Abstract

Introduction of anthropogenic underwater sound (or underwater noise) has been established as an essential stressor for the health of the marine environment, and one of its major components of continuous nature is the noise from vessels. Increasing of commercial shipping traffic but also traffic due to smaller vessels create a challenging soundscape for marine animals, which depend on sound for basic functions. Therefore, scientific research, policy initiatives and bridging between them have been intensified towards an effective management of vessel noise, a good environmental status for the marine waters and a sustainable maritime future. This short review is an effort to demonstrate advances in selected important aspects related to underwater noise from vessels: monitoring with modeling and measurements, impacts on marine life, mitigation measures and potential synergistic effects of underwater radiated noise reduction with energy efficiency measures and greenhouse gas reduction strategies, measurements and standardization, and major international initiatives for monitoring, assessing reducing and regulating underwater noise from vessels. Last, specific emerging and future issues are also considered.

Keywords:Underwater noise; Vessel noise; Shipping noise; Impacts; Marine life; Monitoring; assessment; Noise reduction

Abbreviations: ABS: American Bureau of Shipping; ACCOBAMS: The Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic area; AIS: Automated Identification System; AMSA: Australian Maritime Safety Authority; ANSI: American National Standards Institute; ASA: Acoustical Society of America; AUN: Anthropogenic Underwater Noise; BV: Bureau Veritas; DG ENV: European Commission’s Directorate-General for Environment; DNV GL: Det Norske Veritas Germanischer Lloyd; ECHO: Enhancing Cetacean Habitat and Observation; EOV: Essential Ocean Variables; EMSA: European Maritime Safety Agency; GES: Good Environmental Status; GHG: Greenhouse gases; GloNoise: Global Partnership for Mitigation of Underwater Noise from Shipping; GLUBS: Global Library of Underwater Biological Sounds; GOOS: Global Ocean Observing System; HELCOM: the Baltic Marine Environment Protection Commission, known as the Helsinki Commission; IMO: International Maritime Organization; IQOE: International Quiet Ocean Experiment; ISO: International Organization for Standardization; LR: Lloyd’s Register; MEPC: IMO’s Marine Environment Protection Committee; MSFD: Marine Strategy Framework Directive; NOAA: National Oceanic and Atmospheric Administration; OPUS: Open Portal to Underwater Sound; OSPAR: the Convention for the Protection of the Marine Environment of the North-East Atlantic; PCoD: Population Consequences of Disturbance; PSA: Productivity Susceptibility Assessment; PSSA: Particularly Sensitive Sea Area; PTS: Permanent Threshold Shift; RINA: Registro Italiano Navale; TG Noise: European Union Technical Group on Underwater Noise; TTS: Temporary Threshold Shift; UNEP/MAP: the Convention for the Marine Environment and the Coastal Region of the Mediterranean Sea, formerly known as the Barcelona Convention; URN: Underwater Radiated Noise; UVN: Underwater Noise from Vessels; VMS: Vessel Monitoring System; WMU: World Maritime University; WOA: World Ocean Assessment

Introduction

The marine environment is full of sounds. Underwater sounds originate from natural sources (geophony), such as wind, waves, currents, rain, thunders, earthquakes, ice-cracking, and marine life (biophony), such as vocalizations of marine animals, shrimp snapping etc., as well as from anthropogenic sources (anthrophony), such as shipping, hydrocarbon exploration (geophysical surveys), sonar (both military and civil), explosives, pile driving, dredging, drilling etc.; see, e.g., [1]. Anthropogenic underwater sounds can be categorized into impulsive and continuous, according to their duration. The first category includes sources producing sounds of short duration, such impact pile driving, explosives, seismic surveys, sonar and acoustic deterrents, while the second category mainly includes shipping, dredging, drilling, vibro-piling, and operation of offshore structures and devices (offshore wind farms, marine energy converters, floating storage and regasification units, etc.); see, e.g., [2]. Independently from the sound type or category, the anthropogenic underwater sounds of low-frequency can travel over very long distances (e.g., ship traffic, geophysical surveys).

