Does sand extraction near Sylt affect harbour porpoises ?

  • Diederichs A
  • Brandt M
  • Nehls G
  • 16

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Abstract

We conducted a one year investigation between 2007 and 2008 to assess impacts of sand extrac tion west of Sylt using aerial surveys and passive acoustic monitoring devices (T-PODs). The area was surveyed once every month using a double engine aircraft, and porpoise densities were calculated using distance sampling techniques. Porpoises were present in the area all year round with highest densities (up to 3.5 individuals / km²) and a high number of calf sightings in June and August and much lower densities during the winter months (below 1 ind / km²). This confirms the area’s high importance as a breeding ground. Avoidance of the sand extraction site could not be detected on the basis of aerial surveys. Porpoise densities calculated from sightings during aerial surveys increased with distance to the coast; only few porpoises were sighted close to the coast of Sylt where sand extraction takes place, and high est concentrations were found at distances further than 18 km from the shore. Passive acoustic monitoring devices deployed over a period of 5 months revealed a short term avoidance of the vicinity of the dredging ship by porpoises, possibly due to acoustic disturbance. In comparisons of three reference areas, no signifi cant difference in the long term use of the impact area by harbour porpoises could be detected. Other studies found fish biomass not to be negatively influenced by sand extraction, and thus the area might not decrease in quality as feeding habitat for porpoises. However, acoustic disturbance by the dredging ship might lead to short term avoidance of the area around the ship with a relatively local and short term effect that cannot be detected by aerial surveys

Author-supplied keywords

  • LMER
  • T-POD
  • aerial surveys
  • impact
  • sand extraction
  • windfarm

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Authors

  • A. Diederichs

  • M. J. Brandt

  • G. Nehls

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