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apport and encouragement. R. Williams, C. jtrp, J. Hamill, and H. Williamson improved n early version of the manuscript. C. Karp, ~. Paulin, L. Kaeding, M. Moretti, S. Cranny, nd S. Lanigan participated in abundance ap- roximations. I especially thank the more Ian fifty permanent and seasonal employees Tho assisted with data collection between 979 and 1989. lppendix _pproximations of abundance for adult quawfish in the mainstream Green River are ummarized in the text and presented in Table 9-2. I intentionally excluded tributaries, the 'amp a, White, and Duchesne rivers, and the ;reen River upstream from Echo Park. Simi- If exercises could be conducted for those rreams, but they have not been sampled noroughly or with the same methods for :lnger than a year. Most tributary studies Isted a year or less (e.g., W. H. Miller et aI. 982b, c), extended only to state boundaries e.g., Radant 1982, 1986), or were conducted n limited geographic areas (various workers). :be following exercises used five to seven 'ears of catch/effort data from between river :ilometers 43 and 552 on the Green River, all iuring the same time of year and with the ame equipment. vlethod 1 Jectrofishing is a common tool used by ishery workers to catch large fishes O. B. ~eynolds 1983). Lanigan and Tyus (1989) Ised fish collected using this technique to esti- nate population size of razorback suckers in he upper Green River with mark-recapture nethodology. A population estimate for that :pecies was considered valid because assump- ions of geographic and demographic closure lppeared satisfied, whereas data for Colorado ;quawfish do not meet those necessary as- :umptions. However, the two species were The Colorado Squawfish 401 commonly collected together; shorelines were occupied by squawfish most of the year (Tyus et al. 1984), and by razorback suckers in spring (Tyus 1987). Although electrofishing efficiency was probably different for the two species, I used the estimated size of the razor- back population to approximate standing stock of squawfish. As indicated by Tyus (1987), the catch rate for razorback suckers comprised 20.6% of that for squawfish (0.20 suckers and 0.97 squawfish hr - 1) in the 171 km of Green River downstream of the Yampa River. Assuming the population estimate for razorbacks (948 individuals) by Lanigan and Tyus (1989) is correct, there would be about 984/0.21 = 4515 squaw fish in the reach. This figure in- cludes both larger juveniles (30-40 cm TL) and adults (> 40 cmTL). Based on an adult! juvenile ratio for Green River provided by Tyus et aI. (1987), the adult component amounts to (0.79) (4515) = 3567 fish, or about 21 adults km -1. The estimate for the upper Green River was extended to the entire main stem (552 km) by substituting an aver- age catch rate from the entire main stem of 0.68 squawfish hr-1in place of the average rate of 0.97 hr -1 in the uppermost part (Tyus 1987). This provides an estimate of about 14 fish km-\ jlnd (14 fish km-I) (552 km) 7728 adult squawfish in the main stem. Method 2 If the average dectrofishing catch rate for the Green River is 0.56 adult Colorado squawfish hr-I (Tyus et aI. 1987), and if a boat dec- trofishes one shoreline at a rate of 4.0 km hr-" the average catch would be 0.14 fish km -1 for one shoreline. The number for both shorelines would be double that, or 0.28 fish km - 1. The total number of adult fish would then be approximated by (0.28) (552) = 155 fish, if electrofishing efficiency was 100%. However, efficiency is affected by many fac- tors and varies from 4% to 54% (Ruhr 1957; ,. i I, '~i ~ ~,j*i 111"II:+! , , I' I I I I I' i ", ii 'I I I '.,~ I' '1'1 'I' '.11\' ~ ,.,