Distribution and Abundance of Native and Non-native Fishes of the Colorado River Ecosystem in Grand Canyon, Arizona

Abstract–The Colorado River ecosystem in Grand Canyon is strongly impacted by operation of Glen Canyon Dam. Fluctuating releases of cold, hypolimnetic water from Lake Powell (~8ºC) for peak hydroelectric power generation has had a profound impact on the formerly warm, silty Colorado River. Clear, cold stenothermal conditions preclude successful reproduction and recruitment of most native fishes in the mainstem and intro-duced trout have become dominant species in the fish community. To assess the effects of dam operations on the native fish assemblage, the distribution and abundance of native and non-native fishes of the Colorado River ecosystem in Grand Canyon were assessed in the first year of a two-year research and monitoring pro-gram funded by the Grand Canyon Monitoring and Research Center. Study sites were located along 159 miles of the Colorado River from the Paria River to below Havasu Creek. An intensive area of study was estab-lished from just above the confluence of the Little Colorado River (LCR) at river mile (rm) 60 to Tanner Rapid at rm 68.6 (LCR inflow reach). Additional sampling was conducted in major tributaries (LCR, Bright Angel, Shinumo, Kanab, Havasu). Mini-hoopnets, minnow traps, electrofishing, trammel nets, and seines were used to sample fish during 120 days of fieldwork spread over four seasons between March 1998 and February 1999. Some 4,618 fish were captured in the Colorado River mainstem and tributaries. Non-native species dominat-ed the catch in the mainstem Colorado River (1,515/2,565, 59%) and rainbow trout Oncorhychus mykiss wa