Empirical measures of harbor seal behavior and avoidance of an operational tidal turbine

There is global interest in marine renewable energy from underwater tidal turbines. Due to overlap in animal habitat with locations for tidal turbines, the potential for collisions has led to concern around strike risk. Using data from tagged harbor seals collected before construction and after operation of the SeaGen tidal turbine in Northern Ireland, this study quantifies risks of an operational turbine to harbor seals by taking into account turbine characteristics, tidal state, and seal behavior. We found 68% spatial avoidance (95% C.I., 37%, 83%) by harbor seals within 200 m of the turbine. When additionally accounting for variation in seal occupancy over depth and tidal flows, there is an overall reduction in collision risk from 1.29 to 0.125 seals per tidal cycle (90.3% reduction; (95% C.I., 83%, 98%)) compared to risk calculated under assumptions of uniform habitat use. This demonstrates the need to incorporate environmental conditions to properly assess strike risk. •Strike risk is reduced by slower rotational blade speeds of tidal turbines.•Harbor seals predominately swam at surface and seafloor layers in this tidal region.•Harbor seals were 68% less likely to occupy habitat within 200 m of turbine.•Perceived risk is reduced by 90.3% using empirical data of dive and surface locations.