Measurement and analysis of underwater noise of offshore mussel suspended farming waters

Abstract

Anthropogenic noise pollutions are gradually increasing in the world, especially in coastal waters due to shipping and seismic exploration. However, the need for high-quality aquatic products has led to more and more marine suspended farming, which may add to the new sources of low-frequency anthropogenic noises in the ocean. Waters around the Gouqi Island, one of the major reef islands in the northern Zhoushan Archipelago, are considered as important suspended farming waters of mussel. The units of suspended farming facilities composed of aquaculture lines with attached mussels, or the sleeves, are suspended in the surface layer by buoys. The length of the sleeves is 2.3-2.8 m and the water layer occupied by the aquaculture system is about 3.5 m. These suspended farming facilities can reduce the current speed significantly. In this paper, the underwater noises of mussel suspended farming waters of Gouqi Island were measured by a passive monitor system at two sites (inside and outside mussel farming waters) with depths of 5 m and 10 m, and boat noises were also recorded to determine the noise source. All sounds were monitored prior to record to ensure the minimum self-noise, and at least 10 recordings were made and separated by at least 5 min. Power spectra of sound recordings were presented as 1/3-octave sound pressure level (SPL) in dB. Results showed that the main frequency of boat motor noise was 250 Hz, and its SPL at 5 m (114.7± 1.7) dB was greater than that at 10 m (97.4±0.9) dB when the boat was anchored. There were three peak frequencies (63 Hz, 250 Hz, 500 Hz) of underwater noises generated outside the mussel farming waters, where the SPL of 250 Hz at 5m was greater than that at 10m, indicating that the environmental noise outside mussel farming waters was greatly affected by fishing boat. The average environmental noise SPL was (87.7±13.7) dB inside the mussel farming waters, there were also three peak frequencies (63 Hz, 250 Hz, 500 Hz), where SPL of 500 Hz at 10 m (85.13±0.5) dB was greater than taht at 5 m (179.4±0.9) dB, indicating that the suspended mussel farming facilities were the main source of 500 Hz inside the mussel farming waters. This vertical distribution of marine environmental noise may be due to the blocking effect of mussel suspended farming facilities on tidal currents. Owing to that tidal current velocity was reduced in the surface layer, underwater noise level at bottom was greater than that in the surface layer. The underwater noise from suspended farming facilities can contribute significantly to the local ocean noise, and it is clear that the frequency of the suspended farming noise overlaps with the frequency used for acoustic communication in fish. It is necessary to increase our understanding of marine soundscapes, especially when ocean soundscapes continue to be suffered from human activity. In the future, more studies on the effects of noise from aquaculture facilities on the biology and ecology of fish are needed to fill the missing link in acoustic ecology.