WSP07272

• File Number: 8220.101.10
• Description: Colorado River-Water Projects-Glen Canyon Dam/Lake Powel-Glen Canyon Adaptive Management
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 9/1/1991
• Author: USDA Forest Service
• Title: Native Fishes of Arid Lands-A Dwindling Resource of the Desert Southwest
• Water Supply Pro - Doc Type: Report/Study

formerly extensive range (that included much of the
Colorado River system), Larvae are produced by the
thousands in Lake Mohave, but disappear before
even attaining the juvenile stage. There is no
evidence for recruirment of young fish into rhe adult
population for at least 30 years, and researchers
have concluded thar they are eaten by introduced
predators. Wild'caught and hatchery-produced fish
are fertile, however, and the species readily
responded ro artificial propagarion, .
In rhe United Srates, ir is obvious rhar
modificarions of rivers, drying of reaches below
dams, channelization, and diversion of large volumes
of water from place ro place, destroy habitat and
eliminate the fauna. Stabilization of rivers is
similarly disruptive to natural communities, and
changed temperatures and flow regimes below dams
exclude native fishes. Major impacts have further
been associated with use and misuse of watersheds.
Overgrazing, logging, and other practices have
promoted increased erosion and sediment transport.
Chemical modifications include changes in nutrient
relations due to damming and watershed uses, and
dumping of injurious substances by industrial and
domestic activities.
Modificacions in Mexican river systems are nor yet
so extensive, but are of the same kinds and
magnitudes. For example, the Rio Yaqui system is
not yer as completely dammed and diverted as the
Colorado, Some changes in Mexico are, however,
equally as severe as any in the United States. Heavily
logged watersheds promoted widespread erosion and
pollution reported by Ndo Leopold and others in the
1930s, and the pressure of grazing and other
destructive land,use has long been evident, As in the
United States, diversion dams desiccate long reaches
of channel, effluents from mines and smelters
influence tributaries and mainstream alike, and
domestic sewage pollutes many streams below towns
and cities.
Native fishes tend to remain in places where
physical and chemical modifications are minimal or
non.existent, a.nd natural stream conditions and
native fishes still go hand and hand in Mexico.
However, in the United States, a substantial number
of native species are disappearing from rivers and
creeks that appear in a natural srare, which seems
inconsistent. What differences exist in conditions in
the two countries that cause different biological
resulrs? The introducrion and establishment of
non-native fishes, which replace natives through
competition for food, space, or other resource,
or simply devour rhem, seems the mosr logical
answer.
Hisroric evidence points toward a development of
a depauperate fish fauna in harsh riverine

environments of the American Southwest over the
pasr 5 ro 10 million years, and maybe longer.
Evolution selecred for ecological generalisrs-
specialization through attainment of generalized
traits-wirh parallel development of highly attuned
interactive capabilities among a few species. Non-
native fishes had to carve Qut new niches in their
new world, and in the process replaced the natives.
Perhaps as importantly, native fishes had co,evolved
in communities that lacked ca.rnivores other than
rhose developed from among their own ranks (i.e.,
rhe Colorado squawfish). PredatOry fishes common to
other parts of North America did not occur-gars,
bowfins, pikes, mOst catfishes, sunfishes, and basses
disappeared early in rhe geologic histOry of this
region, or were never present. The native fauna had
never before experienced pressures from piscine
predatOrs.
Since essenrially all rhe introduced fishes were
characteristic of quiet warer, preadapred for life in
artificial impoundments. human stabilization of
rivers and construction of reservoirs in western
United Stares prepared the region for esrablishmenr
of an exotic fauna. Construction of impoundments
was accompanied by srocking of non,native fishes
from eastern North America and elsewhere. Most
game fishes are predators-northern pike, larger
catfishes, largemourh, smallmourh, striped, white,
and yellow basses, crappies, and walleye,
Mosquirofish, introduced for control of pesriferous
insects, is a notorious ptedator on smalJ fishes as
well. Forage fishes srocked ro feed rhe introduced
ptedarors-rhreadfin shad, a whole series of
minnows called shiners (red, redside, golden shiners),
fathead minnows, smaller sunfishes (bluegill, green,
redear), etc.-are potential predarors as well as
probable competitors. The impacts of omnivorous
fishes like common and grass carps, African cichlids
(tilapias), and bullhead catfishes are relarively
unknown"
A tremendous loading of aquatic habirats occurred
and continues, so rhar rhe original fish fauna of rhe
Colorado River has been increased from a roral of
32 species to more than 80. Furthermore, fishes that
were introduced were generally transported from
zones of faunal saturation, places such as the
Mississippi River valley, Atlantic coastal drainages,
and tropical Mexico and Africa, where hundreds
rarher than rens of species were naturally present. It
is logical thar fishes which developed and existed in
association wirh large faunas should be more capable
of dealing with biological adversity than one thar had
not before seen another species of its Own genus,
The native fish fauna, lacking experience with alien
species, was disadvantaged, especially when non'
native fishes became abundant.

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