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We conducted in situ testing of strobe lights as a potential fish deterrent by examining avoidance and acclimation behaviors of rainbow smelt Osmerus mordax in Lake Oahe, South Dakota. Split-beam hydroacoustics were used to assess the effectiveness of the deterrent system by comparing proximal densities of rainbow smelt before and after the device was activated. In summer 2005, strobe lights successfully repelled rainbow smelt to a minimum horizontal distance of 21 m at both 1 h and 4 h postactivation; the model AGL-FH 920 flashhead produced a light intensity of 6,585 lumens per flash. Similarly in 2004, a model AGL-FH 901 flashhead, which produced a light intensity of 2,634 lumens per flash, repelled rainbow smelt to a horizontal distance of 15 m. A comparison of strata 10 m above and below the strobe light system showed that rainbow smelt were vertically displaced to approximately 6 m. We conclude that strobe lights elicit behavioral avoidance by rainbow smelt and may provide an effective means for reducing entrainment losses through Oahe Dam. [ABSTRACT FROM AUTHOR]