Perils Facing the Gila Trout

The plight of Gila trout provides a case study of problems confronting fish inhabitants of mountain headwater streams in the Southwest. This endemic salmonid once occurred in much of the Gila River drainage--a Colorado River basin tributary in New Mexico and Arizona--but by the time the species was first described in 1950, its range had been reduced to a few headwater streams in New Mexico (Propst et al. 1992). The first attempts to conserve and recover populations of this fish occurred in 1923, when the New Mexico Department of Game and Fish established hatchery stocks of Gila trout and prohibited stocking of nonindigenous trout species in stream reaches where the Gila trout occurred. The U.S. Endangered Species Preservation Act of 1966, the U.S. Endangered Species Act of 1973, and the New Mexico Wildlife Conservation Act of 1974 legally protected the remaining populations.

The Gila trout recovery plans of 1979 and 1983 (U.S. Fish and Wildlife Service 1987) identified the need to replicate and safeguard the remaining populations as genetically distinct evolutionary units. These plans provided the guidelines to recovery, requiring the selection of stream reaches suitable for reestablishment, mechanical and chemical removal of nonindigenous trout, and transplanting Gila trout to the renovated systems. Streams selected as donor sites for Gila trout populations were generally headwater reaches within the historical range of the fish. By 1987 recovery efforts had resulted in the establishment of Gila trout populations at nine localities, eight in New Mexico and one in Arizona. After meeting benchmark requirements of the recovery plans, the U.S. Fish and Wildlife Service proposed downlisting the Gila trout from endangered to threatened (U.S. Fish and Wildlife Service 1987); concurrently, the New Mexico Department of Game and Fish delisted the McKnight Creek population and downlisted all other New Mexico populations from endangered to threatened (Propst et al. 1992). No one anticipated the natural events and their repercussions that would transpire over the next 6 months.

Before the end of the comment period for the proposed downlisting, a flood eliminated more than 80% of the Gila trout population from McKnight Creek, a forest fire and subsequent flooding eliminated the Main Diamond Creek population (the type locality), and drought and forest fire eliminated more than 90% of the South Diamond Creek population. The occurrence of these chance events concurrent with attempts to downlist this fish demonstrates the fragility of small populations and dramatically shows the potentially serious problems in recovery strategies implemented during the 20-year recovery period.

Threats to Gila trout were comparable to those identified for other interior North American headwater stream trout--competition, predation, and hybridization by introduced trout such as the rainbow trout and brown trout, habitat loss and degradation, and decreased water quality and quantity. In addition, fragmentation of range and recovery attempts relying on establishment of isolated populations are principal problems for Gila trout. Additional threats identified after the catastrophes of the late 1980's were associated with the preferred habitat of this species or with changes within that habitat (for example, fire suppression and riparian habitat degradation).

The attempt to propagate Gila trout by adhering to the historical approach of multiple but small populations had several flaws. Translocated fish were isolated from other populations in habitats that could support only relatively small populations. The limited area for reintroduction and the small population sizes made Gila trout at these sites extremely susceptible to loss through natural events (fire and flooding) or excessive human-related activities (livestock grazing, timber harvest, and mining). This propagation strategy--where more populations were equated with higher levels of protection-- caused a false sense of security. Another, and more difficult, conclusion reached by Gila trout biologists concerned the potential need to place the survival of the species as a higher priority than the maintenance of genetically distinguishable populations. They concluded that there was a need to investigate transplanting populations to longer and larger streams outside of the current range of the species and to introduce more individuals more frequently (Propst et al. 1992).