Reducing vessel noise: An example of a solar-electric passenger ferry^a)

Concern over the impacts of anthropogenic noise on aquatic fauna is increasing, as is the number of vessels in the world's oceans, lakes, and rivers. Sound signatures of different vessel types are increasingly characterized, yet few reports are available on solar-electric powered vessels. Such data are important to model the sound levels experienced by marine fauna and their potential impacts. Sounds from two vessel types were recorded in the shallow waters of the Swan River, Western Australia, using bottom-mounted OceanInstruments SoundTraps. Multiple passes from two 10-m solar-electric powered passenger ferries and, for comparison, two 25-m conventionally powered (inboard diesel engine) passenger ferries were selected. Analysis was conducted on 58 and 16 passes by the electric ferries (in 2016 and 2017–2018, respectively) and 10 and 14 passes by the conventional ferry (2016 and 2017–2018, respectively) at 5-m range. At 55-m range, analysis was conducted on 17 and 1 passes by the electric ferry (2016 and 2017–2018, respectively) and 9 and 3 passes of the conventional ferry (2016 and 2017–2018, respectively). Measured received levels and modeled sound propagation were then used to estimate monopole source levels (MSL) and radiated noise levels (RNL). At 55-m range, the conventionally powered ferry type produced 156 and 157 dB re 1 μPa²m² MSL and RNL, respectively, while the same metrics for the electric ferry were 12 dB lower. At frequencies below 500 Hz, spectral levels of the electric ferry at a range of <5 m were 10–25 dB lower than those of the conventional ferry, implying a potential benefit for animals that use low-frequency communication, if electric motors replaced petrol or diesel engines.