Giant Sea Farms Leave a 24 Hz Acoustic Fingerprint in the Deep Ocean

Far off the coast of Zhanjiang in southern China, one of the largest fish farms ever bolted to the open ocean has been quietly singing to the sea. A team of researchers from Guangdong Ocean University and the Southern Marine Science and Engineering Guangdong Laboratory has now recorded that song in unprecedented detail, capturing 46 continuous hours of synchronized acoustic data around a massive truss-type deep-sea aquaculture platform. Their findings, published in Frontiers in Marine Science, reveal that these industrial sea farms broadcast a distinctive low-frequency hum into the water column, one that travels not through the air but through the very steel bones of the structure itself.

The platform under study is no small fixture. Stretching 101 meters long and 47.5 meters wide, with an aquaculture draft of 15 meters and roughly 60,000 cubic meters of farming water, it represents the new generation of offshore infrastructure designed to relieve pressure on saturated nearshore waters. Powered by a hybrid system of photovoltaic panels and diesel generators, these truss platforms are becoming the mainstream equipment for deep-sea aquaculture across coastal provinces like Guangdong, Shandong, and Hainan. Yet until now, almost nothing was known about how their operations reshape the underwater soundscape that marine organisms depend on for foraging, communication, predator avoidance, and navigation.

The research team deployed hydrophones at three strategic locations: inside the central net pen where fish are actually farmed, near the propulsion hatch on the platform deck, and at a distant reference station 1.5 kilometers away. A sound level meter simultaneously recorded airborne noise on the deck corridor, while a self-contained current meter and high-precision tide gauge tracked the hydrodynamic conditions. Everything ran continuously for 46 hours under unusually calm sea conditions, with mean wind speeds of just 4.3 meters per second and sea state below Force 2, providing a clean baseline for isolating the platform’s acoustic signature from natural background noise.

What emerged from the data was striking. During generator operation, the underwater near-field exhibited a discrete harmonic skeleton centered on a fundamental frequency of 24.41 Hz, with power spectral density reaching 122.29 dB re 1μPa²/Hz, approximately 16 dB higher than the idle baseline. Sharp narrowband peaks appeared at harmonic multiples including 48.8, 73.2, 100, and 122 Hz, forming a fingerprint unmistakably tied to the platform’s propulsion system. Above 500 Hz, the interference rapidly faded, and beyond 2 kHz the spectral curves before and after startup completely overlapped with the natural environmental background, which remained within the 72 to 86 dB range.

Perhaps the most compelling evidence came from the moment of generator startup and shutdown. The researchers observed pixel-level temporal synchronization between the deck’s airborne sound field and the underwater near-field radiation. When the 25 Hz spectral line abruptly vanished from the air spectrum, the bright high-energy band in the underwater low-frequency range collapsed in a vertical cliff-like manner. Simultaneously, the baseline at the 1.5-kilometer distant station experienced a spontaneous drop to the quietest state of the entire monitoring period. This triple alignment across three independent measurement domains pointed to a single conclusion about how the acoustic energy reaches the water.

Because air and seawater have an acoustic impedance mismatch of roughly 3,600 to 1, atmospheric noise incident at the sea surface experiences significant reflection losses, making direct airborne transmission into the water negligible. Instead, the structural elastic waves excited by the propulsion system travel as solid-borne vibrations through the rigid truss pile foundation, conducted along the steel columns to their underwater outer walls, where they radiate into the water column. This finding has immediate engineering implications: noise control for such platforms should prioritize structural isolation and vibration damping at equipment foundations rather than relying solely on sound-absorbing barriers within deck machinery rooms.

The distant acoustic environment told its own story. Within the observation window, the low-frequency soundscape at the 1.5-kilometer station appeared modulated by semidiurnal tidal dynamics, with noise peaks lagging behind tidal peak levels by approximately 6 hours. During neap tide periods, when current velocity dropped below 0.3 meters per second, the acoustic background fell below -40 dB re 1μPa²/Hz, forming regular low-energy troughs. Conversely, during stronger tidal-current transitions, the low-frequency energy rose in phased association with the surging flow. The researchers interpret this as flow noise excitation driven by turbulent boundary layer pressure fluctuations and vortex shedding across the seabed topography, processes that decay substantially when currents weaken below the empirical threshold.

Day-night differences added another layer of complexity. The team measured sound-speed profiles in the shallow-water channel and found a distinct minimum at approximately 4 meters depth during the day, creating a natural acoustic waveguide that confines sound energy. At night, surface cooling and evaporative heat loss flipped the profile into a positive gradient with upward refraction, forcing sound rays to strike the sea surface repeatedly. Under the rougher nocturnal sea surface, this enhanced incoherent scattering plausibly explains the observed decay of the distant-station characteristic sound after dark, allowing the ambient soundscape to return to its natural quiet trough. The total sound-speed change across the water column at night reached approximately 1.96 meters per second, significantly greater than daytime fluctuations.

The ecological stakes are considerable. Fish rely on acoustic signals for essential life functions, and anthropogenic underwater noise can disrupt these behaviors through auditory masking, physiological stress, and behavioral avoidance. Repeated exposure to vessel noise has been shown to damage sensory hair cells in fish inner ears and trigger irreversible declines in auditory sensitivity, while noise exposure alters daily swimming patterns and escape responses. Because modern aquaculture management increasingly depends on acoustic technologies, including sonar-based biomass estimation, acoustic virtual fencing, and passive monitoring of fish behavior, understanding the platform’s own noise characteristics becomes a core prerequisite for optimizing equipment deployment and improving detection accuracy.

The researchers are careful to frame their conclusions appropriately. These findings derive from a single 46-hour campaign under calm conditions and should be regarded as process-level evidence rather than climatological statistics, applicable only to the observation window and not extrapolatable to other seasons or sea states without further measurements. Limitations include single-range sampling, the absence of direct in-situ structural vibration measurements, and the lack of sediment acoustic parameters. Future work will employ multi-node, multi-range arrays and full-waveform inversion to achieve precise reconstruction of the three-dimensional acoustic radiation, propagation, and attenuation boundaries of large fixed offshore facilities. For now, the study provides foundational data for acoustic environment monitoring, ecological impact assessment, and the optimized deployment of aquaculture acoustic equipment on similar deep-sea platforms worldwide.

Subject of Research: Underwater noise characteristics and tidal modulation of the acoustic environment around a large truss-type deep-sea aquaculture platform

Article Title: A study on the evolutionary patterns of the acoustic environment of large-scale truss-type deep-sea aquaculture platforms

Article References: Wu, D., Qu, K., Yang, B., Deng, X., Chen, X., Ling, W., & Qi, Y. (2026). A study on the evolutionary patterns of the acoustic environment of large-scale truss-type deep-sea aquaculture platforms. Frontiers in Marine Science, 13, Article 1962198. https://doi.org/10.3389/fmars.2026.1962198

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1962198

Keywords: deep-sea aquaculture, underwater noise, truss platform, passive acoustics, tidal dynamics, solid-body transmission, underwater soundscape, acoustic monitoring, marine ecology, Frontiers in Marine Science, evolutionary, patterns