Tidal stream use by black guillemots Cepphus grylle in relation to a marine renewable energy development

Seabirds that forage within tidal streams may be vulnerable to collision or habitat change due to tidal stream turbines. The black guillemot Cepphus grylle is considered to be the seabird species most at risk from tidal stream turbines in UK waters. Using GPS tracking of adult breeding black guillemots, carried out on the island of Stroma, Caithness, in 2016 and 2017, we examined habitat use within the Inner Sound of the Pentland Firth in relation to the MeyGen tidal lease area (MTLA). We found foraging areas of black guillemots within the Inner Sound to be influenced by tidal velocity and seafloor depth. The velocities and depths which black guillemots selected while foraging within a 1 km boundary of the lease area significantly differed from those concurrently occurring within the MTLA (velocities: foraging = 0.79 m s-1, MTLA = 1.57 m s-1; depths: foraging = 24.55 m, MTLA = 32.09 m). This disparity between the used habitat and the conditions within the MTLA may indicate a reduced potential for interactions with turbines. The potential for collision with turbine blades is further reduced by black guillemots predominantly associating with mean tidal velocities slower than the 1 m s-1 cut-in speeds of the MeyGen turbines. However, as more turbines are constructed within the lease area (up to 398 turbines proposed), habitat and hydrodynamic conditions may be altered to become more suitable for foraging, so monitoring black guillemot foraging behaviour post-construction is strongly recommended.