Short-term consequences of Baltic offshore energy system construction for the Harbour Porpoise

By exploiting the immense offshore wind resources, Denmark’s planned Energy Islands are forecasted to provide 5 GW of power, with long-term expansion potential to 10 GW. This study focused on the endangered Baltic Sea Harbour Porpoise population of 500 mammals. This study addresses a gap in the literature: existing agency assessments assume “soft start” makes noise insignificant, but this has not been tested for a critically endangered resident population at 0 km from protected habitats. It was found that based on the expected noise emissions, permanent hearing damage could be caused within a distance of 200–840 m and behavioural changes of the mammals up to 11 km away. Critically, the two planned offshore wind farms are situated adjacent to three Natura 2000 protected habitats, with the Rønne Banke and Adlergrund area located at a distance of 0 km from both development sites. The analysis revealed that while the Danish Energy Agency considers noise emissions insignificant if soft start/ramp up procedures are applied, the actual impact is dependent on variable factors including foundation type, soil conditions, and pile diameter—none of which have been finalized. Furthermore, the application of mitigation measures such as restricting construction to low-prevalence periods presents an inherent trade-off by prolonging overall installation duration and potentially creating chronic disturbance. However, restricting construction to low-prevalence periods creates a trade-off: reduced peak noise versus prolonged disturbance duration. For this resident critically endangered population, prolonged low-level disturbance may be biologically worse than short intense disturbance, as it risks chronic habitat abandonment without a clear return trigger.