2023_programme: Small reductions in cargo vessel speed dramatically reduce noise impacts to marine wildlife

• Session: 20. Underwater noise in the EU Marine Strategy Framework Directive: monitoring and impacts on marine ecosystems
  Organiser(s): Jakob Tougaard, Aristides Prospathopoulos and Emmanuel Skarsoulis
• Lecture: Small reductions in cargo vessel speed dramatically reduce noise impacts to marine wildlife
  Paper ID: 2048
  Author(s): Findlay Charlotte R., Rojano-Doñate Laia, Tougaard Jakob, Johnson Mark P., Madsen Peter Teglberg
  Presenter: Findlay Charlotte
  Abstract: Global reductions in underwater radiated noise (URN) levels from cargo vessels are needed to reduce increasing impacts on marine wildlife. Concern for the increasing and widespread impacts from vessel noise have led regulators to take measures to reduce URN. Three approaches have been proposed: (1) slowdowns; (2) technological modifications; and (3) increasing the distance between vessels and animals. However, the efficacy of these mitigation measures at reducing impacts has rarely been quantified. \n\nWe used a vessel exposure simulation model to examine how reducing fixed pitch propeller vessel source levels through slowdowns and technological modifications, and increasing distance between animals and vessels, can lessen impacts on marine mammals. We used maximum received level, acoustic looming (i.e., peak rate of change in vessel received levels over time), and exposure duration (i.e., time over which vessel received levels exceed the ambient noise) as impact proxies. \n\nWe show that the area exposed to ship noise reduces dramatically with moderate source level reductions that can be achieved with small reductions in speed. For example, a 10 dB source level reduction results in 90% less area instantaneously exposed to URN from a vessel. Slowdowns are an effective way to reduce source levels: a 10 to 50% reduction in speed decreases source levels by 3 to 18 dB at all frequencies if speed exceeds cavitation inception speed. Moreover, slowdowns reduce all impacts to marine mammals despite the longer time that a slower vessel takes to pass an animal. Speed reductions can also be supplemented by routing vessels away from critical habitats, where possible, and by technological modifications to reduce vessel noise.\n\nWe conclude that cumulative noise impacts from the global fleet can be reduced immediately by slowdowns. This solution requires no modification to ships and is scalable – from local speed reductions in sensitive areas to ocean basins.
• Corresponding author: Dr Charlotte Findlay
  Affiliation: Aarhus University
  Country: Denmark
  e-mail: