7601

• UCREFRP Catalog Number: 7601
• Author: Minckley, W. L.
• Title: Native Fishes of Arid Lands
• USFW Year: 1991.
• USFW - Doc Type: A Dwindling Resource of the Desert Southwest.
• Copyright Material: NO

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 recruitment of young fish into the adult
population for at least 30 years, and researchers
have concluded that they are eaten by introduced
predators. Wild-caught and hatchery-produced fish
are fertile, however, and the species readily
responded to artificial propagation.
In the Vnited States, it is obvious that
modifications of rivers, drying of reaches below
dams, channelization, and diversion of large volumes
of water from place to 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.
Modifications in Mexican river systems are not yet
so extensive, but are of the same kinds and
magnitudes. For example, the Rio Yaqui system is
not yet as completely dammed and diverted as the
Colorado. Some changes in Mexico are, however,
equally as severe as any in the Vnited States. Heavily
logged watersheds promoted widespread erosion and
pollution reported by Aldo Leopold and others in the
1930s, and the pressure of grazing and other
destructive land-use has long been evident. As in the
V nited 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, and natural stream conditions and
native fishes still go hand and hand in Mexico.
However, in the V nited States, a substantial number
of native species are disappearing from rivers and
creeks that appear in a natural state, which seems
inconsistent. What differences exist in conditions in
the two countries that cause different biological
results? The introduction and establishment of
non-native fishes, which replace natives through
competition for food, space, or other resource,
or simply devour them, seems the most logical
answer.
Historic evidence points toward a development of
a depauperate fish fauna in harsh riverine

environments of the American Southwest over the
past 5 to 10 million years, and maybe longer.
Evolution selected for ecological generalists-
specialization through attainment of generalized
traits-with parallel development of highly attuned
interactive capabilities among a few species. Non-
native fishes had to carve out 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 carnivores other than
those developed from among their own ranks (i.e.,
the 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 the geologic history of this
region, or were never present. The native fauna had
never before experienced pressures from piscine
predators.
Since essentially all the introduced fishes were
characteristic of quiet water, preadapted for life in
artificial impoundments, human stabilization of
rivers and construction of reservoirs in western
V nited Srates prepared the region for establishment
of an exotic fauna. Construction of impoundments
was accompanied by stocking of non-native fishes
from eastern North America and elsewhere. Most
game fishes are predators-northern pike, larger
catfishes, largemouth, smallmouth, striped, white,
and yellow basses, crappies, and walleye.
Mosquitofish, introduced for control of pestiferous
insects, is a notorious predator on small fishes as
well. Forage fishes stocked to feed the introduced
predators-threadfin shad, a whole series of
minnows called shiners (red, redside, golden shiners),
fathead minnows, smaller sunfishes (bluegill, green,
redear), etc.-are potential predators as well as
probable competitors. The impacts of omnivorous
fishes like common and grass carps, African cichlids
(tilapias), and bullhead catfishes are relatively
unknown.
A tremendous loading of aquatic habitats occurred
and continues, so that the original fish fauna of the
Colorado River has been increased from a total 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
rather than tens of species were naturally present. It
is logical that fishes which developed and existed in
association with large faunas should be more capable
of dealing with biological adversity than one that 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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