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important negative effects on young squawfish, but their elimination is impracticable in a river system as large as the Colorado. Concern should be directed toward preventing introductions of additional fishes to the already large non-native fauna (cf. Tyus et al. 1982). It is generally agreed that hatchery production is important to the Colorado squawfish recovery effort. Selective breeding to encourage phenotypes that grow best under the short growing season of the upper basin might be a means to increase growth potential of Colorado squawfish. Such phenotypes might occur in the wild genome but are unable to express themselves in viable populations because selective pressure on all phenotypes is too strong. ACKNOWLEDGEMENTS Support for preparation of this report was provided largely by the United States Bureau of Reclamation and the Municipal Subdistrict, Northern Colorado Water Conservancy District. We thank Dr. Charles R. Berry, Jr. for his useful comments. REFERENCES Behnke, R. J., and D. E. Benson. 1983. Endangered and threatened fishes of the upper Colorado River basin. Coop. Ext. Serv., Colo. St. Univ., Ft. Collins, Colo., Bull. 503A. Black, T., and R. V. Bulkley. 1985. Growth rate of yearling Colorado squawfish at different water temperatures. The Southwestern Naturalist 30:253-257. Bulkley, R. V., C. R. Berry, R. Pimentel and T. Black. 1981. Tolerance and preferences of Colorado River endangered fishes to selected habitat parameters. United States Fish and Wildlife Service, Completion Report, Contract 14-16-0008-1061 A-2, Utah Cooperative Fishery Research Unit, Logan, Utah, USA. Carlander, K. C. 1969. Handbook of freshwater fishery biology. Volume one. The Iowa State University Press, Ames. Carlander, K. D. 1977. Handbook of freshwater fishery biology. Volume two. The Iowa State University Press, Ames. Cole, L. C. 1954. The population consequences of life history phenomena. Quarterly Review of Biology 29:103-137. -116-