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,~ 1: , 390 Recovery of Long-lived Species other life stages could also be estimated, but it would be far more difficult and the results less meaningful. Smaller squawfish are not vulnerable to sampling methods usually and practically employed (Holden and Wick 1982), and the relative abundance of fish less than 60 mm TL fluctuates greatly with en- vironmental conditions. No population estimate has been published for Colorado squawfish in the upper main- stem Colorado River. However, catch statis- tics in Table 19-1 indicate that absolute num- bers are lower than in the Green River and about an order of magnitude less abundant; perhaps less than 1000 adult fish remain in the upper Colorado mainstream. The impacts of recent stockings of hatchery fish are un- known but apparently have barely affected this approximation. Recent work in Cataract Canyon and upper Lake Powell (Valdez 1988, 1990), and in the San Juan River (Meyer and Moretti 1988; Platania and Bestgen 1988; Platania et al. 1990), indicated that adult Col- orado squawfish persist in even smaller num- bers in those areas. Management toward Recovery General Review Environmental changes in the Colorado River basin have been dramatic since the turn of the century, and far too rapid for genetic adapta- tion by its native fishes. As shown in Figure 19-5, much of the system was significantly al- tered by main-stem impoundments, diver- sions, and other water-resource development. The lower Colorado River was largely con- verted from a natural, fluctuating, turbid sys- tem to a modified (dammed), channelized, water-delivery system. Native fishes in the lower basin have been mostly extirpated from the mainstream and replaced by a new fauna sorted out from forty-four or more introduced forms, twenty of which have become locally or regionally abundant (Minckley 1973, 1982). Fish habitat in the upper basin has also been altered Uoseph et al. 1977), but more than 2000 km of riverine habitat is estimated to remain. Although about the same number of non-native fishes have been introduced (fony-two species; Tyus et al. 1982b), replace- ment of natives has not been complete, and populations persist in the reaches least af- fected by humans. Losses of reproducing populations of Col- orado squawfish and other endemic fishes have been mostly attributed to conversion of riverine habitat to artificial impoundments, replacement of warm-water habitat by cold tail waters of dams, and erection of migration barriers (Vanicek 1967; Joseph et al. 1977; Seethaler 1978; Holden and Wick 1982; Tyus 1984). More insidious impacts, including introductions of non-native fishes, small but cumulative water depletions, and downstream effects of water manipulations, have also been suggested (reviewed by USFWS 1978b, 1989a), but these are more difficult to document or prove. Preimpoundment poisoning of the Green River is also sometimes implicated in this decline (in this volume, see Holden, chap. 3; Rinne and Turner, chap. 14). There is no doubt that squawfish were eradicated in the reservoir area, and above, when it was treated with rotenone in 1962, and fish were killed as far downstream as the mouth of the Yampa River. However, conversion of the treated area to a reservoir with its cold tailwaters, and blockage of potential migratory routes for adults and young by the dam, make the issue academic. With passage of the ESA, federal and state agencies were provided with policy direction and monies to protect and recover remaining stocks of endangered fishes. Federal agencies, who were required to consult with the USFW~ regarding effects of water-development proJ- ects, began funding research to learn more about squawfish and other species. Provisions of the ESA focused national attention on man- t ! i i , j I 1