A non-intrusive tracking technique for dolphins interacting with a pelagic trawl using a sparse array of hydrophones [abstract]

Underwater acoustic tracking techniques mostly rely on an active acoustic source triggering replies from an array of transponders. If the tracking equipment must remain passive, the transponders are replaced by hydro- phones. This research is to develop systems which can passively locate and track echo-locating cetaceans, with a view to reducing the by-catch of cetaceans in fishing nets, particularly in pelagic trawls. A 'flat' array of four hydrophones in two parallel streamers can be used to produce these tracks. The streamers are attached to the top of the trawl and 'fly' in a reasonably stable manner as the trawl is towed through the water. A fifth hydrophone is placed on the trawl headline, slightly above the plane of the two streamers, so as to obviate any ambiguities in the position-fixing computation. The bandwidth limitations of the coaxial cable transmission link to the towing vessel led to the use of a look-up table to determine positions. This paper describes the technology of the system and the position-fixing procedure associated with it.