead in 2004. BIO-WEST surveyed a 14.5-mile (23.3-km) section of the Virgin River from Halfway Wash to the current confluence of the Virgin River with Lake Mead in April 2004. Only one native fish, a young-of-the-year flannelmouth sucker (Catostomus latipinnis) was collected. Additional surveys in July, August, and September 2004 found no native fish. Red shiner (Cyprinella lutrensis), western mosquitofish (Gambusia affinis), and common carp (Cyprinus carpio) dominated the catch throughout the study area. Blue tilapia (Oreochromis aurea) were collected in a 2-mile (3.22-km) section of the Virgin River immediately above Lake Mead in July, August, and September 2004. An area of extremely dense vegetation in the channel upstream from this area appeared to prevent blue tilapia migration upstream. A large flood event in January 2005 radically changed habitat in the Virgin River and helped to end a 5-year drought in the basin. The SNWA contracted BIO-WEST to perform two follow-up surveys from Halfway Wash to Lake Mead in August 2005 and September 2005. The 2005 surveys were conducted to determine the extent to which the floods and high spring runoff altered habitat and changed the fish community below Halfway Wash. Surveys of the Halfway Wash to Lake Mead study area in August and September 2005 showed that habitat had been altered dramatically by flooding and spring runoff earlier that year. However, red shiner and western mosquitofish continued to dominate the catch rate, and two speckled dace (Rhinichthys osculus) were the only native fishes collected. We collected blue tilapia as far upstream as RM 40.5, and we collected striped bass (Morone saxatilis) for the first time in the Virgin River.

Fisheries survey of the Virgin River from Halfway Wash to Lake Mead

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