Design of an Interactive Acoustic Deterrent to prevent by-catch of cetaceans in pelagic trawl fisheries

No abstracts are to be cited without prior reference to the author.

Incidental catches of cetaceans have been reported in several EU pelagic trawl fisheries and are felt to have a significant impact on cetacean populations. Dolphins have been observed deliberately entering trawls to feed, generally exiting the trawl without being caught. However, entrapment can occur and when it does it can effect many animals at once. Efforts have been made to eradicate the problem by adapting passive acoustic deterrents ('Pingers'), originally developed for use on static gears, but with only limited success due largely to the acoustic noise generated by the vessels and gear used in the fisheries as well as the large size of pelagic trawls. As part of research under an EU funded project, a more intelligent acoustic deterrent system has been designed as a simpler solution to the problem of cetacean by-catch in pelagic fisheries. The interactive unit developed has the benefit of being silent until the detection of echo-location activity from dolphins. When this is detected the unit is triggered and instantaneously outputs a sound based on the wide band signals, which have been shown to be very effective at displacing harbour porpoises in the vicinity of gillnets. This serves two purposes, firstly to mask the echolocation returns to the animal, thus preventing foraging, and secondly warn off dolphins thus preventing them entering the trawl to begin with. Studies have been carried out into associated trawl noise, and as a consequence of these preliminary experiments, the unit has been developed to cope with the many noise sources present within a trawl net. The interactive unit has to apply advanced recognition algorithms to the signals received to discriminate between background noise / boat sonar activity and true echo-location activity from dolphins before firing. Initial tests to prove the units ability to distinguish echo location activity from noise have been conducted on captive animals in the dolphinarium, at Kolmarden Wild Animal Park, Sweden. Full interactive experiments with wild dolphins in the Shannon Estuary are planned, followed by extensive trials onboard commercial fishing vessels during the albacore tuna fishing season in late summer.