Bat Activity and Migration in Relation to Weather in the Baltic Sea : Implications for Wind Power Curtailment Schemes

Wind power is an important part of the transition away from the use of fossil fuels. Turbines can, however, have a negative impact on bat fauna as direct collisions or barotrauma from rotor blades cause several fatalities per turbine each year. Interestingly, migratory species are disproportionately affected by wind turbines. The growing number of planned offshore wind farms in the Baltic Sea raises concern for the potential negative effects on bat populations, as little is known about bat abundance and migration in the region.  One strategy to reduce bat mortality is the curtailment of wind turbines – temporarily stopping turbines during periods of high bat activity, for example during migration or specific weather conditions. However, the effectiveness of curtailment is limited by the lack of knowledge on which weather variables can be used to best predict bat activity, and what threshold values should be imposed in models. In this study, we investigated two questions:1) which bat species are active in the Stockholm archipelago and how does their abundance differ temporally, and 2) how does bat activity relate to different weather variables?  From June to November 2024, automatic bat recording devices were placed in the Stockholm archipelago. Recordings were manually analysed to identify species and examine bat abundance and temporal activity patterns. For the same period, data of air pressure, precipitation, temperature, visibility and wind speed were collected from the Swedish Meteorological and Hydrological Institute. A negative binomial generalized linear model was used to analyse correlations between bat activity and the weather variables.   We recorded seven bat species from June to October. Notably, the detections of Pipistrellus pygmaeus in September indicate migratory activity over the Baltic Sea. Overall, activity decreased with wind speed and increased with air pressure, although there was a difference in the response to different weather variables between species. These results give insight into the bat abundance over the Baltic Sea and contribute to our understanding of how bat activity correlates to weather conditions. This is in turn useful for the modelling of curtailment schemes for offshore wind farms in the Baltic.