Horns Rev offshore wind farm was constructed on Horns Reef in the northern German Bight in 2002. As part of a large environmental monitoring program 21 harbour seals were caught in the period 202-2005 on the island Rømø and equipped with satellite transmitters. In addition to satellite transmitters, 21 seals were equipped with a sophisticated datalogger in a cooperation with the University of Kiel. These loggers are capable of collecting high resolution information on the diving behaviour and movement of the seals. The loggers fall off the animals after a couple of months. To get the data the loggers have to be retrieved from the coast, where they wash up. At present, 7 of the deployed loggers have been retrieved.
The primary aim of the investigations was uncovering the importance of Horns Reef as foraging area for harbour seals from the Danish Wadden Sea. A secondary aim was to determine whether seals were present in the wind farm after construction and whether their behaviour was affected by the presence of the turbines.
Foraging of harbour seals from the Wadden Sea The study has documented that harbour seals from the island Rømø are foraging primarily outside the Wadden Sea in the period September to July. Individual seals appear to have strong preference for smaller, confined areas, which they will return to again and again on their foraging trips. The combined picture of many seals however, shows a more or less even distribution of seals primarily in an area from Rømø out to approximately 100 km from shore, stretching from Holmsland Klit in north to south of the Danish-German border. Similar results have been found in telemetry studies in Germany and the Netherlands and confirm that the entire eastern part of the German Bight is the primary foraging habitat for harbour seals from the International Wadden Sea. Horns Reef and thus also the wind farm is located in the centre of the foraging area of the seals from Rømø and the area is thus of importance to the seals. Nothing seems to indicate however, that the reef or the wind farm area is of greater importance than the surrounding areas.
Effects of construction and operation of the wind farm The accuracy of the positions retrieved from satellite transmitters and dataloggers turned out to be insufficient to conclude with certainty on the degree to which the construction of the wind farm has affected the seals. However, it is close to certain that one or more of the tagged seals were inside the wind farm area during the period the transmitters were active. Visual observations from ship surveys, conducted as part of the monitoring program on harbour porpoises, supports this, as seals were observed inside the wind farm area in numbers not readily different from the surrounding waters. An exception from this was the construction period in spring and summer 2002, where very few seals were observed inside and in the immediate surroundings of the wind farm. Seals were most likely staying away from the construction site due to the very high levels of underwater noise generated by the pile driving operations and the associated mitigation.
Underwater noise from the turbines appearsto be the only potential negative source of impact of practical relevance. The scale of this impact is considered to be marginal, based on measurements of the emitted noise from the turbines and compared to the other sourced of underwater noise in the area, caused by e.g. ship traffic. It is believed that the artificial reef formed on the foundations and scour protection potentially will benefit the seals in the area through an increase in food availability.