Populations of Blueback Herring Alosa aestivalis, an important anadromous forage fish in the northeastern USA, have declined from historic levels. Measures to reduce mortality from many sources, including entrainment by hydroelectric turbines, are cited by fishery managers to be important to restoring populations. At the Crescent Hydroelectric Project (Crescent) on the Mohawk River, New York, pulsed ultrasound (122–128 kHz) was used to deter adult and juvenile Blueback Herring migrating out to sea from entering the intake channel to the headrace and turbines, where mortality may occur. To increase the deterrence rate, the sound field was extended further upriver to expose juvenile Blueback Herring to an increasing sound gradient as they migrate downriver and allow them more time to avoid the intake channel. When juvenile Blueback Herring were present upstream of Crescent and exposed to an ultrasound field, mean catch per unit effort (CPUE) by pelagic trawling in the main channel downriver of the ultrasound was 94% of the mean CPUE in the upriver trawl region and 250% of the mean CPUE in the intake channel. Repeated mobile echo sounder surveys revealed that the abundance of juvenile Blueback Herring averaged 35 times higher in the downriver main channel region than in the intake channel region. During the peak migration period of September 20 through October 14, 2012, continuous fixed‐location horizontal echo sounding showed the percentage of net downstream passage of juvenile Blueback Herring that bypassed the intake channel significantly increased from 31.1% to 76.5% after redirecting the ultrasonic field upriver. These results demonstrate that pulsed ultrasound, when properly directed, can be used effectively as a fish deterrent system to provide safe downstream passage of juvenile Blueback Herring at hydropower dams. At Crescent, this could increase annual survival by tens of thousands of fish or more. Received October 4, 2013; accepted August 26, 2014