The unwanted sound in the marine environment from anthropogenic sources is usually called anthropogenic underwater noise (AUN). Scientific research has documented, increasingly in the last two decades, that when AUN exceeds certain thresholds (a combination of intensity, duration, and spatial coverage), it can have adverse effects on the hearing abilities, behavior, and, finally, well-being of marine life (mammals, fish, turtles, birds and even seagrass), both at individual and population levels. Its impacts on the marine life vary from reduced communication range and masking of biologically relevant sounds, to changes in feeding or habitat areas, altered diving and breathing patterns, disruption of reproduction, chronic stress, temporary or permanent hearing loss, and even death on an individual or group scale (e.g., mass strandings of marine mammals).

As a result, there is a growing concern, both from the scientific community and policy makers, of AUN reduction to mitigate the aforementioned adverse effects. At the European level, the European Commission has adopted thresholds for AUN under the Marine Strategy Framework Directive, requiring cooperation at regional level, including the Regional Sea Conventions (the Baltic Marine Environment Protection Commission, known as the Helsinki Commission (HELCOM), the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR), the Convention for the Marine Environment and the Coastal Region of the Mediterranean Sea, formerly known as the Barcelona Convention (UNEP/MAP)), while leading countries that have adopted AUN-related policies and actions are the United States, Canada, Australia and several (mostly northern) European countries.

Shipping is recognized as the main source of continuous underwater noise, primarily due to propeller noise from cavitation and engine noise from vibrations transmitted through the hull. Numerous scientific studies have shown that this noise affects the communication and behavior of marine organisms, which may display behavioral changes, chronic stress, communication masking, and potential hearing loss due to increased levels of underwater noise from shipping; see Section 4 for more details. For this reason, it is essential to monitor changes in ship traffic, to examine the noise it generates and the propagation of the latter in the marine environment, and to consider mitigation measures. At the international level, the International Maritime Organization (IMO) plays a key role, having issued guidelines for “quieter” ships and organized relevant workshops as well as initiatives to increase awareness in developing countries.

Over the past 50 years, there has been a significant global increase in the size and number of vessels in commercial fleets. During the same period, increasing levels of ambient noise have been observed in certain ocean areas. Studies attribute this rise in noise levels to the growth of global shipping; see, e.g., [3-5]. Recently, Vakili et al. [6] estimated the rate of increase in maritime trade to range from 40% to 100% between 2020 and 2050, translating to an average annual growth rate of about 1.21% to 2.33%. Assuming the worst-case scenario of 2.33%, the average increase in shipping noise levels was estimated globally in relation to the rise in maritime trade and the reduction in noise that will be required to meet the goals set by the Okeanos Foundation; see Figure 4 in [6].

During 2020, shipping was among the many human activities affected by the COVID-19 pandemic; see, e.g., [7]. However, the pandemic also presented a unique experiment for the scientific community, offering an unprecedented opportunity to better explore the impacts of ship noise on the underwater environment in many parts of the world; see, e.g., [5,8-11].

In the following sections, an effort will be made to summarize advances in some important issues related to underwater noise from vessels (UVN). The topics selected were the monitoring of UVN with modeling and measurements, the impact of UVN on marine life, the UVN mitigation measures and potential synergistic effects of underwater radiated noise (URN) reduction with energy efficiency measures and greenhouse gas (GHG) reduction strategies, the measurements and standardization of URN from vessels, as well as UVN-related European and international initiatives. Let us note that, although commercial shipping dominates the noise from vessels (especially in low frequencies), the use of the term ‘vessels’ has been used over the term ‘ships’ to include both large and small vessels.

Monitoring UVN with Modeling and Measurements

Acoustic monitoring aimed at assessing the effects of anthropogenic noise - particularly from commercial shipping, seismic surveys, and military activities - emerged in the 1970s in response to growing environmental concerns. By the 1990s, increased awareness and technological advancements enabled more rigorous research, supported by the development of digital hydrophones and remote acoustic monitoring systems for measuring underwater ambient noise. During this period, the concept of “ocean noise pollution” began to take shape, as anthropogenic sound was increasingly recognized as a threat to marine ecosystems. Since the 2000s, modern monitoring programs incorporating both permanent and mobile stations have been established to evaluate oceanic ambient noise levels, investigate long-term trends in both natural and anthropogenic sources, and assess their impacts on marine life..

