Underwater Noise Mapping of Small Vessels Without Automatic Identification System Transmitters

Anthropogenic underwater noise impacts marine life across many taxa and life stages. Underwater noise mapping is an important tool in estimation and mitigation of this impact. The present study develops a framework for noise mapping of continuous underwater noise from vessels without Automatic Identification System (AIS) transmitters. The framework is applied to a study area in the Kosterhavet Marine National Park on the Swedish west coast. A geographical density of small vessels, estimated from satellite observations, is used to predict underwater noise levels. Source levels are estimated by refining a recently published source model for small vessels. Extensive propagation loss measurements support development of a semi-empirical propagation loss model. Predicted noise levels display good agreement with measured levels at two sites, while at two other sites, levels are underestimated, likely due to failure to resolve local boat density variations and insufficient consideration of local bathymetry. Our results show that underwater noise mapping of small vessels without AIS transmitters is feasible, and that levels of underwater noise due to small vessels significantly exceed the natural background noise in our study area. Furthermore, the framework is used to demonstrate the effectiveness of enforcing local speed regulations in reducing underwater noise levels.