This report describes a study of the abundance of harbour porpoises (Phocoena phocoena) in the waters north of the island Sprogø in the central Great Belt. The study was conducted in the years 2008-2010, before, during and after construction of a small offshore wind farm.
The Great Belt is together with the Little Belt and the northern Sound home to some of the highest densities of harbour porpoises in European waters and together with other areas designated as specially protected areas as part of the Natura2000 network. This also means that there is increased attention surrounding construction of an offshore wind farm within the boundaries of the Natura2000 area.
The wind farm consists of 7 turbines, each 3 MW and placed on concrete gravitational foundations, held in place by ballast rocks. They are placed on a line, separated by 450 m, running parallel to the coast of Sprogø.
The abundance of porpoises and the possible impact of construction and operation of the wind farm was studied by means of passive acoustic dataloggers (T-PODs) in a BACI-design (Before-After-Control-Impact), in line with previous studies in other offshore wind farms in Denmark and abroad. Four measuring stations were established; two in the wind farm area (impact) and two in a reference (control) area further north in the Great Belt, in an area with comparable bathymetry and hydrography.
From the collected registrations of porpoise echolocation clicks four statistical indicators were derived, again in line with previous studies. These indicators were porpoise-positive minutes (PPM), expressing the fraction of a day, counted minute by minute, in which porpoise clicks could be detected; clicks per PPM, expressing the average number of porpoise clicks per minute, for minutes where at least one click was detected; Encounter duration, being the duration of groups of clicks, never separated by more than 9 minutes of silence; and finally waiting time, being the intervals between acoustic encounters.
The data collected from spring to autumn in the three monitoring years 2008 (baseline, 2009 (construction) and 2010 (operation) showed systematic variation with month of year, reaching lowest abundance in August and with parallel changes in abundance in the wind farm area and the control. There was a consistent and statistically significant difference in acoustic activity between the two areas, indicating a general higher abundance of porpoises in the central Great Belt, compared to the more northerly reference area.
With respect to effects of the wind farm, no significant BACI-effect was found for any of the four indicators. This indicates that no significant increase or decrease in acoustic activity in the wind farm area not explained by general variations in the entire Great Belt could be observed.
Thus, the study documents a large and stable occurrence of porpoises in the central Great Belt, in line with several other studies where different methods were used. Secondly, no statistically significant effect of the wind farm on abundance of porpoises could be shown. This is in contrast to the studies at other wind farms and also runs counter to the expectations of the environmental impact assessment, where a partial displacement from the wind farm area was expected during construction. The absence of a statistically significant effect can be due to several factors, not mutually exclusive:
- Insufficient statistical power (Type II error). This is unlikely to be the case in the present dataset, as a very large amount of data was collected and fairly evenly distributed across stations and monitoring years. Furthermore significant seasonal patterns and general differences between areas support the notion of a robust dataset with good statistical power.
- The small size of the wind farm (7 turbines) compared to the much larger wind farms previously studied (72 turbines or more).
- Absence of particularly noisy activities during construction. In contrast to the three other wind farms studied, no pile driving of sheet piles or monopiles were conducted at Sprogø.
- High levels of ship traffic, leisure boat traffic and also bottom trawling in the area means that the general level of disturbances is high as is the expected background noise level.
- Prior history of a major construction work in the area (the Great Belt Connection), which may mean that some animals have prior experience and thus can habituate faster.
Thus, any disturbance caused by construction of the wind farm appears to have been sufficiently small to be unnoticeable at the measuring stations, located about 500 m from the nearest turbine. Thus the impact on porpoises, both short term and long term appears to have been negligible.
Due to the special conditions surrounding this particular offshore wind farm in this particular location it is not safe to extrapolate this result to larger wind farms and/or wind farms located in other areas with different levels of other disturbances.