Impact of ship-radiated noise on marine mammals in the South China Sea: Source-level estimation and safety-distance assessment

With the expansion of maritime trade, commercial-vessel underwater radiated noise (URN) has become an important anthropogenic stressor in marine environments. This study introduces a measurement-constrained framework that integrates acoustic observations from 268 quality-validated vessel passages with source-level reconstruction, BELLHOP propagation modeling, and auditory weighting to estimate vessel-category- and hearing-group-specific safety distances in the South China Sea. Cargo ships and tankers dominated the dataset. Auditory-weighted sound exposure levels were calculated for six hearing groups, and distances associated with temporary and permanent threshold shifts (TTS and PTS) were estimated. The modeled distances varied among vessel categories and hearing groups, with TTS distances consistently exceeding PTS distances. Low-frequency cetaceans were particularly relevant because their hearing range overlaps with vessel-dominated low-frequency noise. The results provide comparative, screening-level estimates under the specified propagation, bandwidth, depth, and exposure-duration assumptions. They may support the preliminary identification of vessel-noise scenarios requiring further assessment. This measurement-constrained framework links regional vessel-noise observations with standardized acoustic-impact criteria and provides a basis for future site- and species-specific assessments.