Abstract
Offshore renewable energy developments have grown substantially over the last 20 years, yet there remains a large knowledge gap in the effect they have on the movement of seabirds. Being able to detect seabird movements and changes in abundance is a crucial step in understanding the impact to populations of sensitive and vulnerable species. Careful survey design is one way of enhancing the detectability of seabird displacement. This study used red-throated diver (Gavia stellata) survey data from the Outer Thames Estuary Special Protection Area to investigate how the statistical power of a survey to detect a given displacement rate was influenced by spacing between transects, the density of red-throated diver and the number of survey days. This was done through the use of a Before-After study design, comparing mean densities of red-throated diver before (pre-impact) and after (post-impact) the construction of a wind farm. The results indicated that transect spacing minorly affects the statistical power of being able to detect a given displacement rate. The number of survey days can have an impact on the power to detect displacement, however the variable able to make the largest improvement to the survey power was the displacement rate to be detected. A sufficiently powerful survey can require an impractically large number of post-impact survey days, particularly where low displacement rates are to be detected, which would be unreasonably costly. Naturally, higher displacement rates, a large number of survey days, higher densities of red-throated diver, and combinations of these factors were all more likely to result in higher statistical power of a survey. However, some of these variables are not alterable, such as the density of red-throated diver, and others have vast cost implications, such as increasing the number of survey days. Therefore, careful consideration of these variables at an early stage of designing a survey can help ensure desired displacement can be detected.