ABSTRACT
Throughout the last century, the Colorado River’s natural flow regime was drastically altered by the infrastructure of water regulation. The once dramatic seasonal and annual variations in water flows, which shaped the life histories and unique morphologies of its native fishes, have been subdued by the many impoundments throughout the river. Bluehead sucker, flannelmouth sucker, speckled dace, bonytail, Colorado pikeminnnow, humpback chub and razorback sucker are fish native to the Colorado River Basin, the latter four being critically endangered endemics. The native fish face two major threats: (1) habitat degradation, caused by cold, sediment-starved, and relatively flat-lined water releases, and (2) nonnative fishes, introduced since the late 1800s. Both of these threats work together in precluding the recruitment of the natives. The cold, clear, hypolimnetic releases degrade the natives’ backwater rearing habitat and cause near-zero growth rates in their young-of-year offspring, while the introduced fishes predate upon the young-of-year and juveniles. The current and proposed management strategies for restoring and/or perpetuating the fish native to the mainstem of the lower Colorado River include experimental flows, mimicking the natural flow regime, tailwater warming, nonnative fish control, and artificially-maintained populations. A holistic, ecosystem-based approach, using a combination of these management options, will likely be needed if the sustainable recovery of the lower Colorado River’s native fishes is ever to be achieved.