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RESPONSE OF NATIVE FISH HABITAT TO HYDRODAM PEAKING POWER RELEASES IN THE COLORADO RIVER. Richard A Valdez, Senior Aquatic Ecologist, BIO/WEST, Inc., Logan, Utah. The construction of 13 major dams on the Colorado River of the southwestern United States has segmented native fish habitat contributing to the extirpation and endangerment of some species. Of 13 fish species native to the mainstem Colorado River, four are federally endangered - Colorado squawfish, humpback chub, bonytail chub, and razorback sucker. Each has speck habitat requirements and I'rfe history aspects that evolved over 5 million years and have been disrupted by habitat fragmentation and altered natural flow regimes. Proper water-release management of these dams, particularly the hydropower units, is vftal to preserving remaining habitats and populations. Properly-timed flow releases, durations, ramping rates, and flow magnitudes can minimize detrimental impacts, enhance existing habitats, and aid species recovery. Obtaining sound biological information to describe these needs requires innovative methods and techniques necessary to cope with the remote whitewater nature of the turbid Colorado River. The ecological requirements of the humpback chub are currently under investigation in the Grand Canyon as part of Bureau of Reclamation's Glen Canyon Environmental Studies. Radiotelemetry is the primary tool for gathering movement and behavioral data relative to various flow conditions, and habitat is described with the aid of aerial photography, mapping, sonar, and staff gages. The sedentary nature of this species and the need for the Grand Canyon population to access spawning tributaries dictates specific flow requirements. This paper describes the principles of data collection for the four endangered species and contrasts their I'rfe history requirements as affected by the major hydropower dams of the Colorado River, with the focus on the humpback chub of the Grand Canyon. FISHBIOLOGICAL INVESTIGATIONS OF A DANUBE IMPOUNDMENT (ALTENWORTH, AUSTRIA). Herwig G. Waidbacher, Dep. of Hydrobiology, Fisheries and Aquaculture, Univ. of Agriculture, Feistmantelstr. 4, A-1180 Vienna, Austria. From 1985 to 198'] fishbiological investigations have been performed in impounded and free-flowing areas of the Danube. Seventy percent of the Austrian Danube section is already impounded for hydropower generation. The impoundment of "Altenworth" is located about 50 km west of Vienna, its length is approximately 30 km, the theoretical water exchange 10 to 15 hours. Stream flow is reduced from 1.5 m/s to 0.4 m/s. Seventy-five percent of the impoundment is lined by earth dams, the shoreline is secured with a monotonous riprap. Due to reduced current speed, increased depth, silty to muddy sediments and high benthic biomass, the impoundment conforms more to a habitat for limnophilic fish species. However, the relatively low average annual temperature of the Danube, the lack of shoreline structure and the low plankton density inhibit better development of lacustrine fish associations. Distinct riparian macrophyte groups which serve as spawning ground, nursery and feeding place for juvenile fish, are seriously affected by flood events or even constant high average discharges and in such a case lose their protective functions. Mainly two fishing methods were used. In the shore area, samples were taken with an electrofishing boat (Coffelt system, 10 anodes in front of the boat). Longlines (50 - 300 m) were used to sample fish from the river bottom. The original dominant rheophilic fish fauna was represented in the impoundment by adult individuals only, which obviously use the rich food resources. Corresponding juvenile fish could only be detected in the free-flowing section except a few individuals in the uppermost part of the impoundment. For typical rheophilic fish species there are no spawning grounds in the impoundment. The remaining free-flowing section upstream significantly influences the species composition of the impoundment as indicated by the unexpectedly high number of species recorded (38).