However, given their local validity, measurements are often not adequate to characterize and assess the underwater acoustic status of large marine areas and practically is not possible to spread measuring systems throughout such areas to achieve an adequate coverage. To this end, two monitoring methods are complementarily used to assess the acoustic status of a smaller or larger marine area: simulation models and underwater sound measurements. In the first case, the appropriate models should be first verified and then validated with field measurements, while in the second case, measurements can be supported by modeling. It should be emphasized that exclusive use of either modeling or measurements can lead to uncertainties. The European Union Technical Group on Underwater Noise (TG Noise) advises that the combined use of measurements and models is the best way for Member States to determine the levels and trends of ambient noise in the relevant frequency bands; see [12], pp. 21, 40).

According to [13], Annex 5, in the case that UVN monitoring is based on numerical models supported by measurements, modeling is used to produce soundscape maps on a grid to assess both the temporal and spatial distribution of noise, and three types of models are involved in the procedure: models (empirical, semi-empirical, hybrid) estimating the distribution and levels of the sound emitted from the acoustic source (Underwater Radiated Noise, URN), sound propagation models calculating the noise levels on the predetermined grid, and models estimating sound levels from natural sounds (wind, waves, currents, etc.) on that grid.

For most vessels, individual source power is unknown. Thus, statistical models have been developed to describe the source levels from different vessel categories and provide a statistical representation of source power for specific frequencies or frequency bands, considering either a monopole or a dipole source. A not-exhausting chronologically-ordered list of such models includes [14-26]. An important issue is the availability of parameters required for the implementation of those models, which varies from a number of basic parameters (e.g., the vessel type, length and speed) to several parameters including propeller and hull as well as vessel engine characteristics. The user has to consider the assessment framework and the marine area under assessment, and to compromise between data availability and accuracy.

Quite a lot underwater acoustic propagation numerical models have been developed, mainly categorized with respect to the suitability of handling bathymetry (shallow vs deep water), frequency range (low vs high frequencies) and variability of the marine environment with respect to the distance from source (range independent vs range-dependent); see, e.g., [27,32]. Specific modifications allow for treatment of particular environments, such as ice-covered areas. The input parameters for these models mainly include information on the characteristics and distribution of the sources and the environmental conditions (bathymetry, sound speed profiles, seabed composition and properties). Basic information on ship characteristics and distribution is provided by Automated Identification System (AIS) data, while the corresponding information for (some) fishing vessels is provided by Vessel Monitoring System (VMS). Considerable gaps may exist in vessel distribution data due to incomplete AIS data (inadequate coverage offshore, switching off AIS, etc.) and significant lack of distribution data for smaller vessels, therefore methods filling such gaps could result in significant corrections. In general, availability and accuracy of source and environmental data play a crucial role in the quality of model results. Apart from the legendary book of Jensen et al. [28], representative reviews of underwater acoustic propagation models can be found in [29-31], while a considerable number of models have also been developed for specific environments and frequency range; see, e.g. [32].

The contribution of natural sounds to the overall underwater soundscape is usually considered with the implementation of an empirical model for wind contribution. A description for relevant techniques can be found in [33]. Validation of models that estimate the wind contribution is difficult since it must take place in areas with very low levels of vessel traffic and very low presence of vocalizing animals.

A modeling system for prediction of vessel noise including natural sound contributions (mostly wind) can be efficiently used for assessment after validation of its results with field measurements. However, before model validation, demonstrating that the models are correctly implemented in software and calculate the correct outputs for specified inputs (verification; see, e.g., [34]) is a beneficial procedure, and can be implemented by comparing models to understand differences between model predictions for identical inputs; see, e.g., [35].

Model calibration, meaning adjustment or tuning of model parameters, follows validation, and may be a recurrent procedure in order to achieve a satisfying convergence between model results and measurements for various time periods; see, e.g., [36,37]. During the validation/verification process, sources of uncertainty and variability should be identified. The key aspects and parameters involved in numerical modeling of UVN are summarized in Table 1.

