Wildlife and Anthropogenic Changes in the Arid Southwest - Design with the Desert: Conservation and Sustainable Development - page 178

recent warming trend in the temperature of the Colorado River because of drought, a sig-
nificant effort being launched to remove non-native fish and because of moving the species
into habitat previously unoccupied (i.e., translocation efforts).
The razorback sucker was listed as endangered in 1991 as a result of population declines
and range contractions (Figure 9.10). As with other southwestern fishes, these problems
were largely caused by the establishment of non-native predatory fishes and habitat
alterations associated with water development. Historically, this fish was widespread in
the Colorado River basin, ranging from Wyoming to the Gulf of California in Mexico.
Currently, wild riverine populations only exist in small and isolated patches in the upper
Colorado River basin above Lake Powell. 71 The largest remaining lacustrine population
occurs in Lake Mohave, although this population has also suffered precipitous population
decline, plummeting from the hundreds of thousands to only 44,000 in 1991 and fewer
than 3,000 in 2001. 72 As of 2008, fewer than 50 wild adults are thought to be remaining
in Lake Mohave. 73 The primary reason for their dramatic decline in Lake Mohave is that
natural recruitment is precluded by predation on early life stages by non-native fishes.74,75 74,75
Although biologists have undertaken concerted efforts to bypass the natural recruitment
process and to maintain wild genetic diversity via capturing wild razorback larvae and
growing them to larger size classes in hatcheries and in isolated off channel habitats, fol-
lowed by release back into Lake Mohave, the problems with severe non-native predation
may be nearly insurmountable.
The Rio Grande River, the largest of the Chihuahuan Desert, has been subjected to alter-
ation by multiple impoundments, channel incision, and water diversions in order to meet
the ever growing demands of civilization. The Rio Grande system is operated to reduce
flood threats and to supply water for irrigation and municipal and industrial uses via a
complex system of dams, ditches, and conveyance channels; as a result, substantial por-
tions periodically dry over the past few decades. 76
The Rio Grande silvery minnow was declared an endangered species in 1994. Historically,
this small fish occurred from northern New Mexico to near the Gulf of Mexico as well as in
the Pecos River, a tributary of the Rio Grande. Today, the silvery minnow is thought to only
occur in a 168 mile section of the middle Rio Grande between Cochiti Dam to Elephant
Butte Dam, which passes through the metropolis of Albuquerque, New Mexico. 77 This is
about 7% of its former range, in a stretch of river split into four discrete reaches by three
dams. Four other native fishes that have been extirpated in this reach of river include the
speckled chub, Rio Grande shiner, phantom shiner, and bluntnose shiner, all thought to
have shared similar ecological attributes with the silvery minnow. 78
Like the Colorado River, the historic Rio Grande was characterized by a diverse native
minnow assemblage. However, unlike the humpback chub, the silvery minnow is short-
lived (1-2 years), and its population dynamics is likely more susceptible to short-term
adverse environmental changes, such as a few years of low river flows. Like most south-
western native riverine fishes, the silvery minnow appears dependant upon annual spring
runoff to cue spawning activity, followed by a sustained period of high river discharge
that inundates complex shoreline habitat and creates backwaters where growth into larger
size classes occurs. These natural attributes are often lacking in today's riverine systems,
where artificial floods (dam releases) to improve habitat for native fishes may only last
for a day or 2 in order to conserve water. In addition, smaller fish species that can rapidly
undergo population growth and decline are sometimes artificially augmented by stocking
during periods of low abundance. Sometimes, this appears to be helpful, but more often
stocking activities can lead to a wide variety of detrimental genetic issues. 79, 80 Finally, rivers
that are kept at low volumes via man-made activities can concentrate non-native predator