Largest Tidal Energy Test to Take Place This Summer

Scientists are working off the coast of Scotland to install the largest tidal turbine to date in freezing waters and 50 foot waves. If it works, it could renew wider efforts to start using tidal power (similar efforts have been abandoned in the US). From the looks of the thing below, it can do some damage too.

I'm not familiar with this stuff, what's the environmental impact of these turbines on marine life?