The method to reduce underwater noise from a vessel engaged in trawl and trawl-acoustic surveys
The sound waves radiated in water by research vessels can distort estimations of fish abundance by trawl or trawl-acoustic surveys. Several methods to reduce this noisiness for decreasing its influence on fish behaviour are reviewed. New technique of the air-bubble screen arrangement is presented that improves its sound-absorbing and sound-dispersive capabilities and provides its spatial continuity in the frequencies of the maximal acoustical sensitivity for the main commercial fishes. The proposed air-bubble screen differs from the screens traditionally used in fishery by ability of fitting its parameters, as diameter of calibrated orifices in the blast pipe arranged around vessel, distance between them (that determines the screen density), and air overpressure in the pipe, in dependence on acoustic sensitivity of certain fish species to the noise frequencies. Some examples of the air-bubble screen parameters calculation for various fish species are presented.