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Report 5- Potamodromy and reproduction of Colorado squawfish in the Green River Basin, Colorado and Utah. (Tyus 1990) Movements of Colorado squawfish Ptvchocheilus lucius were studied by radiotracking and recapturing tagged fish in the Green River basin of Colorado and Utah, 1980-1988. Of 153 fish tracked, 63% were highly mobile: 41% migrated to known spawning sites, 11% migrated to suspected spawning sites, and 11% moved to other locations. Nonmigratory behavior of tracked fish (18%) was linked with non-annual spawning or sexual immaturity. Contact was lost with 14% of the fish, but five that were lost one year migrated in subsequent years. Some fish (5%) were radiotagged after the spawning season. Spawning migrations began about 28 days after highest spring flows with water temperatures of 9°C or greater. Movement of fish to known spawning areas (N=63) averaged 140.7 km (range 32-372.8 km) in downstream (73%) and upstream (23.8%) directions, and some fish (3.2%) moved downstream in tributaries, then upstream in the mainstream. An autumn-to-spring home range was indicated by radiotelemetry and recaptured fish. Colorado squawfish spawned as summer flows decreased and water temperatures increased, and capture of 13 ripe females indicated that spawning occurred in mean water temperatures of 23°C (range 22- 25°C). Most ripe fish were males, and their mean total length (TL) was smaller than females (males, 555 mm, N=194; females, 654 mm, N=14). The possibility of stock differentiation was suggested by radiotracked and recaptured individuals that spawned at the same site for more than one year, and by migrations to a spawning reach from both upstream and downstream areas. Innate and environmental factors influencing reproduction and recruitment should be fully considered in management of this endangered fish. (Abstract) Report 6- Response of young Colorado squawfish to water flow and light intensity. (Paulin et al., in prep) We used an advection-diffusion model to evaluate directional and random movements of 1-, 3-, 6-, and 36-week-old Colorado squawfish Ptvchocheilus lucius with three water flows and two light levels. Fish were placed in the center of a seven-chambered tank and their u - and downstream locations recorded at water flows of 0, 21, and 237 cm~/s. Tank hydraulics represented riverine conditions: a meandering mainstream and associated quiet (backwater) areas. Fish age, flow rate, and light level (light and dark) all influenced the extent and direction of fish movement. Larval Colorado squawfish exhibited a diel pattern of downstream movement. Rate of downstream drift was inversely related to fish size, and relative swimming ability was correlated with body length. Juvenile fish tended to move at random through the experimental tank at all flows. A knowledge of drift mechanisms can be used in management and recovery activities for this endangered fish. (Preliminary Abstract) Report 7- Population size and status of the razorback sucker in the Green River basin, Utah and Colorado. (Lanigan and Tyus 1989) The status of the razorback sucker Xyrauchen texanus in the Green River, Utah, was evaluated with capture-recapture data collected from 1980 to 1988. The razorback sucker population in the upper Green River (river kilometers 282-555) was estimated at 948 fish (95% confidence interval, 758-1,138), based on a total of 410 fish captured (68 recaptured). Razorback suckers in the 10