Tidal current turbines extract kinetic energy from tidal current in much the same way as wind turbines do with wind. Tidal current velocities are by nature slow and variable, whereas electricity generation typically requires fast and steady rotary motion. This article investigates the performance of a hydraulic transmission system based on Digital Displacement™ technology, which allows variable speed of the tidal current turbine rotor while maintaining constant generator shaft speed. The case study of a generic horizontal axis tidal turbine is considered. Control strategies based on rotor variable speed are derived to optimize yearly power generation and to cope with short-term variations in stream velocity.