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H. Phenotype The physical manifestation of the interaction of an organism's genetic information with its environment which results in a unique physical, physiological or behavioral trait. PROBLEMS CONTRIBUTING TO THE DECLINE OF THE SPECIES MffiHOWnmYMmBEmG~AGrn A. Present or threatened destruction. modification or curtailment of the species' habitat or range: Young (1995) determined that introductions of non-native salmonids may have had the greatest effect on Colorado River cutthroat trout. Stocking of these non-native salmonids has been widespread since before 1900. Non-native salmonids affect populations of Colorado River cutthroat trout in different ways. Brook trout are known to replace most subspecies of inland cutthroat trout when in sympatry, especially at lower elevations and in low-gradient streams (Oberholtzer 1990, Fausch 1989, Behnke and Zam 1976, Eiserman 1958). Competition is often suspected as the mechanism leading to replacement, but this has not been demonstrated (Fausch 1988; Griffith 1988). Nonetheless, water temperature can affect the outcome of competitive interactions between these species (DeStaso and RahelI994), and this may confer a competitive advantage to brook trout at lower elevations. Rainbow trout and non-native subspecies of cutthroat trout readily hybridize with Colorado River cutthroat trout and produce fertile offspring (Martinez 1988, Behnke and Zam 1976, Snyder and Tanner 1960). Introductions of non-native salmonids into existing populations of native trout populations have ceased, and do not represent an ongoing practice or expanding threat. A wide variety of land management practices have been suggested to affect populations of Colorado. River cutthroat trout, including overgrazing (Binns 1977), heavy metal pollution (Oberholtzer 1987, Jespersen 1981, Quinlan 1980), and water depletion and diversion (Jespersen 1981). Some of these practices have served to isolate upstream populations of Colorado River cutthroat trout and protected them from invasion by non-native salmonids, but they also serve to fragment streams, restricting movement between formerly connected populations and creating small, isolated populations that may be more liable to go extinct. Even when the effects of land management are discemable, the consequences for fish may be unknown. Young (1995) describes an example of differential. habitat effects where production of juvenile trout benefited at the expense of adult habitat. Behnke and Benson (1980) have described the Colorado River cutthroat trout as the "canary in the mine" with regard to h8hitat degradation, but it has also persevered in suboptimal habitats. Binns (1977) found that Colorado River cutthroat trout persisted sometimes in marginal. and degraded habitats, and often as the only fish species. Behnke and Zam (1976) reported that Colorado River cutthroat trout persisted in such habitats despite introductions of rainbow trout. Habitat problems are viewed as site specific and not an overall threat throughout the range. March 1999 15