Tidal streams, fish and seabirds: understanding the linkages between mobile predators, prey, and hydrodynamics

Driven by the necessity to decarbonize energy sources, many countries are targeting tidal stream environments for power generation. However, these areas can act as foraging hotspots for marine top predators, such as seabirds. Thus, it is important to understand the ecological interactions influencing predator behavior and distribution in these areas, to determine the potential ecological implications of marine renewable devices. This study used concurrent observations of foraging seabirds, physical hydrodynamics, and prey presence across a tidal stream environment, before and after the installation of a commercial turbine array close to the island of Stroma, Scotland. There were three main findings: First, benthic foraging seabirds showed a clear preference for certain sections around Stroma where sandeels were detected, while pelagic foraging seabirds were seen all around Stroma. Second, there was a positive effect of water velocity on the number of pelagic foragers and common guillemots. Third, there was a positive effect of the presence of fish schools on the number of pelagic seabirds and common guillemots, in both the same and the previous transects. Thus, it is possible that seabirds target areas of predictable food sources during periods where prey might be easily accessed (e.g., periods of fast flows). Given the difference in the distribution between seabird categories, it is likely that marine renewable devices will impact each category differently. We conclude that any impact on sandbank locations, sandeels preferred habitat, due to the presence of tidal turbines is likely to alter the distribution of benthic foraging seabirds. For pelagic foraging seabirds and common guillemot, changes in prey presence and accessibility (depth and level of aggregation/disaggregation) will have a stronger effect on seabird presence. This study highlights the need to include concurrent physical and biological data when assessing the ecological impacts of tidal turbines.