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4 • While occupying Yuma Cove, adult razorback suckers were observed throughout the day widely dispersed within the backwater. When disturbed, the adults moved into the vegetation. During late June and July, when water temperatures passed 25° C, adults were often observed in cooler waters near the bottom of the cove during the day, and at night would cruise slowly above the vegetation near the water surface. Juvenile suckers over 25 mm were seldom observed during daylight hours in Yuma Cove. At night, they were commonly observed along the bordering vegetation. WATER QUALITY: ASU and USBR closely monitored environmental conditions and followed fish growth. Water quality data were collected using a Hydro-Lab and data sondes and plankton samples were collected in Yuma Cove, Arizona Bay and Hammer Head Cove. Yuma Cove proved far more productive than the reservoir. Conditions in Yuma Cove deteriorated in July and August as water temperatures exceeded 30° C in the shallow cove. Thermal stratification combined by biological oxygen demand by decaying plant material caused dissolved oxygen in the bottom 18 inches of the cove to drop below 1 mg/L by late July. No fish kill occurred, however, it was evident suckers were being stressed. Declining reservoir elevations combined by deteriorating water quality conditions prompted researchers to start removing and transferring suckers to Davis Cove, where water quality mimicked reservoir conditions. Aquatic plants were a major obstacle in collecting fish and contributed to deteriorating -water quality conditions. Sago pondweed which -had grown into long. fronds during June. and .July, totally covered the surface of Yuma Cove by the end of July making seining or netting impossible. As the water, level in the backwater declined during the summer, the long fronds laid on the surface forming a mat. Channels and tunnels were noted under the mats, and fish moved through these. A mechanical weed cutter was required to manually cut corridors through the pondweed to allow netting in these channels. This proved to be effective, and some 30 juveniles were captured during the last week of July using this technique, but it was also extremely labor intensive and degraded dissolved oxygen concentrations by disturbing bottom sediments. `?