The introduction of unwanted or excessive sound into the marine environment by human activity is defined as noise pollution. This pervasive form of pollution fundamentally disrupts the ocean’s natural soundscape, which is a composite of biological, physical, and human-generated sounds. Marine organisms, from invertebrates to large mammals, depend heavily on sound for survival in ways terrestrial animals rely on sight. The sustained rise in human-generated noise, often referred to as “acoustic smog,” is a growing concern because of its potential to interfere with life processes across entire ecosystems.
The Unique Acoustics of the Underwater Environment
The physics of sound transmission underwater differs significantly from that in air. Water’s density and incompressibility allow sound to travel nearly five times faster than it does through the atmosphere, at approximately 1,500 meters per second. Because light rapidly attenuates with depth, making visual cues limited to shallow waters, sound serves as the primary sense for long-range perception and communication in the deep ocean.
This efficient transmission means a single source of human noise can project its influence over vast distances. Low-frequency sounds, such as those generated by large vessels, are particularly efficient at traveling thousands of kilometers with minimal energy loss. Variations in water temperature and pressure create specific layers, like the SOFAR channel, which acts as an acoustic waveguide, further propagating sound across immense areas.
Behavioral Changes and Communication Masking
Acoustic masking occurs when human-generated noise drowns out the natural sounds that marine animals use for essential biological functions. The persistent low-frequency drone from commercial shipping can interfere with the songs of baleen whales, which use these vocalizations to find mates and navigate. This interference affects communication necessary for survival.
In response to acoustic interference, many species exhibit compensatory behaviors, such as the Lombard Effect, where they increase the amplitude or pitch of their calls. Dolphins, for instance, increase their call volume and duration in noisy environments, which is an energetic cost that reduces their overall fitness. Fish and invertebrates that rely on acoustic signals for spawning have also been shown to alter their calling frequency or cease vocalizing entirely when exposed to vessel noise.
Chronic noise exposure often leads to avoidance and displacement from habitats. Species like beluga whales have been documented abandoning traditional feeding or breeding grounds when exposed to noise from icebreakers or construction activities. This displacement forces animals into less productive or less suitable areas, potentially disrupting their migratory routes, foraging success, and reproductive cycles. Altered foraging behaviors are common, such as when marine mammals prematurely end dives or change search patterns, leading to a reduced catch rate.
Physiological Harm and Acute Physical Damage
Beyond behavioral shifts, noise pollution causes biological harm, ranging from stress responses to fatal tissue damage. Prolonged exposure to elevated ambient noise leads to a chronic stress response. This manifests as a sustained elevation of stress hormones, such as cortisol, which has been documented in both marine mammals and fish species like the giant kelpfish.
Sustained high cortisol levels can suppress the immune system and divert energy from reproductive processes, decreasing overall health. More intense, acute sound sources, such as military sonar or seismic airgun surveys, can injure the auditory system. This damage may be a Temporary Threshold Shift (TTS), a temporary reduction in hearing sensitivity, or a Permanent Threshold Shift (PTS), which represents irreparable hearing loss.
The most intense, sudden noise events, like those from explosions or pile driving, can inflict acute physical damage known as barotrauma. This extreme pressure change can rupture internal organs, cause hemorrhaging, and damage gas-filled structures such as the swim bladders in fish.
Beaked whales are particularly sensitive to intense mid-frequency sonar. Exposure has been linked to mass strandings and internal injuries consistent with rapid, panicked ascent.
Reducing Anthropogenic Noise
Mitigating ocean noise pollution requires a combination of technological innovations and regulatory frameworks. A primary focus is on reducing the noise generated by the largest contributor, commercial shipping. Quieter propeller designs, including those with optimized shapes and larger diameters that turn more slowly, significantly reduce the bubble formation responsible for the loudest noise.
Operational changes, such as “slow steaming,” where vessels reduce their speed, have been shown to drastically lower the acoustic footprint of individual ships. The development and installation of Energy Saving Devices (ESDs) on hulls and propellers can improve vessel efficiency while simultaneously reducing cavitation noise. Regulatory measures are also employed, including the establishment of designated quiet zones or implementing time-of-year restrictions on noisy activities like seismic testing during sensitive migration or breeding seasons. Acoustic monitoring using hydrophones allows for real-time tracking of noise levels and animal presence, enabling authorities and operators to adjust activities and minimize impact.