According to [13], Annex 5, when assessing underwater noise primarily with measurements, the locations and number of measurement stations are crucial to ensure results that accurately reflect the soundscape conditions of the area, following guidelines such as those in [12]. In low vessel traffic areas, measurements may be sufficient on their own due to generally low sound levels. However, in areas with moderate to high vessel traffic, measurements alone cannot reliably define the Reference Condition (acoustic status without or with very low vessel traffic, where natural sounds prevail). Assessing acoustic conditions in complex environments like archipelagos is particularly challenging due to their geomorphology and the presence of many untracked recreational vessels. In such cases, a combination of strategically placed measurements, acoustic propagation modeling, and vessel traffic data (AIS, and VMS where available) can still yield meaningful insights.

Impacts of UVN on Marine Life

The number of scientific studies today is sufficient to support with high certainty that anthropogenic noise has negative impacts on marine animals. A synthesis of the negative effects of noise pollution on marine life is presented by [1], based on the systematic literature review conducted in [38]. According to this review, vessel noise appears to negatively affect marine animals, with 94.9% of the studies reporting statistically significant impacts of UVN on marine fauna; see Figure 4 in [1].

Impacts of UVN on marine animals are discussed in several reviews: [39] and [40] focus on marine mammals, which are the most extensively studied group; [41] address fish; [42] examine effects on invertebrates; [43] provides a synthesis for all marine animals; [44] explores the implications for marine biodiversity in general. Additionally, [45] focuses on fish and invertebrates, while an example of a regional review is [46] that specifically targets Mediterranean fish and invertebrates. New studies continue to demonstrate the potential impacts of UVN on marine organisms; see, e.g., [47-53].

The impacts of UVN on marine mammals, as reported in the review [39], include changes in both physical and acoustic behavior, masking of communication and echolocation sounds, and stress. More specifically, the review highlights the effects of UVN on mysticetes (baleen whales), odontocetes (sperm whales, beaked whales, dolphins, porpoises, orcas, etc.), sirenians (dugongs, manatees), and pinnipeds (various seal species). The review also identifies and discusses common issues and challenges related to physical aspects (e.g., sound exposure assessment, recording and reproduction of URN), biological aspects (e.g., experimental design, distinction between disturbance and biological relevance), and research needs, including species coverage, geographical scope, vessel types, and impact types. As mentioned in [40], UVN can negatively affect individual marine mammals’ vital rates, and when a significant portion of the population is exposed, these effects may escalate to population-level consequences. The Population Consequences of Disturbance (PCoD) framework was developed to understand how nonlethal human disturbances, like noise, impact marine mammal populations. While this model is widely used, there remains a lack of empirical evidence linking short-term behavioral changes to long-term biological impacts. Nevertheless, repeated or chronic exposure to vessel noise - especially in heavily trafficked or industrialized areas - poses a significant risk to marine mammal health and population viability.

Over 800 fish species have been found to communicate acoustically [54]. According to [41], increased continuous noise (e.g., shipping or boat noise), can significantly disrupt critical fish behaviors by masking biologically important sounds used for foraging, predator avoidance, migration, habitat selection, and reproduction. Increased noise levels may impair predatorprey interactions, hinder navigation during migration, and interfere with reproductive communication, potentially affecting population success. Although most predator avoidance studies have been conducted in captivity, there is a clear need for research on wild fish in natural environments. Additionally, some evidence suggests that fish may alter their own acoustic signals to avoid being masked by anthropogenic continuous noise, highlighting the widespread behavioral impacts of the relevant form of pollution. While much research on underwater noise has focused on individual animals, the greater concern for fish may lie in its population-level effects, which remain poorly understood. Existing models like the Population Consequences of Disturbance (PCoD) framework, developed for marine mammals, offer a structured way to assess how sound impacts individual survival and reproduction, potentially affecting population abundance. For fish species with limited ecological data, alternative tools like the Productivity Susceptibility Assessment (PSA) have been used to evaluate vulnerability to human activities and may also be useful for assessing the risks of noise exposure on fish populations.

Regarding the impacts of UVN on invertebrates, [42] in Tables 4-7, details the relevant studies that have examined the effects of underwater noise on various invertebrate categories. These tables include the species and developmental stages studied, the observed effects, the acoustic source, and the corresponding noise levels. Studies related to UVN can be isolated by searching the noise source with keywords such as ‘ship’, ‘vessel’, ‘boat’, ‘high ambient noise’, ‘continuous’, and ‘noise eggs’, and for noise levels by the keyword ‘low frequency’. In summary, evidence of URNrelated effects on invertebrates includes behavioral responses (mainly changes in movement patterns, settlement behavior, and protective reactions) and physiological impacts (primarily increased stress, reduced bioaccumulation, slowed growth rates, and morphological effects) [43].

