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1979 THE ENDANGERED SPECIES: A SYMPOSIUM 49 effects of habitat modifications are complex, often having been developing for more than a century, and always difficult to quantify or even specifically identify. The problems identified and briefly examined here for the Virgin River have numerous counterparts throughout the West, as is obvious from the fact that 97 percent of the western fishes list- ed herein are on this list in part because of the present or threatened destruction, modifi- cation, or curtailment of their habitat or range. Disease and Parasitism Wilson et al. (1966) and Seethaler (1978) have suggested that parasitism may place sig- nificant stress on western fishes being sub- jected to other alterations in their environ- ments. Examination of museum specimens of Crenichthys baileyi collected since 1938, sup- plemented by examination of both museum 30 25 20 15 10 5 20 30 40 50 specimens and individuals taken in the field in 1965 and 1966, yields interesting insights into responses to stress. Crenichthys baileyi occurs in warm springs along the course of the Pluvial White River of eastern Nevada. During the early 1960s various exotic or non- native species were established in some Cre- nichthys habitats (Deacon et al. 1964, Hubbs and Deacon 1964). Figure 10 and Table 3 show the incidence of parasitism by Lemea on Crenichthys bail- eyi populations living. in Crystal Spring and in the warm headwaters springs of the Moapa River from 1938 to 1966. All avail- able data are presented in Table 3. Only data resulting from an examination of 20 or more individuals are plotted in Figures 10 and 11. During this period no nonnative fish were es- tablished in Crystal Spring. The poulation re- mained abundant and virtually free of para- sitism by Lemea. In the headwaters of Moapa River, the .--. 26 Nov. '77(N=383) D--<'l 28 Sept. '78(N=427) 60 80 90 70 Fig. 7. Length frequency of woundfin in the lower Virgin River, fall 1977 and fall 1978.