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Environmental Biology of Fishes Vol. 22, No. 4, pp. 287-298, 1988 Q Kluwer Academic Publishers, Dordrecht. Interaction of slow growth and increased early-life mortality: an hypothesis on the decline of Colorado squawfish in the upstream regions of its historic range ~~ ~z~ Lynn R. Kaeding' & Douglas B. Osmundson United States Fish & Wildlife Service, Colorado River Fishery Project, 529 25-1/2 Road, Grand Junction, CO 81505, U.S.A. Received 26.5.1987 Accepted 21.12.1987 Key words: Cyprinidae, Piscivore, Riverine, Population dynamics, Longitudinal effect, Temperature regime, Recruitment, Endangered, Fish Synopsis The Colorado squawfish, Ptychocheilus Lucius, the principal native piscivore of the Colorado River basin, was once widespread and abundant in large rivers and their major tributaries. It occurs today only in the upstream regions of its historic range and is threatened with extinction. Growth rate of the species there is much slower than its potential rate and the rate that might once have been typical in lower-basin rivers. We develop the hypothesis that the interaction of slow growth and increased early-life mortality is an important cause of the decline of Colorado squawfish in the upper basin. We use agrowth-rate versus temperature relation for Colorado squawfish to compare temperature regimes of historic and present habitats, and we describe the strong, positive relation between our measure of temperature-regime suitability and first-year growth of Colorado squawfish in upper-basin rivers. The unusually small size of the age-0 fish going into winter might be an important factor affecting recruitment to the adult stock. Simulations showed how the effect of increased early-life mortality can be especially significant on populations of slow-growing fishes. Predation by introduced fishes, as well as other man-induced causes of increased early-life mortality, probably contributed importantly io the decline of Colorado squawfish in the remaining habitat. Manage- ment efforts that might help this endangered species to recover include water management to enhance temperatures for growth, and the control of important introduced fishes. Introduction The Colorado squawfish, Ptychocheilus Lucius, was once widespread and abundant in large rivers and major tributaries of the Colorado River basin (Jor- dan 1891, Evermann & Rutter 1895, Jordan & Evermann 1896, Gilbert & Scofield 1898). Today, however, this principal native piscivore of the basin occurs naturally only in upstream regions of its historic range (Fig. l) and is threatened with extinc- tion (Seethaler 1978, Holden & Wick 1982, Tyus et al. 1982). Understanding both the causes of the population decline and the factors limiting the pop- ulation is problematic yet essential to programs intended to recover the species. Although the ex- tirpation of Colorado squawfish from its former ' Senior author