Underwater Noise Pollution

Sound as Life: Why Acoustic Ecology Matters

Water transmits sound approximately 4.5 times faster than air and over vastly greater distances. In the deep ocean, a phenomenon called the SOFAR channel (Sound Fixing and Ranging channel)—a layer of minimum sound speed at roughly 1,000 metres depth—acts as a natural acoustic waveguide, allowing low-frequency sounds to travel thousands of kilometres with minimal attenuation. Blue whales exploit this channel, communicating across ocean basins. Fin whales produce 20 Hz calls that once carried from Iceland to the Azores. Sperm whales use complex biosonar clicks to hunt in the lightless deep. For these animals, acoustic communication and echolocation are not supplements to other senses—they are primary survival tools. Disrupting the acoustic environment is therefore not merely a nuisance; it is an attack on the biological infrastructure of marine life.

The 2021 Duarte review synthesised decades of research across taxa, concluding that noise pollution affects organisms across the entire animal kingdom in the ocean, from zooplankton to great whales [1]. The review identified three primary mechanisms of harm: direct physiological injury (temporary or permanent hearing threshold shifts), masking (noise overlapping with biologically important signals, preventing reception), and behavioural disruption (animals altering foraging, migration, reproduction, or communication in response to noise). All three pathways can reduce survival and reproductive success, with cascading effects on population dynamics.

Commercial Shipping: The Dominant Broadband Source

More than 90% of global trade moves by sea, aboard an estimated 90,000 commercial vessels at sea at any given moment. These ships generate continuous broadband noise dominated by frequencies between 10 and 1,000 Hz—precisely the range used by baleen whales for long-distance communication. A 2012 acoustic analysis of modern commercial vessels by McKenna and colleagues at Scripps Institution of Oceanography quantified source levels and identified propeller cavitation—the formation and collapse of vapour bubbles at propeller blade tips—as the dominant noise mechanism, followed by machinery vibration transmitted through the hull [7]. Container ships and bulk carriers are among the loudest vessels per unit, while slower or better-designed ships can be dramatically quieter.

Estimates from underwater acoustic monitoring arrays suggest that low-frequency ambient noise in major shipping lanes has increased by approximately 15 dB—a factor of roughly 32 in pressure—since the 1950s, coinciding with the explosive growth of global trade. A 2017 study in *Frontiers in Marine Science* measured source spectra of ships in the Baltic Sea over long recording periods, confirming that even semi-enclosed regional seas with moderate traffic are experiencing sustained elevated noise floors [2]. The consequences for species that rely on low-frequency communication—blue whales, fin whales, right whales—are profound: their effective communication range may have shrunk by an order of magnitude.

Masking and Communication Breakdown

Acoustic masking occurs when a noise source overlaps in frequency and time with a biologically important signal, reducing its detectability. For North Atlantic right whales—among the most endangered large mammals on Earth, with a population below 360 individuals—shipping noise in the low-frequency band directly masks their contact calls used for mother-calf communication. A study by Rolland and colleagues using automatic call detectors demonstrated that right whales in areas with reduced shipping traffic (during port closures after 9/11) showed significantly lower levels of stress hormones, providing some of the strongest causal evidence that chronic ship noise produces measurable physiological harm [8]. Some cetacean species have shown remarkable plasticity, shifting call frequency or timing to avoid overlap—but such shifts carry energetic costs and may not be sufficient compensation.

Seismic Surveys: Impulsive Noise Across Ocean Basins

Seismic air-gun surveys are used by the oil and gas industry to map sub-seafloor geology. Arrays of pneumatic air guns are towed behind vessels, firing compressed air pulses every 10–15 seconds for days or weeks. Each pulse generates an impulsive sound with peak levels exceeding 250 decibels (re 1 µPa at 1 m), detectable thousands of kilometres from the source. A 2018 study published in *Marine Pollution Bulletin* examined the cumulative acoustic contribution of multiple simultaneous seismic surveys in Baffin Bay, Greenland, and found that the entire basin was exposed to elevated noise levels throughout the survey season, with potential implications for bowhead whales and narwhals using the area [3].

A systematic literature review of the effects of marine seismic surveys on free-ranging fauna, published in *Frontiers in Marine Science* in 2023, found consistent evidence of behavioural disruption across fish, marine mammals, and sea turtles within a radius of tens to hundreds of kilometres from survey vessels [4]. Effects included temporary and permanent hearing threshold shifts, altered vocal behaviour, avoidance of feeding and breeding areas, and changes in diving profiles. A controlled exposure study on narwhals in East Greenland documented dramatic escape responses—sustained high-speed swimming and deep diving—that persisted hours after exposure, representing a significant energetic cost for animals managing the demands of Arctic seasonality [5].

