Sonar, an acronym for Sound Navigation and Ranging, is a technology that utilizes sound waves to detect objects underwater or map the seafloor. Active sonar systems emit a sound pulse and analyze the returning echo to determine the location, speed, and characteristics of submerged objects, a method used by military, commercial, and research vessels alike. The use of sonar has raised questions about its effects on marine life, particularly fish. Investigations focus on whether these powerful sound transmissions cause immediate physical harm, how different types of sonar vary in their threat, and what non-lethal, long-term impacts they may have on fish populations and their ecosystems.
Mechanisms of Physical Harm
The most severe physical danger sonar poses to fish stems from rapid pressure fluctuations inherent in intense sound waves, which can lead to a condition known as barotrauma. This injury is caused by sudden and intense changes in pressure, directly affecting the gas-filled swim bladder present in many fish species. This internal organ, which fish use to regulate buoyancy, is highly vulnerable to rapid expansion or compression when exposed to a powerful acoustic pulse.
An intense sound pulse can cause the swim bladder to oscillate violently or expand beyond its structural limit, resulting in rupture or massive hemorrhaging. This severe internal injury can lead to immediate mortality or delayed death due to organ failure. Vulnerability depends on the swim bladder’s design: Physoclistous fish cannot rapidly vent gas and are thus more susceptible to barotrauma than physostomous fish, which can expel gas quickly. Direct mortality has only been observed in controlled laboratory settings at extremely high sound exposure levels, sometimes exceeding 207 decibels.
Distinguishing Sonar Frequencies and Types
The potential for sonar to cause harm depends highly on the system’s operational frequency and acoustic power output. Sonar systems are categorized by frequency: low-frequency systems operate below 1 kilohertz (kHz), mid-frequency systems between 1 and 10 kHz, and high-frequency systems above 10 kHz. Lower frequencies attenuate less in seawater, allowing them to travel much farther than higher frequencies, making them suitable for long-range detection.
Low-Frequency Active (LFA) sonar, often used by military vessels for submarine detection, emits signals at high intensity, typically between 100 and 500 Hertz. Because these low frequencies can couple with and resonate the gas in the swim bladder, LFA sonar is often perceived as the greater threat for widespread physical impact. However, studies exposing fish to high-intensity LFA sonar have generally found no evidence of mortality or non-auditory tissue damage. The most observed effect was temporary hearing impairment, which typically recovered within 48 hours.
Mid- and High-Frequency sonar, commonly used in commercial fish finders and navigation echo sounders, are less capable of long-range propagation but offer high resolution. While the sound energy from these systems dissipates quickly, they can still cause localized effects in fish close to the source. The specific risk depends on whether the sonar’s frequency overlaps with the species’ hearing range, which varies widely.
Impact on Fish Behavior and Ecology
Sonar transmissions can significantly affect fish behavior and overall population health through non-lethal stressors. Noise intrusion can immediately trigger avoidance behavior, causing fish to flee an area and disrupting their normal distribution and migration patterns. When fish are displaced from feeding grounds or critical spawning habitats by the sound, it can reduce their overall fitness and reproductive success.
Chronic exposure to sound can initiate a physiological stress response in fish, even if the levels are not high enough to cause physical injury. Studies on species like salmon have shown that exposure to low-frequency sound can elevate plasma cortisol levels, a common indicator of stress. Over time, this chronic stress can lead to neurological changes and impact growth, immune function, and reproduction.
Sonar can also interfere with the complex communication systems utilized by many fish species. Sonar signals can mask or drown out natural sounds that fish rely on for fundamental life processes. Interference with acoustic cues, such as the “spawning choruses” used by some species to synchronize reproduction, can disrupt mating and recruitment, leading to population-level consequences. The cumulative effect of these behavioral and physiological disruptions presents a serious challenge to the long-term health and stability of fish stocks.