Abstract:  Passive acoustic monitoring is carried out during a continuous day and night for cage culture fish in a large semi-submersible platform, named ‘Penghu爷. Basing on the results of time-frequency analysis, the noise frequency band and sound pressure levels of four moments, e. g, manual feeding, automatic feeding by machine, ship disturbance, and quiet state in the middle of the night, are obtained. The results show that the activity of fish measured under large-scale automatic feeding is obviously higher than that of other states, and the sound intensity is about 30 dB higher than the background noise of the marine environment. When the ship passing by, the fish noise intensity is about 9 dB higher than that of the artificial feeding moments. Fish are in a quiet state and not active during late night, and noise sound pressure level is about 70 ~ 75 dB. The time-domain signal is decomposed by VMD( Variational Mode Decomposition). The obtained Hilbert spectrum analysis shows that IMF1 is the high-frequency noise component caused by fish flapping and swimming noise. IMF2 is the vocalization of golden pomfret with a frequency band of 1 100-3 000 Hz. The frequency band of grouper sound is 300 ~ 1 100 Hz, which is the main component of the third order IMF (Intrinsic Mode Function) component. The peak of Hilbert marginal spectrum is ranged in the 600 ~ 700 Hz, which is the frequency band that the highest energy proportion of bio-noise produced by fish. It is expected to provide a basis for bait regulation and population classification of the fish cultured in the large cage by studying the relationship between the vocal characteristics, behavioral state and environmental background.

Key words: fish noise, sound pressure level ( SPL), time-frequency analysis, variational mode decomposition (VMD), cage farming