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Risk assessment of scientific sonars

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Burkhardt, E. , Boebel, O. , Bornemann, H. and Ruholl, C. (2007): Risk assessment of scientific sonars , International Conference, The Effects of Noise on Aquatic Life, August 13-17, 2007, Nyborg Denmark. .
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RISK ASSESSMENT OF SCIENTIFIC SONARSELKE BURKHARDT1, OLAF BOEBEL1, HORST BORNEMANN1 AND CHRISTOPH RUHOLL11 Alfred Wegener Institute for Polar and Marine Research, P.O. Box 12016, 27515 Bremerhaven, GERMANY. Elke.Burkhardt@awi.de1. Introduction Scientific sonars form an important asset to conduct oceanographic, geophysical and biological research and are hence installed in many research vessels. Multibeam deep-sea echosounder map the sea-floor topography at high resolution, enabling researchers to identify sites of environmental importances such as cold water coral reefs and sea mounts, to locates suitable sites for oceanographic, geophysical and biological studies such as deep water passages and ice-berg grounding sites, and to develop navigational charts, also in the context of defining marine protected areas. Sediment echosounder explore the upper sediment layer stratification to determine location and thickness of sediment layers for coring sites for paleooceangraphic and sedimentological studies and to map the sediment distribution for paleo-oceanographic and paleo-biodiversity studies. Scientific fish finders are used to map the fish and krill distribution across large areas and at resolutions unachievable by catch data. This study analyses potential risks of these echosounders' usage with special emphasis on cetaceans present in the Antarctic Treaty region, i.e. the ocean south of 60° South. 2. Methods The study first presents a comprehensive description of the echosounders' technical parameters such as sound pressure level, effective pulse length and beam pattern, and continues by analysing three possible impact scenarios:a) Risk of direct damage due to immediate effects of sound;b) Risk of direct damage due to behavioural response;c) Risk of indirect damage due to changes to the habitat.3. Results For cruising ships (R/V Polarstern particularly), the study quantifies that the cumulative risk of type (a) (i.e. the risk of causing a temporary threshold shift, TTS) during simultaneous operation of Hydrosweep and Parasound is estimated to be less then 2 % of the risk of a collision between ship and whale. The risk of causing a permanent threshold shift (PTS) will be smaller. For both, cruising ships and ships on station, the risk of behaviourally induced damage (type b) appears unlikely for cetaceans in the Antarctic Treaty region. This is due to fundamental hydroacoustic differences between the scientific echosounders and the naval mid-frequency sonars associated with recent atypical stranding events elsewhere, and certain physiological and behavioural adaptations of the species involved. Similarly, the risk of indirect damage due to changes to the habitat (type c) appears unlikely. However, while we consider these risks to be significantly less likely than the risk of type (a), a quantification of these latter risks is outside the scope of this study. Figure 1: Critical volume of Hydrosweep Fan-Beam echosounder4. Discussion For ships on station (zero velocity), the non-zero risk of ensonifying a marine mammal at TTS levels obviously exceeds the risk of collision, as the latter becomes zero.The study revealed that only for ships on station, a finite risk of enfonifying a whale with critical sound levels. To mitigate possible negative effects, the Alfred Wegener Institute implemented the following mitigation policy: - shut off of scientific echosounders when on station, unless scientifically need - in later case, implementation of observer, shut-off if whales within 100m radius 5. Acknowledgments This research was supported by Atlas Electronics and WTD 71.6. ReferencesNoise exposure Criteria Group, et al. (2004), Noise Exposure Criteria, paper presented at Second Plenary Meeting of the Advisory Committee on Acoustic Impacts on Marine Mammals, Marine Mammal Commission, Arlington, Virginia, 28- 30 April 2004.Wartzok, D. (2004), Marine Mammal Populations and Ocean Noise: Determining when noise causes biologically signifficant effects, in 4th Plenary Meeting of the Advisory Committee on Acoustic Impacts on Marine Mammals, edited by M. M. Commission, Marine Mammal Commission, New Orleans, Louisianna. http://www.mmc.gov/sound/plenary4/pdf/wartzok.pdf

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