Sonar and Mass Strandings: A Documented Link

The most dramatic and disturbing evidence of acoustic harm to marine mammals comes from the association between military mid-frequency active sonar (MFAS) and mass strandings of beaked whales. Since the 1960s, numerous multi-species beaked whale strandings have occurred during or shortly after naval exercises involving MFAS. The pathological signatures—haemorrhaging in acoustic fat deposits and around the ears, nitrogen emboli in tissues consistent with decompression-like injury—point to a behavioural cascade in which whales ascending rapidly to avoid intense sonar exposure suffer physiological harm analogous to decompression sickness. The Canary Islands (2002), the Bahamas (2000), and the Greek island of Kyparissia (1996) are among the best-documented events. Subsequent naval agreements to avoid exercises near beaked whale habitat have been associated with reduced stranding rates in some affected areas.

Fish and Invertebrates: A Broader Biological Toll

Vertebrate mammals receive the most scientific and public attention, but fish and invertebrates are far more numerous and ecologically consequential in marine food webs. Fish rely on sound for spawning aggregation, predator detection, and territory defence. Many species, including Atlantic cod, European sea bass, and several reef fishes, produce and detect sounds in the frequency range most contaminated by shipping and construction noise. Slabbekoorn and colleagues characterised this as a 'noisy spring,' documenting how anthropogenic noise impairs the acoustic ecology of fish across taxa and geographies [10]. Laboratory and field studies have documented disrupted predator-prey interactions, reduced larval settlement success on reefs near noisy areas, and impaired schooling behaviour. Cephalopods—squid and octopus—use statocysts to detect low-frequency particle motion and have been shown to suffer statocyst damage from seismic air-gun exposure. The Duarte review concluded that the ecological consequences of noise pollution likely extend far beyond the charismatic megafauna that dominate research attention [1].

Regulatory Frameworks: Progress and Persistent Gaps

The International Maritime Organization (IMO) first addressed underwater noise in 2014 with the adoption of MEPC.1/Circ.833, non-binding guidelines recommending that ship designers and operators consider noise reduction through propeller optimisation, hull form design, vibration isolation of machinery, and operational measures such as speed reduction [6]. These guidelines were revised and strengthened in 2024 as MEPC.1/Circ.906/Rev.1, reflecting a decade of accumulated research and technological progress. However, the non-binding nature of IMO guidelines means compliance is voluntary, and uptake by shipowners has been uneven. Speed reduction is among the most immediately available and cost-effective interventions: a 10% reduction in ship speed reduces radiated noise by approximately 2–4 dB and fuel consumption by roughly 19%, simultaneously delivering noise reduction and emissions benefits.

Several major shipping lanes now feature voluntary slow-steaming zones during periods of high cetacean activity—most notably in the Stellwagen Bank National Marine Sanctuary and the Salish Sea. Propeller design advances, including blade skew and improved tip geometry, can reduce cavitation noise at source by several decibels. Weilgart's comprehensive 2007 review of anthropogenic ocean noise impacts on cetaceans concluded that a suite of technological and operational interventions was available and that the principal barrier was not technical capability but regulatory will and economic incentive [9].

A Recoverable Crisis

Unlike many forms of ocean pollution, noise is not persistent. When a ship passes, its noise fades within minutes at distance. When seismic surveys end, the impulsive noise ceases. This means that unlike chemical contaminants that accumulate in sediments and tissues for decades, the acoustic environment has the potential for rapid recovery if noise sources are reduced. Duarte and colleagues highlighted this reversibility as a crucial feature of ocean noise pollution—and a reason for cautious optimism [1]. Reductions in global shipping during the COVID-19 pandemic provided a natural experiment: within weeks of traffic dropping by approximately 20%, underwater noise levels measurably declined in multiple ocean regions, and some cetacean species expanded their vocal ranges back toward historical norms.

Unlike chemical pollution, ocean noise is reversible. Reduce the source, and the ocean begins to recover within days.

— Duarte et al., Science, 2021 (doi:10.1126/science.aba4658)

The acoustic future of the ocean is not predetermined. It depends on the technological choices made by shipbuilders, the regulatory decisions taken by the IMO and national governments, the operational choices of shipping companies and oil explorers, and the willingness of coastal nations to enforce noise limits in their exclusive economic zones. The scientific case for action is now robust. The tools for mitigation exist and many are cost-neutral or cost-positive. The remaining gap is not knowledge but political will.