Advances in tidal power development indicate that many coastal straits and bends with rapid currents will be equipped with in-stream turbines in the future. Considering the variety among turbine designs it is reasonable to assume both large and small rotors will be utilized. Each design implies a different way of interfering with local environment. However, there are very few available reports on fish behaviour in the presence of in-stream turbines. In this paper fish swimming behaviour is investigated in relation to the risk of colliding with different tidal turbine rotors. Based on an existing model and Monte-Carlo simulations with biological data from a field survey the blade-strike probability is calculated for three different and widespread fish taxa (damselfishes, requiem sharks, and barracudas). The results indicate that small turbines carry higher risks, as long as fish are assumed not to detect and actively avoid the rotor. However, the following paper indicates that improvements to the model may alter these results. Furthermore, it is shown that the potential hazard of tidal turbines varies strongly between different fish taxa and individuals. And seemingly it is ‘the bad’ that has the most reasons to be afraid.