For the protection of marine animals from underwater noise, acoustic thresholds and criteria have been proposed in relation to exposure to pulsed and non-pulsed/continuous noise and specific effects such as mortality, occurrence of permanent threshold shift (PTS), occurrence of temporary threshold shift (TTS), and the appearance of behavioral responses. These thresholds can be found in [55,56] for marine mammals, [41] and [57] for fish and marine turtles, and [58] for all of the above taxa.

Mitigation Measures of UVN and Synergistic Benefits

The concern over the increasing and widespread impacts of UVN, especially on marine ecosystems, has led regulatory authorities to take measures aimed at reducing underwater noise emissions from shipping. According to [40] and relevant references therein, three general approaches to UVN mitigation measures have been proposed:
a) technological modifications to reduce levels of radiated noise
b) reduction of noise generated through operational solutions, such as vessel slowdowns
c) reduction of received noise by implementing alternative routing measures.

An illustrative depiction of UVN mitigation measures can be found in [59], which categorizes mitigation measures based on their complexity (from the simplest/operational actions to more complex measures involving new design interventions), their cobenefits (e.g., reduction of greenhouse gas emissions, decreased ship strikes on cetaceans, improved efficiency, economic benefits), and the categories of stakeholders involved (IMO and associated Member States, shipowners and operators, managing authorities, and other stakeholders). These are the following:
a) Speed reduction (slow-down or slow-steaming) aimed at decreasing cavitation. Due to its simplicity, it is the most extensively studied measure, with many relevant publications available (see, e.g., [60-62]). It can be particularly effective at a local scale.
b) Re-routing to avoid important breeding and feeding habitats and migration corridors. It can also be effective mainly at a local scale, though there is evidence that the problem may be displaced to adjacent marine areas [63].
c) Cleaning and maintenance of propellers and hulls.
d) Use of flow control devices (e.g., for drag reduction), installed on the hull or elsewhere on the vessel, intended to mitigate the effects of cavitation.
e) Retrofit/modification of propellers and engines.
f) Assistance or replacement of propulsion using alternative energy sources, such as wind power.
g) Design of new, quieter ships.

A short review of most of the aforementioned measures can be found in [64], Annex 2, which includes the mitigation measures by IMO [65], ACCOBAMS [66], and the guidelines by HELCOM [67]. It should be noted here that the IMO guidelines for the reduction of underwater noise emissions from ships to address the adverse impacts on marine life were updated with [68]. Also, a relatively recent review of the methods and technologies for reducing underwater radiated noise from ships is provided in [69].

The co-benefits of synergistic actions aimed at reducing underwater radiated noise from ships and lowering greenhouse gas (GHG) emissions as part of green shipping—along with other beneficial effects—are increasingly being researched and developed; see, for example [70-73], [6].

Improving the ship design process is an important shortterm initiative that can be supported through optimization of the ship hull form and the use of systems and components aimed at reducing fuel consumption. Among other elements, these include hull structures, propulsion systems, energy systems, control systems, and operational protocols. Experience from a number of European projects [74], focused on mitigation measures (d)-(f) above, with the primary goal of reducing GHG emissions but also having a direct positive impact on reducing underwater radiated noise from ships, resulted in important findings regarding hull form optimization, exploration of utilization of propeller and rudder devices, integration of various energy-saving devices with advanced propeller design, retrofitting flow control devices onto existing vessels or incorporation into new designs, as well as investigation of several hydrodynamic enhancement models for specific devices along with exploitation of marine renewable energy.

In September 2023, the IMO [75] organized a workshop involving participants from technical, regulatory, and policymaking fields related to GHG and underwater radiated noise (URN), in order to ensure broad industry participation with practical experience in the implementation of both GHG emission reduction programs and URN mitigation practices. The key messages from the workshop were: Intentional design and technology integration are needed to optimize GHG and URN reduction; GHG and URN emission measurements are necessary to support innovation; Operational considerations can significantly influence GHG and URN reduction; Ongoing information sharing, collaboration, and stakeholder engagement are essential to advance GHG and URN reduction efforts; Decision support tools are needed to foster increased integration of GHG and URN reduction strategies. In the recent IMO-WMU workshop [76], during the session ‘URN Mitigation Solutions, Transferrable Practices and Role of Stakeholders’, several presentations addressed the co-benefits of reducing URN and GHG emissions—not only avoiding a loss of energy efficiency, but in fact enhancing it [72,77-79].

