Underwater noise is a threat to marine life

The oceans are full of sound, there is underwater noise. Waves, earthquakes and breaking icebergs all contribute to the deep sea soundscape. But so do human activities, and this can be a problem for marine life as it can seriously affect their physiology, behavior, reproduction and even survival.

Being able to produce and detect sound in an environment where light penetrates only a few hundred meters is crucial for animals to communicate, feed, avoid predators and navigate vast underwater habitats.

Large whales generate low-frequency communication calls that can travel thousands of kilometers. While the snapping shrimp, native to the western Atlantic, can produce a loud clicking sound capable of stunning and killing its prey.

Noise produced by humans

Human-generated noise alters the natural acoustic environment of our oceans, and our ability to produce it is increasing. This is often an unintended by-product of transportation, infrastructure and industry development.

However, noise can also be deliberately produced. The navy uses sonar to detect ships and submarines, while geologists scan the seafloor for oil and gas using seismic air guns. The noise produced by an air gun can exceed 200 decibels (louder than a gunshot from a meter away).

Sound travels farther and four times faster in water than in air (at a speed of nearly 1,500 meters per second). Therefore, man-made noise can extend over considerable distances underwater. These sounds can be relatively constant, like the noise produced by a ship’s engine and propeller, or sudden and high-pitched in the case of marine sonars and seismic air guns.

Can underwater noise kill?

The sound produced by a seismic airgun can cause permanent hearing loss, tissue damage, and even the death of nearby animals.

Evidence of the lethal effects of noise can be difficult to document in the open ocean. But seismic surveys have been linked to mass mortality of squid and zooplankton. In 2017, research revealed that a single air gun caused the zooplankton death rate to increase from 18% to 40-60% in a 1.2 km stretch of ocean off the south coast of Tasmania.

The use of naval sonar has also been linked to the mass strandings of several species of whales in the Caribbean, Europe and East Asia. Mass stranding events involve entire herds of animals stranded simultaneously.

Examination of the dead whales revealed that they had suffered trauma similar to decompression sickness. This is believed to have been caused by sudden changes in its deep diving behavior after exposure to sonar.

Development halted by underwater noise

Over the past two decades, research has also revealed the widespread impact of chronic noise exposure on animal behavior and physiology. These impacts can extend far beyond the source of the noise and affect vast areas of the ocean.

laboratory studies in the sea hare, a sea slug, revealed that exposure to ship noise led to a 21% reduction in successful embryonic development. Pups also suffered a 22% higher mortality rate than sea hares that were not exposed to ship noise.

These findings demonstrate the negative effects that a common source of underwater noise can have on animal development and survival. If these lab results can be applied to natural environments, these impacts could threaten entire populations of marine species in specific areas.

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Behaviors disrupted by marine noise

Observing the movements, feeding, communication, resting and social interactions of animals provides scientists with a method for exploring the effects of noise.

The impacts of noise on the behavior of marine mammals are particularly well studied due to conservation concerns and their reliance on sound for communication, foraging and navigation. Many of these species travel great distances, and long-range communication is crucial for coordinating social interactions and reproduction.

But the sounds produced by large marine mammals are in a low-frequency range similar to much of the noise produced by humans. The noise produced by ships tends to be below 2 kHz, which overlaps with the vocal frequencies produced by many large mammals. Blue whales, for example, produce vocalizations at frequencies below 100 Hz, meaning their calls can be lost in background noise.

marine mammal vocalization

The noise of ships caused marine mammals to change their vocalization patterns. This includes making longer, repetitive calls or waiting until noise levels subside before calling. Research has shown that noise from ships within 1,200 meters of humpback whales has caused the whales to reduce or stop singing in the waters around Japan’s remote Ogasawara Islands.

Despite these vocal adaptations, underwater noise can negatively affect the animals’ feeding behavior and increase physiological stress. The investigation found that a reduction in shipping after the 9/11 terrorist attacks led to a six decibel drop in noise levels in the Bay of Fundy on Canada’s Atlantic coast. This coincided with lower levels of physiological stress detected in North Atlantic right whales when the researchers measured stress hormones in the whales’ floating faeces.

The COVID-19 lockdowns have also led to previously occupied waterways being used more frequently by large marine animals. For example, numbers of dolphins, including the endangered pink dolphin, have increased in the waters around Hong Kong following temporary restrictions on ferry traffic.

Protect and restore natural soundscapes

Noisy oceans are having a profound negative impact on marine life. Taking action to protect and restore natural soundscapes is a conservation priority.

The good news is that noise is removed from the environment as soon as the sound source is turned off or turned down. Technological advances in ship design, such as reducing propeller cavitation (the formation of air bubbles on the surface of a propeller), have already reduced the noise produced by ships.

Small adjustments in speed can also substantially reduce engine and propeller noise. The investigation found that a reduction from 15.6 to 13.8 knots in the average speed of commercial ships can reduce underwater noise pollution by more than 50%.

But global awareness of the impact of underwater noise on ocean health needs to improve, and policies to manage sound and implement technological solutions need to be more stringent. These are readily available solutions that promise a brighter, calmer future for our oceans.

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