How tolerent are harbour porpoises to underwater sound?

The marine environment is not as quiet as one might think. Numerous sound sources exist underwater which constantly create a natural background noise of varying intensity and composition. The most prominent physical sound source are waves and rain while snapping shrimp or singing whales amongst the loudest or best known biological sound producers. A new category of sounds is introduced into the sea by the increasing use of the seas by humans. The anthropogenic sound resulting from shipping, industrial and military activities or many other sources has lead to an substantial increase in the overall background noise in the oceans over the past decades. The North and Baltic Sea are amongst the most intensively used and subsequently also the loudest marine areas of all. Currently another large-scale anthropogenic activity is envisaged for these areas. Large numbers of wind turbines are planned to be build in offshore areas of the North and Baltic Sea as part of the efforts to increase the use of renewable energies and thus meet the goals of the Kyoto protocol and its successors. While this effort per se would be regarded as very positive especially from an environmental point of view, its execution nevertheless rises concern about potential adverse effects on the marine environment. One of the key issues in this respect is the noise emitted during the construction of the turbines. As these structures will be build in water depths of up to 40 m and will stretch to over 120 m above sea level, they have to rest on very reliable structures in the ground. Current designs comprise fundaments consisting of single or ‘Mono’-piles and those resting on three piles, the so-called Tri-Pods. All these piles need to be driven to a depth of 25 m and more into the ground and most often this will be achieved by using impulsive pile driving. The emissions produced during this process reach up to intensities which could cause a variety of effects in the marine fauna, from behavioural reactions and stress to physical damage and injury. The published source levels emitted during the pile driving for a wind turbine reach from 225 to 246 dB re 1µPa at 1 m (Ødegaard & Danneskiold-Samsøe A/S 2000; Nedwell et al. 2003, Robinson et al. 2007).