Measurements and Standardization of Underwater Radiated Noise (URN) from Vessels

Efforts to reduce ship noise, including quiet vessel certification programs, aim to mitigate these impacts. However, inconsistencies in measurement standards across classification societies and the challenges of conducting accurate underwater radiated noise (URN) measurements - especially in shallow water - limit the comparability and effectiveness of such certifications. Standardizing and improving measurement methods could enhance global adoption of quieter ship practices and facilitating communication between appropriate stakeholders.

URN measurement methods are described in the silent ship certification procedures of five classification societies: American Bureau of Shipping (ABS), Bureau Veritas (BV), Det Norske Veritas Germanischer Lloyd (DNV GL), Lloyd’s Register (LR), and Registro Italiano Navale (RINA); [80], Table 2. These five procedures result in five different values of underwater radiated noise level (in deep water, the ABS and RINA procedures are almost identical). All five procedures use either Source Level (LR), Radiated Noise Level (RINA), or a modification of these (ABS, BV, DNV GL).

On the other hand, international standards regarding quantities and procedures for description and measurement of URN from ships in deep water (ASA/ANSI S12.64-2009 [81], ISO 17208-1:2016 [82], ISO 17208-2:2019 [83]) and the umbrella of ISO terminology for underwater acoustics (ISO 18405:2017 [84]) provide significant standardization guidance, while the ISO/ FDIS 17208-3 [85] for shallow water measurements is under development. According to the URN Standardization Support program [86], which aims to assist in developing the ISO/FDIS 17208-3 standard by providing measurements of URN in shallow water, the proposed measurement geometries concern a deep water geometry, a horizontal array for performance evaluation in very shallow water, vertical arrays at two distances as well as a three-element horizontal array for evaluating geometries at intermediate water depth, while adjustments are needed in measurements for specific frequencies and hydrophone array configurations; see Figure 6 in [80].

International Initiatives for Monitoring, Assessing Reducing and Regulating UVN

The ecological implications of UVN are increasingly being recognized as a critical factor in ocean health. The Biology and Ecosystems Panel of the Global Ocean Observing System (GOOS) has formally identified (ambient) sound as a cross-disciplinary Essential Ocean Variable (EOV), acknowledging its ecological importance and its sensitivity to human-induced pressures [87]. Marine traffic is explicitly mentioned as one of the main ocean noise contributors in the Second World Ocean Assessment [88] of the United Nations, and –among others– issues regarding relevant mitigation measures, standardization and AIS coverage are addressed. In response, global efforts for management and regulation have been intensified towards the broader goals of marine ecosystem conservation and sustainable human activity. This international momentum is reflected in policy literature. In [89] International Treaties and their link with underwater noise are described, and [90] provides a detailed synthesis of organizations, relevant resolutions, decisions and reports demonstrating international interest in addressing anthropogenic underwater noise.

