Chemical removal of blue tilapia,Oreochromis aureus, from the South Fork of the Muddy River, Moapa, Clark County, Nevada

Authors: Kevin Guadalupe

Year: 2012 (xliv)

Abstract

Chemical removal of blue tilapia, Oreochromis aureus, from the South Fork of the Muddy River, Moapa, Clark County, Nevada Blue tilapia, Oreochromis aureus, was first discovered in the Muddy River in 1991. Tilapia quickly invaded the Warm Springs area after dam removal in 1995. Tilapia has been reported to prey on native fishes including the Moapa White River springfish, Crenichthys baileyi moapae and the endangered Moapa dace, Moapa coriacea. Tilapia chemical removal was first attempted in the South Fork Muddy River on December 9, and 16, 2009. This barrier isolated headwater stream to the Muddy River was believed clear until tilapia were rediscovered during Moapa dace surveys on August 3, 2011. The South Fork of the Muddy River is a low gradient; low flow velocity impounded stream supplied entirely by numerous warm springs and seeps throughout. Thermal stream conditions combined with dense underwater and emergent vegetation makes it unfavorable to the mixing and carrying of rotenone throughout the system. Complex habitat increases the potential that rotenone did not reach lethal concentrations in all locations, resulting that some fish may have survived during rotenone applications in 2009. Intensive vegetation removal, reservoir application techniques, and four rotenone treatments spaced over two months in 2011 allowed for physical conditions to change, changing exposure conditions for the target fish. Adequate spacing also allowed potential tilapia eggs time to hatch and become vulnerable to subsequent treatments. Snorkel surveys throughout 2012 show that tilapia are currently absent from the South Fork of the Muddy River. Removal of tilapia is an essential step in removing threats to Moapa dace and other Muddy River native fishes.