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Abundance and distribution of dolphins and porpoises

• Line transect method has been used for the abundance estimation of cetaceans. Observers conduct visual survey from a moving ship cruised on the pre-fixed lines and count the number of surfaced cetceans.
  
• However, visual survey can not be conducted during nighttime, foggy or roughweather conditions. Visual observer likely to miss animals in the water.
  
• Acoustic line transect by receiving sonar sounds of dolphins and porpoises is helpful solution. Towing A-tag fixed behind the moving ship gives us the direction of a sound source (an animal). Feasibility of this method has been tested for finless porpoises in the Yangtze River, China first and widely used for the observation of echolocating odontocetes in marine and freshwater systems now.

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