Now that tidal energy looks like it may be commercially viable in certain locations, there is increasing pressure to ensure that devices such as subsea turbine blades create as little damage as possible to the local fish population (the danger is that thrashing blades might well form currents strong enough to drag fish towards them and chop up significant numbers).
What is needed is an underwater scarecrow. Rather than just use a source of loud noises (whose vibration probably disturbs marine mammals too), today’s invention is a simulated predator.
This would take the form of a sinuously moving multi-section raft on the water surface, moored to the turbine in question. This could be caused to wave backwards and forwards by a small motor and would look from below like the silhouette of a large hunting fish, such as a shark.