Researchers in Denmark have found that, while harbour-porpoise (Phocoena phocoena) activity in and around a large-scale offshore wind farm in the Danish western Baltic sea is recovering, it is only 29% of the baseline level before construction of the wind farm more than 10 years ago.
The findings are in direct contrast to earlier work that looked at a wind farm off the Dutch coast. This research found that the wind farm acted as a safe haven for porpoises and that porpoise activity increased during wind-farm operation when compared with a baseline level of activity before the wind farm was built.
“The Nysted wind farm in the Baltic is in a very different environment to the Egmond aan Zee wind farm in the Netherlands,” explained Jonas Teilmann from Aarhus University. He told environmentalresearchweb: “The Dutch coastal area is very busy with shipping traffic and trawlers churning up the sea floor. The Egmond aan Zee wind farm offers porpoises a safe haven from all this activity, whereas in the Nysted wind farm there is not as much shipping traffic and no trawling. Winds are weaker and so the relative noise from the wind farm is higher. We also know that the Dutch porpoises leave Dutch waters in winter, whereas the Baltic porpoises stay in the area for longer.”
Teilmann and his colleague Jacob Carstensen collected acoustic data on harbour-porpoise echolocation activity for more than 10 years in the Nysted wind farm, which has 72 × 2.3 MW turbines, as well as a reference area some 10 km away. They found a strong effect during construction of the wind farm – animals left the wind-farm area almost completely despite the fact that minimal pile driving was undertaken. There was also a negative effect at the reference site. Since then, activity has slowly recovered but is still only 29% of the original level.
The reasons for these differences between the wind farms are unclear. Differences in construction methods, signal-to-noise ratios, fishing activity and other disturbances may have played a part. Another potential cause has to do with how the porpoises perceive the different habitats. “One possible explanation to the stronger response at Nysted may be that the porpoises at Horns Reef and Egmond aan Zee may be more tolerant to disturbance,” said Teilmann. “Satellite tracking data from Nysted porpoises has shown that the porpoises are not particularly interested in the area and may simply avoid the area if disturbed.”
Teilmann believes that more research is needed in order to fully understand the different behaviours. “It is clear from our research that different populations of porpoises in different environments may react differently to similar disturbances such as wind farms,” he said. “This is an important conclusion in future monitoring of wind farms.”
The scientists published their study in Environmental Research Letters (ERL).
Nadya Anscombe is a freelance science journalist based in Bristol, UK.