June 23, 2011

It is often assumed that offshore wind farms have a negative impact on marine animal populations but researchers from Denmark and the Netherlands have found that, in some cases, wind farms can act as a haven for certain species.

The researchers, from Wageningen University and Research in the Netherlands and Aarhus University in Denmark, monitored harbour porpoise (Phocoena phocoena) activity on a Dutch wind farm and found a significant and surprising increase in activity compared with levels before the wind farm was built.

Meike Scheidat and her colleagues used acoustic monitoring to study porpoise activity prior to construction of Egmond aan Zee, the Netherlands’ first off-shore wind farm. They then used the same technology to monitor activity during the wind farm’s operation, expecting to find that porpoise activity would return to normal or decrease, as previous studies had shown.

“But we were very surprised to find a distinct increase in activity inside the wind farm during operation, over and above the general increase in population that has been recorded in Dutch waters over the last two decades,” said Jakob Tougaard, co-author of the study, which is published in Environmental Research Letters (ERL). “We believe that there could be two possible causes for this increased activity – an increased food availability inside the wind farm (reef effect) and/or the absence of vessels in an otherwise heavily trafficked part of the North Sea (sheltering effect),” he told environmentalresearchweb.

Egmond aan Zee was constructed using steel monopile foundations. While the percussive pile driving required has been shown to have a dramatic negative effect on porpoise activity in the surrounding area, the use of large boulders around the base of these piles creates an artificial reef on the otherwise homogeneous sandy sea floor. This creates better foraging opportunities for the porpoises.

The wind farm also provides shelter from the heavy shipping and fishing traffic because all vessels, except a few maintenance and research vessels, are prohibited within the farm as well as in a marginal 500 m buffer zone.

Free-swimming porpoises in the wild vocalize almost constantly, rarely remaining silent for more than a minute at a time, so acoustic activity monitoring has been shown to be a good direct indicator of the presence of porpoises.

The detectors used by the researchers consist of a hydrophone, an amplifier, a number of band-pass filters and a data-logger that continuously logs echolocation click-activity of porpoises. This processes the recorded signals in real-time and only logs the time and duration of sounds that fulfil a set of acoustic criteria chosen by the user to match the specific characteristics of porpoise echolocation-clicks. Porpoise click trains are recognizable by a gradual change of click intervals throughout a click sequence, whereas boat sonars and echosounders have highly regular repetition rates.

Tougaard and his colleagues have conducted similar studies at two other wind farms and found conflicting results. “Our investigations at Horn Reef on the northern border of the German Bight and Nysted in the Western Baltic found the wind farms had either no effect or a negative effect, respectively, on porpoise activity,” said Tougaard. “These wind farms are located in different habitats or were constructed using different methods, so it is clear that many factors govern porpoise behaviour in and around wind farms and we cannot extrapolate from one single study.”

By Nadya Anscombe (environmentalresearchweb)

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Source: environmentalresearchweb, June 23, 2011; Image: noaa