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In February 2010, 3 experimental tetrapod fields were established north of Helgoland parallel to the shoreline and approximately 400 m from the coastline. The fields were introduced at 5- to 7-m depths based on the mean low-water spring (MLWS) according to hydrographic charts (Fig. 1). The 3 fields were placed 80 m from each other to reduce reciprocal impacts while ensuring similar environmental conditions. One field consisted of 6 tetrapods arranged in 2 rows of 3. The size of one experimental field was approximately 7 × 4.5 m, and the fields were approximately 2 m in height (Fig. 1). The area surrounding the tetrapod fields was dominated by rocky to pebbly substrate. A detailed study of the substratum typology of this IPI-145 area is provided in Wehkamp and Fischer (2012).
2.2. Survey methods
The assessment methodology was identical in 2009 (without tetrapods), 2010 and 2011 (with tetrapods). Visual census was conducted on SCUBA. Transect lines were laid out in a thorax cross-like pattern (Fig. 1) at the beginning of the tetrapod fields (in 2010 and 2011) or at the proposed position of the tetrapod fields (in 2009), which was subsequently termed the 0-m sampling station. Sampling was performed every 5 m (0, 5, 10, 15 and 20 m) along each transect by counting and identifying all fish in a 1 × 1 m square to the left of the transect line and a 1 × 1 m square to the right of the transect line. The diver used a 1-m PVC rod to determine the size of the 2 × 1 m rectangle. At the 0-m sampling station, we counted the fish in a 1 × 1 m square to the immediate left and in a 1 × 1 m square to the immediate right of the tetrapod foot were the transect line was fixed.
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