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The Colorado Squawfish 3 89 Estimated population sizes Range Average or mean Fish km-1 ,1 7728 14.0 2 775-3875 2325 4.2 3 8611 15.6 4 7369-8840 8105 14.7 5 4226-12,134 9764 17.7 6 3091-44,160 11,150 20.2 Averages 3865-17,252 7947 14.4 Relative proportions of these values are .... supported by interagency collections of adults , and young, respectively, by spring electro fish- ~, ing and autumn seining in 1986 and 1987 (USFWS 1987e, 1988c). Collections in reaches of densest populations of these two life stages (Archer et a1. 1986) averaged 1.01 fish per hour electrofishing time for adults in the Green, and 0.31 fish per hour in the Colorado rivers. Seining caught 26.5 and 7.0 young per . 100 m2 in the Green and Colorado rivers, re- spectively (USFWS 1987e, 1988c). Although perhaps not directly comparable because of different habitat size and turbidities, the Yampa and White rivers of the Green sub- basin yielded electrofishing catches of 0.7 and 0.2 adults per hour. Absolute numbers of Colorado squawfish are unknown, since rarity and lack of infor- mation on movements, recruitment, and mor- tality have precluded use of standard popula- tion estimation methods. However, many years of study allow some meaningful interpreta- tions regarding standing stock, and crude ap- proximations of adult population size (Table 19-2) were therefore developed from (I) com- parisons with catch rates of razorback sucker (Xyrauchen texanus), for which a population estimate is available (Lanigan and Tyus 1989); ~""-~ (2) use of electrofishing catch rates, converted to number of fish per kilometer and corrected for an assumed catch-rate efficiency; (3) use of electrofishing catch rates corrected by an estimated efficiency for razorback suckers; (4) a crude capture-recapture estimate, gener- ally disregarding usual population estimation constraints; (5) an opinion survey of experi- . enced fishery workers to determine relative numbers of razorbacks and squawfish observed per unit distance electrofished; and (6) a simi- lar survey using the number of squawfish esti- mated per unit distance based on electrofishing catches. A computed overall average summed the outputs of all the above methods. The ra- tionale and computations for these approxi- mations are given in the appendix to this chapter. Despite strong reservations, I believe the re- sulting range of averages-3875-17,252 adult fish-is realistic for the Colorado squawfish population of the main-stem Green River, and the grand average of all approxima- tions-about 8000 adults-is also reason- able. Accuracy remains unknown; however, the similarity among results of sampling tech- niques implies either an inherent bias com- mon in all methods attempted or a reasonably accurate grand average. The abundance of .j i