Project CETASEL: tracking echo-locating dolphins in the vicinity of a trawl

The by-catch of marine mammals has become a serious environmental issue to which the European Commission has responded by funding projects under its AIR-III Programme. One of these, the CETASEL project, had the aim of studying the by-catch problem in the context of pelagic trawl fisheries. Several species of cetaceans (dolphins, porpoises and whales) were of particular interest, the bottlenose dolphin (Tursiops truncatus), harbour porpoise (Phocoena phocoena), common dolphin (Delphinus delphis), Atlantic white-sided dolphin (Lagenorhynchus acutus) and white-beaked dolphin (Lagenorhynchus albirostris). Studies were carried out on these species, either in captivity or at sea, or both in the case of the first two species. To study the behaviour of such animals unobtrusively without influencing their behaviour, a passive acoustic tracking system was developed and a position-fixing algorithm was applied to track their echo-location clicks in the vicinity of a trawl. This paper describes the design of the hardware of the tracking system and the basic principle of the algorithms. Particular attention was paid to the hardware because this had to work in the harsh environment of a fisheries research vessel, FRV Tridens, operated by The Netherlands Fisheries Institute (RIVO) in the Atlantic Ocean. For the system to be of practical use, it had to be capable of computing and displaying the tracks of echo-locating cetaceans in three-dimensional space