Estimating the traffic rates of bats migrating across the North and Baltic Seas to develop efficient mitigation measures at offshore wind energy facilities

Bats migrating across the open seas are at high risk of colliding with offshore wind turbines. Despite recent advances on how to conduct offshore environmental impact assessments for bats, there is still a lack of standardized methods to estimate the extent of offshore bat migration. We here present a method for estimating the number of migrating bats from long-term acoustic monitoring data collected at remote offshore structures like buoys and platforms. As an example, we apply the method to the German Seas, using ultrasound recordings from multiple offshore structures. We show that high bat migration traffic rates of 1500 bats per km and year, related to a 1 km line perpendicular to the general migration direction, occur closer to the shoreline of the German North Sea. In the German Baltic Sea, bat migration rates ranged from 900 to 4600 bats per km and year. These findings underscore the urgent need to protect migrating bats from collisions with rotating rotor blades of offshore wind turbines in German seas and beyond. Our method may also be suitable for assessing the extent of offshore bat migration in other regions. It provides the opportunity to define threshold values at which protective measures through spatial planning and mitigation measures like curtailment schemes during times of intense offshore migration of bats should be taken. • We estimate the offshore bat migration traffic rate from acoustic data. • High bat migration traffic rates appear in the whole German Baltic Sea. • In the German North Sea high bat traffic rates occur nearer to the shore. • Offshore migrating bats are vulnerable at offshore wind turbines. • Protective measures at wind farms are urgently needed.