Different approaches have been developed internationally to assess and manage UVN and mitigate its impacts on marine life. Major initiatives will be described below for Europe, U.S., Canada and Australia, while several countries worldwide have adopted national measures and regulations. Europe’s response consists of the following main initiatives: a) the Marine Strategy Framework Directive (MSFD) 2008/56/EC [91], establishing ‘introduction of energy, including underwater noise, into the marine environment’ as one of the eleven qualitative descriptors for determining Good Environmental Status (GES). MSFD was accompanied by the Commission Decision 2017/848 [92] for criteria and methodological standards on Good Environmental Status of marine waters and specifications and standardized methods for monitoring and assessment, in which a specific criterion is defined for monitoring and assessment of continuous low-frequency anthropogenic sound. The Technical Group on Underwater Noise (TG Noise) - established by the European Commission in 2010 to support Member States in implementing the MSFD - proposed a methodology and specific thresholds [13,93], and in late 2022 the European Commission approved these first EU-wide thresholds; b) UVN is part of the policies and actions of Regional Sea Conventions ([67], Strategic Objective 8 in [94], Ecological Objective 11 in [95]). Following the aforementioned TG Noise methodology and thresholds, Klauson et al. [96] performed the first regional assessment for shipping noise for the Baltic Sea; c) the European Union has supported research on underwater noise from ships, their effects on marine life, and potential mitigation measures through a series of research program calls (DG ENV, Horizon 2020, Horizon Europe, INTERREG, LIFE). Relevant project compilations can be found in [97] and [98], while on-going projects under the JPI Oceans (www.jpi-oceans.eu/en/underwater-noisemarine- environment) and the INTERREG frameworks, as well as the Sustainable Blue Economy Partnership, update those lists; d) the European Maritime Safety Agency (EMSA) provided strategic leadership in the implementation of two key scientific programs, the SOUNDS project [43] on an inventory of existing policy, research and impacts of continuous underwater noise in Europe, and the NAVISON program [99] where calculation and analysis of shipping sound maps for all European Seas from 2016 to 2050 was performed, also providing forecasts for 2030, 2040 and 2050 under four different mitigation scenarios.

The U.S. strategy regarding underwater noise from ships includes multiple actions and initiatives: a) the Maritime Administration under Marine Energy Technology Evaluation & Advancement program (U.S. Department of Transportation) - mandated by the 2020 and expanded in the 2023 National Defense Authorization Acts - actively researches technologies to reduce vessel underwater radiated noise (URN), including quieter propeller designs, hull forms, and operational shifts. Maritime Administration also investigates the synergy between quieter ships and improved energy efficiency (https://www.maritime. dot.gov/innovation/meta/vessel-generated-underwaternoise); b) National Oceanic and Atmospheric Administration (NOAA) Ocean Noise Strategy towards development of soundmapping tools, decision-support systems, and long-term acoustic data programs (oceannoise.noaa.gov); c) the SanctSound: a collaborative NOAA–Navy project (2018–2022) that installed and analyzed sound monitoring stations across seven National Marine Sanctuaries (sanctsound.ioos.us); d) the Ocean Noise Reference Station Network in major U.S. regions, monitoring ambient and vessel noise trends from 2014 onward (https://www.pmel.noaa. gov/acoustics/noaanps-ocean-noise-reference-station-network); e) the Quiet Sound program, a voluntary slowdown program (from October 2022), targeting commercial vessels in Southern Resident killer whale habitats (quietsound.org); f) vessel noise best practices, where NOAA, in collaboration with industry and IMO representatives, promotes quieter vessel operations through speed reductions, optimized route planning, hull maintenance, and propulsion design - all aiming to minimize URN as a cobenefit with energy efficiency; g) active support of IMO revised guidelines by the U.S. delegation; h) NOAA Fisheries mandates for noise assessments and mitigation to protect listed species Under the Marine Mammal Protection Act and Endangered Species Act; i) the Navy’s Ship Silencing Program, focusing on reducing radiated noise from naval vessels and supporting quieter naval engineering practices.

Canada’s approach combines substantial funding, sciencedriven monitoring, regulatory guidance aligned with IMO standards, and community engagement, particularly in areas crucial for endangered whales. The result is a comprehensive and evolving framework to assess and reduce underwater noise from shipping vessels [100]. Representative large-scale actions are: the flagship Transport Canada’s program ‘Quiet Vessel Initiative’, targeting to create opportunities to engage indigenous groups about quiet vessel solutions and include traditional knowledge and participation in the project, research or testing projects to evaluate “quiet” technologies on marine vessels, and monitor underwater noise to assess the effectiveness of operational and technical mitigations aimed at addressing underwater noise; the Enhancing Cetacean Habitat and Observation (ECHO) Program, aimed at better understanding and managing the impact of shipping activities on at-risk whales throughout the southern coast of British Columbia with the long-term goal to develop mitigation measures that will lead to a quantifiable reduction in potential threats to whales as a result of shipping activities; the leading role in the adoption of the IMO 2023 revised guidelines, see [101]. It is worth noticing that the structure of the draft version of Canada’s Ocean Noise Strategy [102], organized around 3 themes, 3 objectives and 20 recommendations, closely aligns with the NOAA’s Ocean Noise Strategy Roadmap and the noise mitigation component of the European MSFD.

