7952 ‎(2)‎

• UCREFRP Catalog Number: 7952
• Author: Miller, R. R. and B. Chernoff.
• Title: Status of Populations of the Endangered Chihuahua Chub,
• USFW Year: 1979.
• Copyright Material: YES

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competition as the causative agents ,fqr the.de.cline of ths,.chub. Discussion
of our observations on the ecology of Gila nigrescens-axe relevent here.
Among the eight stations where taken, adult chubs- were present only in those
habitats. that had some combination of pools at least 1 m deep, shade, and
undercut banks or cover (~e.g., downed logs-, submerged shrubs). Juveniles
were found in shallower Habitats, with or without cover. This- seems to suggest
that the habitat requirements of the adults are the limiting factor for the
survival of the species. That is, as the natural waterways are being continuously
modified by hydrological projects, it is the adult habitats that are diminished.
For example, it is obvious that habitats such as we observed in the .Rio
San Miguel at Ignacio Zaragoza, with a maximum depth of SQ mm, could not support
adult Chihuahua chubs, whose body depth. is nearly as great as this. A 'constant
flow of water is not necessarily important to adult survival and spawning.
Rather it is the-maintenance of those critical features listed above that is.
necessary. For example, the presence of adult and juvenile Gila nigrescens
in the small, isolated pool at the base of a bridge in the Bus illos. basin may
indicate- that this species: can survive i'n intermittent environments asp long
as-the pools persist tfirougliout the. dry seasons. This description also
characterizes the situation at Arroyo de Aguila, Minckley-(1973,108) reported
this: type of ecological adaptation ,for the Sonora chub, Gila ditaenia Miller.
It may be argued that large. volumes of water would. be best for the survival -
of the Chihuahua chub.. Our observations support this- contention as-long as
ample. pools, etc., are maintained. For example., the population in the Rio
Santa Maria above. Buenaventura is abundant, b.ut further modification of the
habitat that would increase.._currents:and decrease the number of pools, undercut
banks and embayments~ would drive the population to extinction. Such a ~'
scenario may have occurred in-the Mimbres River.
The marked reduction in abundance and distribution of Gila ni'grescens
thus may be attributed to habitat alteration or loss from groundwater mining,
channelizati.on, water diversion, deforestation, and pollution. We also noted
the frequent occurrence of exotic species. However, what role they may play
has-not b.een~determined and we can only assume that five introductions (carp,
longfin dace, black bullhead, mosquitofi'sh, and rock bass) into an area
characterized by a depauperate fish fauna can scarcely enhance the survival of
the native fishes-. Gila niQrescens is not reproducing successfully in the
Mimbres River (only 12 small to large adults have been taken since 1975), whereas
long~in dace. (Agosa chrysogaster), are. excessively abundant; it is tempting to
infer that this exotic not only eliminated the native Notropis formosus but
also may be suppressing the survival of the. Chihuahua chub. However, that
hypothesis must be tested be.~ore competition is inferred.
We recommend that (1) the Chihuahua chub. be officially designated an '
endangered species, for both the U.S. and Mexico; (2):a rehabilitated refuge be
selected for it, with.adeduate protection, in the Mimbres River basin; (3) a
stock of the Mimbres population, now being developed at llexter National Fish
Hatchery, be reestablished in the Mimbres- refuge when appropriate; (4) additional
populations of the species be sought and monitored in the Rio Casas Grandes,
Chihuahua - the genetically closest stock to that of the Mimbres River; and
(5) an attempt be made to designate a refuge for the species in Mexico so
as to assure perpetuation, should the Mimbres- River population be extirpated.