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digestion of some of the centrarchid fishes was not appreciated. Langhorst and Marsh (1986) found that razorback sucker larvae were only distinguishable in stomachs of green sunfish (Lepomis cyanellus) for about 30 minutes. After that time the larvae were dissolved. It is now thought that introduced nonnative fishes are the most important biological threat to the razorback sucker. At one time, there was concern about hybridization between the razorback sucker and other riverine suckers (e.g. Wick et al. 1982). Although recent work has largely dismissed that concern that hybridization poses a threat to the existence of the razorback sucker in the present system (reviewed by Minckley et al. 1991), the potential remains. The exotic parasitic copepod Lernaea cyprinacea (anchor worm) has been implicated as a factor in lack of successful razorback sucker reintroduction efforts in the Verde River (Clarkson et al. 1993), but there is no evidence that diseases or parasites have played a major role in its endangerment (Flagg 1982). However, the possibility of further introduction of foreign parasites and diseases remain. Finally, competition for food also may be a mechanism by which nonnatives limit the success of razorback sucker populations (Papoulias and Minckley 1990). Relative Importance of Physical. Chemical .and Biological Factors Native big river fishes have disappeared from about three-fourths of their original range during a time when there have been major alterations to physicochemical and biological conditions in the Colorado River system. Thus, the relative importance of physical, chemical, and biological changes in producing a decline in the fish is uncertain. However, even in the present system there are locations where physical habitat has been altered relatively little, such as in the Yampa River, but the abundance of native fishes has declined while nonnative fishes have become abundant. This suggests that natural physical habitat conditions are a necessary, but not a sufficient condition for recovery of the razorback sucker in its present environment. Although it is obvious that suitable physical habitat is a requirement for the native fishes, the suitability of the physical habitat is not the only issue. Most suitable physical habitat now is occupied by introduced species, including many that are predaceous and highly competitive, and therefore harmful to the native fish fauna (Minckley 1982, Tyus et al. 1982, Carlson and Muth 1989, Tyus and Saunders 1996). An increasing number of chemicals have entered the Colorado River system, but the effect of chemicals on the decline of razorback suckers is not very n the system is not understood. 22