Australia has taken specific key steps to assess and mitigate underwater noise from shipping: the Australian Maritime Safety Authority (AMSA) has issued marine notices (e.g., Jan 2025, April 2024) promoting the 2023 IMO revised guidelines, see amsa.gov. au; under the National Environmental Science Program’s Marine Biodiversity Hub, Australia funded Project E2 (2018-2021), mapping spatial and temporal patterns of shipping noise in its EEZ and World Heritage areas and producing a technical report cataloging ship noise signatures based on long‑term recordings from Australia’s Integrated Marine Observing System; supports IMO-designated Particularly Sensitive Sea Areas. AMSA manages these zones, where shipping noise considerations are factored in; additionally, inquiries into reef ecosystem health, such as the 2021 Senate review, have recommended speed reductions and routing adjustments to lower noise in critical habitats like the Great Barrier Reef.

It has become evident so far that IMO plays a catalytic and ongoing role in boosting the international regulatory efforts for UVN reduction to address adverse impacts on marine life. Main IMO’s initiatives could be summarized as follows:
a) guidelines for the reduction of underwater noise from commercial shipping [65,68]
b) adoption of Particularly Sensitive Sea Areas (PSSAs), which are considered to deserve special protection, due to their recognized ecological or socio-economic or scientific significance, and which may be vulnerable to damage by ships. Ship routing measures can be proposed for adoption in connection with a PSSA, to protect marine life
c) joining forces with the United Nations Development Program and the Global Environment Facility to launch the Global Partnership for Mitigation of Underwater Noise from Shipping (GloNoise) [103]. The general goal of the GloNoise Partnership is to establish a truly global stakeholder collaboration, with a strong focus on developing countries, in order to address the major environmental issue of underwater noise. The IMO and the World Maritime University (WMU) organized a workshop [76] related to this initiative
d) In late 2024, the Marine Environment Protection Committee (MEPC 82) approved a URN Action Plan for the reduction of underwater noise from commercial shipping, to be reviewed and revised as necessary. The URN Action Plan aims to address barriers to the uptake of the Revised URN Guidelines in order to further prevent and reduce URN from ships.

Last but not least, large-scale long-term international scientific programs, such as the International Quiet Ocean Experiment (IQOE) (iqoe.org), aiming to promote research, observations, and modelling to improve understanding of ocean soundscapes and effects of sound on marine organisms, are opportunities for international collaborations, offer multi-level valuable insights to policy makers and raise awareness to the public. Among the running IQOE activities, one could highlight the actions to help implementation of the Ocean Sound Essential Ocean Variable Implementation Plan, the “Global Sounds: Low-Cost Hydrophone Project”, the expansion of the Open Portal to Underwater Sound (OPUS), the implementation of the Global Library of Underwater Biological Sounds (GLUBS), and the sponsorship of the second World Ocean Passive Acoustic Monitoring Day.

Conclusion and Further Considerations

Specific important aspects of underwater noise from vessels were selected for reviewing relevant advances: modeling and measurements, impacts on marine life, mitigation measures and co-benefits with other reduction strategies, measurements and standardization, and international initiatives. The potential adverse effects of increasing underwater noise on marine life, especially from commercial shipping, have boosted international efforts for noise reduction measures and management and regulatory actions. Moreover, some specific points of ongoing and future scientific and policy interest are highlighted:
a) Integrating energy efficiency and underwater radiated noise reduction strategies in commercial shipping towards a sustainable maritime future [6]
b) A growing tendency towards underwater noise being thoroughly incorporated within Marine Spatial Planning ([104] for Europe) c) The future contribution of ships to the underwater soundscape under the climate change and ocean acidification [105].
d) The recent efforts to evaluate noise from smaller vessels [106,107].
e) The concern on the impacts, management and governance options of underwater noise from shipping in polar environments, mostly in the Arctic [108-110], but also in Antarctica [111,112].
f) The cumulative impact on marine life from multiple anthropogenic pressures [113].

Acknowledgment

The author acknowledges support by the project NAVGREEN, implemented in the frame of the National Recovery and Resilience Plan “Greece 2.0” with funding from the European Union - NextGenerationEU (Project code: TAEDR-0534767).

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