A method for discriminating fish from debris in sonar counts of out-migrant salmonids was tested in the Trinity River, California. The method used induced fish movements to distinguish fish from drifting debris. Electricity and light served as stimuli and video and split-beam sonar were used to measure movements of fish (mainly juvenile chinook salmon) and debris (mainly tree leaves). Differences in fish and debris behavior were clearly observable with underwater video. Many fish darted or slowed and most fish dove, whereas debris drifted passively. Fish responded to the electric field inconsistently, and an apparent positive phototaxis was the most consistent response to stimuli. Lack of matched sonar and video observations of individual targets prevented direct testing of sonar’s ability to differentiate fish and debris in the Trinity River. However, analysis of sonar data from a similar situation in the Seton River, British Columbia, indicate that fish responses measured in the Trinity River by video were within the resolution of split-beam sonar. Split-beam position measurement error averaged ≤ 0.06 m within 5° of the acoustic axis, compared to a mean diving reaction of 0.11 m by salmonids observed with video. Proper transducer deployment, improved sonar analysis methods, and perfection of stimuli to elicit obvious and consistent fish responses are key to the success of this technique. With suitable development and validation, the stimulus-response method could become a useful tool for apportioning sonar counts among fish and debris.