Effect ranges of underwater noise from anchor vibration operations in the Wadden Sea
Anchor pipe vibration embedment operations during the construction of seed mussel collectors were performed in the Wadden Sea, a designated World Heritage Site by UNESCO in 2009. We recorded 200 min of underwater noise during the construction of seven anchor pipes. Underwater noise was recorded simultaneously at three positions with a water depth of 9 m with increasing distance to the construction site to assess the disturbance potential to the marine fauna. The recorded vibration embedment noise was a continuous sound with durations of 2–55 s, with most energy below 1 kHz and peak frequencies around 900 Hz. Background noise level at a distance of approximately 1 km increased around 13 dB at frequencies between 800 and 1000 Hz. We estimated the sound propagation by a non-linear logarithmic regression by means of the intercept, slope and attenuation factor, which allowed us to evaluate the received sound levels that reach an animal in certain distances from the construction site. The estimated sound exposure level (SEL) of the source was 148.2 dB re 1 μPa2s and the median SEL ranged from 120 to 99 dB re 1 μPa2s at distances between 394 and 2288 m, respectively. Behavioural thresholds for indigenous species of marine mammals in the Wadden Sea as well as representative fish species were used to determine effect radii of vibration embedment noise. Our study showed that the detected anchor pipe vibration embedment noise might exert a behavioural reaction on a local scale. Marine mammals could be affected by the construction operations up to a distance of 375 m and fish up to a distance of 766 m. These zones of responsiveness for vibration embedment operations are relatively small, compared to pile driving, which is regularly used during construction operations. Our study shows that it is important to monitor and assess any kind of noise introduction to verify, whether a sustainable human use with respect to the complied guidelines is ensured without affecting the marine fauna. That is the first step to maintain a good environmental status as implemented in the MSFD.