Tidal-stream turbines are a promising source of renewable electricity worldwide. These technologies are sufficiently new that only single test devices have been deployed with arrays imminent. Being new, their interactions with marine organisms are poorly understood and the risk of large marine vertebrates colliding with their moving blades is a consenting and ecological concern. Operational noise is also considered a disturbance threat but under what circumstances is poorly defined. Further, the threats of collision and turbine noise may be inversely correlated with animals needing to hear turbines to avoid them. Consequently, there have been proposals to add extra-noise by fitting turbines with acoustic deterrents to warn or scare animals away. In this talk we examine the acoustic interactions between marine mammals and tidal turbines. The interactions are complex and depend on turbine source levels, ambient sound, propagation in moving water, sensory abilities, swim speeds and diving behaviour. In addition, the occurrence of turbines in arrays adds further complexity as responses to one turbine will impact collision risk with another. We then consider the options for and implications of adding additional warning sounds but such quick fixes might have unintended consequences that either increase collision risk or lead to undesirable avoidance.