Gila seminuda

Gila seminuda formerly was included as one named and one unnamed subspecies of Gila robusta. DeMarais et al. (1992) raised seminuda (including the Muddy [Moapa] River population) to species status, an action that has been accepted by the American Fisheries Society (Nelson et al. 2004) and USFWS. The origin of G. seminuda may have included hybridization between G. elegans and G. robusta (DeMarais et al. 1992, Gerber et al. 2001).

A number of factors limit the range of the Virgin River Chub: non-native fish, drought, altered stream-flow regimes, diversions, elevated water temperature, decreased turbidity, water management events, and a decline of optimum spawning and rearing habitat (Hardy et al. 2003, USFWS 2008a, Huizinga and Fridell 2012). Collectively, these factors have severely impaired ecosystem function in the Virgin River Basin, causing precipitous declines in Virgin River Chub. Additionally, these stressors and perturbations may be further exacerbated by extreme natural events, changing climatic trends, and periods of extended drought. Virgin River Chub monitoring data indicate that adult populations continue to persist and when conditions are favorable and are able to reproduce and recruit into the population. Over the past 15 years, with the exception of a few outlier years, drought conditions have persisted in the Virgin River Basin. Extreme water years, both high and low, can act as environmental stressors on native fish populations, limiting reproduction, recruitment, and survival.

The Virgin River Chub currently occurs in the mainstem Virgin River, upstream of La Verkin Springs in southwestern Utah, downstream to the confluence of Beaver Dam Wash in Arizona with very small numbers occurring sporadically downstream of Beaver Dam Wash in the Nevada portion of the Virgin River (USA). The occurrence of Virgin River Chub is uncommon in the lower Virgin River and is largely dependent on reproduction and downstream drift from the chub population above the Washington Fields Diversion (UDWR 2020). Additionally, a 30 km reach in the Muddy River between Warm Springs Bridge and Wells Siding Diversion persists in Nevada (USFWS 2000, Page and Burr 2011, UDWR 2019).

This species is represented by what can be regarded as two major populations, one in the upper Virgin River mainly in southwestern Utah, and an isolated population in the Muddy River in southeastern Nevada.

On-going threats to the Virgin River Chub include widespread modification and reduction of habitat; de-watering by agricultural diversion, drought, and water development; increased temperature, salinity, and turbidity of the Virgin River; increasing human population in surrounding area.

Introductions of non-native fish and parasites, particularly the Red Shiner, Cyprinella lutrensis have been particularly problematic in the lower portions of the Virgin River in both Arizona and Nevada. The Red Shiner has been eliminated in the Utah portion of the Virgin River above the Washington Field Diversion. Most of the habitat destruction that threatens the species occurred between 80 and 100 years ago. Hoover Dam was completed in 1935 creating Lake Mead, which inundated the lower 80 kilometers of the Virgin River and the lower 8 kilometers of the Muddy River. Subsequent stocking of nonnative species (illicit or authorized) in Lake Mead to develop a sport fishery and their unrestricted access to the lower reaches of the Virgin and Muddy Rivers introduced a new threat. Additional water development projects on tributaries to the Virgin River (Santa Clara, Ash Creek, and Beaver Dam Wash) continued through the latter half of the 1900s. In more recent times, the Quail Lake project (completed in 1985) and Sand Hollow Reservoir (completed in 2002) have replaced older diversion structures in the upper river and further modified Virgin River hydrology (USFWS 2008).

Errors in detoxification during an attempt to eradicate red shiners via rotenone poisoning resulted in lethal concentrations of piscicide passing through an additional 50 kilometers of stream; chubs subsequently produced through natural recruitment in poisoned reaches exhibited deviations from the original pattern of genetic variation (DeMarais et al. 1993).

Decline of the Muddy River population may have been related to cumulative effects of parasitism; changes in flow, water quality, and substrate; channelization; and the establishment of non-native fish species (USFWS 1995).

Potential threats include further water removal, desalinization, urban growth, sedimentation, pollution, and channel alteration. The naturally limited range of this species makes it vulnerable to extensive losses from localized events. One concern with non-native fish control efforts is the use of fish barriers to isolate stream reaches and prevent upstream fish migration. While fish barriers prevent upstream nonnative fish movement, they also reduce genetic diversity by preventing Virgin River chub subpopulations in the upper and lower basins from exchanging genetic material. In response, the USFWS recommended periodic relocation of chub from the lower basin to the upper basin to maintain genetic fitness.