Ocean noise pollution is defined as unwanted or excessive sound introduced into the marine environment by human activities. Sound is a fundamental sensory mechanism for marine life, but it behaves differently in water than in air. The higher density of water allows sound to travel approximately 4.5 times faster than in the atmosphere. Low-frequency sounds can propagate over thousands of kilometers with minimal energy loss, meaning human-generated sounds can blanket vast areas of the ocean. This disrupts the natural acoustic habitat that marine organisms rely on for survival.
Sources of Underwater Sound
The origins of excessive underwater noise stem from varied industrial and military activities. Commercial shipping is one of the most pervasive sources, contributing a continuous, low-frequency background hum. This noise primarily results from propeller cavitation—the formation and rapid collapse of tiny vapor bubbles near the rotating blades. Machinery and engine vibrations also contribute significantly to this low-frequency sound profile, which often peaks between 20 and 500 Hertz.
Another intense source comes from seismic surveys and geophysical exploration, typically conducted to locate oil and gas reserves. These surveys utilize airguns that fire high-pressure compressed air into the water, creating extremely loud, short-duration pulses. Designed to penetrate the seabed and return echoes, these impulsive sounds are energetic enough to travel hundreds of kilometers. Their high intensity and repetitive nature make them a concern across wide oceanic regions.
Military operations introduce loud, intermittent sound through naval sonar systems. Mid-frequency active (MFA) sonar, operating between 1 and 10 kilohertz, emits loud pulses used for detecting submarines. MFA sonar is implicated in numerous marine mammal strandings and is widely deployed by navies globally.
Coastal and offshore construction projects, particularly the installation of fixed structures like wind farms, are a source of highly localized, high-intensity noise. Pile driving, which involves hammering large foundation piles into the seabed, generates powerful impulsive noise. This noise can be audible to marine life over tens of kilometers. Other construction activities, such as dredging and the operation of support vessels, add to the localized noise.
Biological Impacts on Marine Species
The introduction of excessive sound has a wide range of documented adverse effects on marine fauna, including mammals, fish, and invertebrates. One direct consequence of intense sound exposure is auditory damage, ranging from temporary threshold shift (TTS) to permanent hearing loss. This damage reduces the ability of marine life to detect natural sounds, mates, and predators. Extreme sound sources, like military sonar or close-range seismic surveys, can also cause physical injury to internal organs, sometimes leading to hemorrhaging or disorientation.
A more pervasive effect is communication masking, where anthropogenic noise obscures biologically relevant sounds. The low-frequency continuous noise from shipping overlaps directly with the calls of baleen whales and many fish species. This masking reduces an animal’s “listening space,” making it harder for them to coordinate hunts, find partners, or maintain group cohesion. Some species, such as killer whales, attempt to overcome the noise by increasing the duration or shifting the frequency of their calls, a reaction known as the Lombard effect.
Noise pollution also triggers significant behavioral changes in marine life. Animals may avoid areas near loud activities, leading to the abandonment of important feeding, breeding, or migratory grounds. Whales, for example, alter their migration routes or change diving patterns in response to sonar exposure, risking stranding or decompression sickness. Fish and invertebrates also show alarm responses, decreased foraging efficiency, and uncoordinated schooling behavior when exposed to noise.
The chronic presence of noise induces physiological stress, impacting health and reproduction. Studies show that marine organisms, including fish, exhibit elevated levels of stress hormones, such as cortisol, when exposed to persistent human-made sounds. When sustained, this stress response can weaken the immune system, lower growth rates, and negatively impact reproductive success. Invertebrates, such as squid, have shown cellular damage to their statocysts—organs responsible for balance and orientation—when exposed to low-frequency noise.
Strategies for Noise Reduction and Mitigation
Addressing ocean noise pollution requires technological innovation, operational adjustments, and international policy action. Technological solutions for quieting commercial ships focus on reducing propeller cavitation, the dominant noise source. This includes designing and installing highly skewed propellers, which distribute pressure more evenly and delay cavitation. Other engineering measures involve isolating machinery noise using vibration-dampening foundations or specialized hull coatings to reduce flow noise.
Operational changes are immediate and effective mitigation strategies, particularly for the shipping industry. Reducing vessel speed is a proven method for decreasing noise, as a small reduction can significantly lower the source level radiated from the propeller. For example, a 20% reduction in speed can decrease a vessel’s mean source level by about 6 decibels. Other operational tactics include rerouting vessel traffic to avoid biologically sensitive areas and conducting regular maintenance, like hull cleaning, to reduce drag.
For highly impulsive sources like offshore construction, specific localized mitigation technologies are employed. The use of bubble curtains, which pump a continuous stream of air bubbles around the piling site, creates an acoustic barrier. These systems reduce noise radiating into the water column, achieving reductions of 10 to 12 decibels and shrinking the disturbance zone. Alternative foundation types, such as drilling or gravity base structures, are also being explored to replace impact pile driving entirely.
Policy and regulatory frameworks aim to standardize noise limits and encourage quieter industry practices. The International Maritime Organization (IMO) has developed voluntary guidelines promoting the reduction of underwater radiated noise from shipping. Although not mandatory, these guidelines provide a framework for new ship design and operational best practices. National and regional legislative responses are also emerging, setting specific noise exposure thresholds for impulsive sources like seismic surveys and pile driving.