EMSA commissioned a study to consolidate information on the subject of continuous Underwater radiated noise (URN) from shipping, in order to derive recommendations for a future multi-stakeholder strategy within Europe.
he study focussed on four main subject areas, with noise sources, environmental impact and policy providing the basis for the main goal, mitigation.
Main source of URN
The main source of URN from shipping is broadband propeller cavitation, radiating noise over a large frequency
range. Machinery, primarily main propulsion engines, can also have an important contribution.
These are typically the mechanisms targeted by mitigation measures. Both measurements and modelling of ship noise are important for effective mitigation, with the standardisation of terminology and procedures, as well as uncertainty quantification, being the focus of recent and ongoing work
While deep water conditions have been treated thoroughly in the past, work on shallow water propagation effects is in progress, with final results expected in 2024. However, the limited access to Automatic Information System (AIS) data, required for experimental analyses and modelling purposes, also contributes to uncertainties, and should preferably be expanded.
In addition, the report notes that the simplification of ship noise measurements is a rapidly developing subject, with several studies on the use of onboard sensors and drones recently published.
Such technologies could help in increasing the amount of data available to researchers and policy makers in the future, as well as reducing costs for ship owners, when performed either on a voluntary or mandatory basis
In terms of underwater noise monitoring, many researchers go beyond the suggestion of the MSFD to consider the 63 and 125 Hz one-third octave (OTO) bands, often analysing higher-frequency bands (up to 50 kHz), or broadband levels, related to the communication frequencies of specific marine fauna.
There is also evidence that recreational craft might be the dominant noise generators in certain EU coastal waters, with these vessels typically producing sound at higher frequencies than larger merchant ships.
It is therefore recommended that future efforts on policy and mitigation take a wide range of frequencies into account
Looking to the available information regarding hearing ranges and the use of sound by different species, an overlap with the most relevant noise sources from shipping is unequivocal. Responses to underwater noise levels have been observed for the main groups of species, marine mammals, fish and invertebrates.
They correspond mainly to behavioural changes, masking and physiological responses. Behavioural responses, depending on the group, are reflected as for example, on changes of swimming and diving pattern, displacement or changes in the vocal behaviour, reflected on changes on the acoustic signals the animals produce
Furthermore, although most of the studies evidence responses to underwater noise from shipping, few of them provide a proper description of the source levels or the received levels at which the responses were observed. This presents a constraint on the assessment of the impacts in the presence of different vessels, according to EMSA.
Underwater noise policies
So far, several policies are being developed and implemented at different levels for managing underwater noise.
These are being developed under the auspices of the Regional Sea Conventions, International Multi-party agreements and at European level.
Nevertheless, only the guidelines released at IMO, providing recommendations for underwater noise reduction from shipping, are exclusively dedicated to the shipping industry.
The Marine Strategy Framework Directive is the only piece of regulation implementing binding actions at European level
Other policies that can be implemented are related to incentives. In fact, while numerous classification societies offer “Quiet Class” notations, few merchant vessels have, so far, been built with noise requirements. For this reason, ship owners could be encouraged to obtain such notations through incentive schemes, which may also accept voluntary sustainability certification as evidence of noise performance.
As for economic incentives, they have had some success in North America, with possible implementation in Europe requiring further attention.
Numerous possible solutions exist, whose impact in terms of, for example, GHG emissions and financial aspects, should be evaluated
the report said.
Considering, the wide range of activities being performed by multiple stakeholders in relation to ship URN, and the diverse data sources required for modelling purposes, the efficiency of research & development work, as well as noise management could be significantly enhanced by establishing a common data repository.
Managing underwater radiated noise from ships is a multi-sectoral challenge which requires coordination between different policies and stakeholders to reach the main goal of underwater noise reduction