Mexican native trouts: a review of their history and ... - Webspace

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observations made between 1975 and 2000. We present newly discovered historical evidence that leads us to
conclude that a “lost” cutthroat trout, a lineage not previously known from Mexico, was collected more than a
century ago from headwaters of the Río Conchos (a major tributary of the Rio Grande (= Río Bravo)), a basin not
previously considered to harbor a native trout. We review the last century of regional natural resource management
and discuss our own observations of trout habitats. Impacts of logging, road building and overgrazing are
widespread and expanding. Many streams suffer from heavy erosion, siltation and contamination, and though
long-term hydrologic data are generally not available, there is evidence of decreased discharge in many streams.
These problems appear related to region-wide land management practices as well as recent regional drought. Trout
culture operations using exotic rainbow trout have rapidly proliferated throughout the region, threatening genetic
introgression and/or competition with native forms and predation on them. Knowledge of distribution, abundance,
relationships and taxonomy, not to mention ecology and population biology, of native trouts of the Sierra Madre
Occidental remains inadequate. Vast areas of most mainland drainages are still unexplored by fish collectors, and
even rudimentary information regarding basic biology, ecology and population structure of stocks remains lacking.
Concentrated exploration, research and management of this long overlooked and undervalued resource are all
urgently needed. The history of natural resources exploitation that placed so many native trouts of the western
United States on threatened and endangered species lists is repeating itself in the Sierra Madre Occidental. Without
concerted action and development of region-wide socio-economic solutions for current, largely non-sustainable
resource management practices, native Mexican trout gene pools will soon be in grave danger of extinction.
Introduction
Unambiguously native Mexican trouts range from near
the U.S. border in Baja California and headwaters
of mainland Pacific drainages of the Sierra Madre
Occidental from the Río Yaqui southward to at least
the Río Culiacán (Needham and Gard, 1959), and
disputably native or introduced populations (Lindsey,
1960; Miller, 1960; Miller and Smith, 1986; Minckley
et al., 1986) extend further southward in mainland
Pacific drainages to at least the Presidio and perhaps
Baluarte and Acaponeta river drainages (Figure 1).
Other possibly native populations occur in endorheic
basins that are part of the Casas Grandes or Guzmán
system on the east side of the continental divide
in Chihuahua (Behnke, 1992; Needham and Gard,
1959) and, though voucher specimens are lacking,
a trout may be native to the upper Río Conchos, a
major tributary of the Rio Grande (Río Bravo), which
drains eastward to the Gulf of Mexico. Nearly half a
century has passed since these interesting fishes were
comprehensively studied. In the interim, relatively
few collections have been made and habitats have
been altered in many ways. Logging has expanded
considerably and continues to do so, with sawdust
and other waste products often contaminating streams
as well as contributing to increased erosion, siltation,
flow variability and decreased stream shading. Nonnative
fishes have been widely introduced, including
rainbow trout (Oncorhynchus mykiss) nativetothe
western United States, which many studies on other
native trouts of the southwestern U.S. tell us will
likely interbreed with native forms (Behnke, 1992;
Behnke and Tomelleri, 2002; Carmichael et al., 1993;
Carmichael et al., 1996; Dowling and Childs, 1992;
Leopold, 1918; Propst et al., 1992; U.S. Fish and
Wildlife Service, 1987), have been widely introduced.
The construction of small hatcheries and
growout facilities for this non-native continues to
expand rapidly throughout the region. In general,
many lines of evidence indicate that trout habitats in
the Sierra Madre Occidental are more geographically
restricted than they were when Needham and Gard
and their colleagues sampled and there is no indication
that habitat conditions will improve in the near
future. Much of the historic range of Mexican native
trouts remains unsampled and several taxa remain
undescribed. Knowledge of these fascinating, littlestudied,
and potentially valuable fishes remains inadequate
to allow managers to make sound management
decisions, or even to simply assess conservation status
of the various taxa and populations. These relatively
unstudied trouts should probably all be considered
threatened or endangered, yet, surely as a result of
lack of information and formal taxonomic description,
only two of them, the Mexican golden trout (Oncorhynchus
chrysogaster) and Baja California rainbow
trout (O. mykiss nelsoni – also sometimes called
Nelson’s trout), have been included in major lists of
government or international agencies. Mexican golden
trout has been listed by the Mexican Government
as threatened (“amenazada”) (Secretaría de Desar-