Mobile hydroacoustic surveys of 11 stations in J. Strom Thurmond (JST) Lake from February 1986 to October 1988 provided data to aid in identifying critical areas, times, and water release regimes for fish relative to proposed pumpback of water from JST Lake to Richard B. Russell (RIiIR) Lake. Mean relative biomass (volts2/m2 ) usually was significantly higher in the tailrace (0-450 m below RBR Dam) than in the tailwater (1-7 kmbelow RBR Dam), tributary, or lake areas in spring and summer; whereas in fall and winter, biomass was often lower than or did not differ from that in other areas. Highest numbers of fish occurred in the tailrace from May through September, probably because ofblockage of upstream spawning migrations in spring and attraction to cold, oxygenated water released from RBR Dam in summer. Mean relative biomass in the tailwater 1-7 km below RBR Dam either was lower than that in warm-water tributaries and lake areas, or means did not differ significantly. Overall, mean relative biomass was significantly lower in the tailwater 1-3 km below RBR Dam than it was in the tailrace upstream or in the tailwater 4-7 km below RBR Dam. Mean densities in the tailrace during night nongeneration surveys usually were higher than those observed during either daytime surveys, under any operational regime, or during night postgeneration surveys. Fish densities in the tailrace generally were highest in areas over deep water within 50 m of RBR Dam. Predicted pumpback velocities and the relative biomass offishes in the tailrace and tailwater suggest that the tailrace is the most critical area for entrainment of fish, and blueback herring and other c1upeids are most likely to be adversely affected.