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

Abstract

SynopsisThe Colorado squawfish, Ptychocheilus lucks, 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 a growth-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 to the decline of Colorado squawfish in the remaining habitat. Management efforts that might help this endangered species to recover include water management to enhance temperatures for growth, and the control of important introduced fishes.

Do Endangered Razorback Suckers Have Poor Larval Escape Performance Relative to Introduced Rainbow Trout?

“…We concur with the conclusions of previous researchers who have studied the effects of temperature on razorback sucker life history (e.g., Marsh 1985;Clarkson and Childs 2000); it appears that warmer discharges from the dams on the Colorado River (particularly during the spring-summer reproductive period) are necessary to allow successful recruitment of the razorback sucker. However, we realize that increased water temperatures may also benefit nonnative warmwater fishes in the river (Kaeding and Osmundson 1988;Childs and Clarkson 1996). Thus, a complex regime involving nonnative fish removal as well as increased water temperatures will be necessary to reestablish the razorback sucker in the Colorado River.…”

Effects of ambient Lake Mohave temperatures on development, oxygen consumption, and hatching success of the razorback sucker

“…Kaeding & Osmundson (1988) suggest that recruitment of Colorado River squawfish, Ptychocheilus lucius, in the upper river may be limited by temperature that reduces growth rates needed to secure a minimum overwintering size. Minimum size for 0+ fish is critical to their survival through the first winter (Gutreuter & Anderson 1985, Wicker & Johnson 1987.…”

Growth, survival, and starvation resistance of Colorado squawfish larvae

“…Low summer water temperatures caused by hypolimnetic releases from these dams may curtail reproduction of Colorado squawfish and reduce growth, survival, and recruitment of early life stages (Vanicek et al . 1970, Kaeding & Osmundson 1988, Tyus & Haines 1991 .…”