Immediate mortality of juvenile alosids, American shad Alosa sapidissima and blueback herring A. aestivalis, passed through the 17-MW Kaplan turbine at Holyoke Dam on the Connecticut River was estimated with mark-capture methods. Turbine-induced mortality was 62% at 16.5-MW generation, 82% at 12 MW, and 82% at 5.5 MW. The significantly lower mortality of fish at full power output is thought to be related to greater turbine efficiency.