Underwater noise

You may not realize it only by looking at the calm sea from the perspective of your towel stretched on the beach, but the Baltic Sea is very busy… and quite noisy. Commercial shipping, fishing vessels, recreational boats, and ferries – they all contribute to the heavy marine traffic and generate an invisible form of pollution – underwater noise. While it may be barely noticeable to humans on the surface, the sound generated by boat engines and propellers can travel long distances underwater and affect marine animals that rely heavily on sound to survive.

For sailors and recreational boaters, understanding this issue is an important step toward protecting the fragile and unique ecosystem of the Baltic Sea.

Sound underwater – it’s all physics but let’s make it easy

To understand why high levels of noise should be perceived as environmental problem, we should look more closely at how sound behaves underwater. The physics of sound propagation in water differs significantly from what we experience in air, and these differences are key to understanding its impact on marine life.

Sound is a mechanical wave - a vibration that travels through a medium, causing particles to oscillate. In water, these oscillations take the form of compressions and rarefactions (areas of higher and lower pressure). Unlike light, which is quickly absorbed or scattered, sound propagates efficiently in water because water molecules are densely packed and transmit vibrations effectively.

What does it mean in practice? Sound travels roughly four times faster and can reach much greater distances in water than in air. It’s worth noticing that sound speed is not constant and it depends on three main environmental factors: temperature, salinity, pressure (depth). In warmer, more saline water and at greater depths, sound speed is higher.

The Baltic Sea soundscape

Just by analyzing these variables, it becomes clear that the Baltic Sea, being relatively cold and brackish, has slightly different acoustic properties than open oceans. Still, sound travels efficiently enough to cover long distances. Moreover, the Baltic Sea has several characteristics that shape its unique soundscape. Relatively shallow depth of the Baltic Sea promotes cylindrical spreading of sound - confining it between the surface and the seabed. Seasonal water stratification (layered water column structure, presence of halocline) increases sound wave bending, called refraction. Additionally, the dominance of soft sediments, particularly in the southern Baltic area, influence sound absorption and scattering.

International organizations such as HELCOM (the Baltic Marine Environment Protection Commission) monitor underwater noise as a growing environmental pressure. Human-generated noise is now recognized as a form of marine pollution, alongside chemical contamination and marine litter. The current state of underwater noise in the Baltic Sea reflects moderate but spatially uneven environmental pressure, strongly linked to shipping intensity and coastal human activity. According to HELCOM assessments, overall levels of continuous underwater noise are classified as “good status” at the basin scale for marine mammals, but local and regional impacts can be significant, particularly in areas with dense vessel traffic (HELCOM, 2023). The highest noise levels occur along major shipping routes and narrow transit corridors, especially in the south-western Baltic Sea (e.g. the Danish Straits and Arkona Basin).

 Sources of underwater noise from recreational boating

Most underwater noise from recreational boats originates from two main sources:

1. propellers which, while rotating, create turbulence and cavitation (formation and collapse of air bubbles);
2. engines which generate low-frequency vibrations that propagate through the hull into the water.

The intensity of this noise often increases with boat speed and engine power, which means that faster vessels typically produce more underwater noise ^[1].

While commercial shipping remains the dominant source of underwater noise, recreational boating is an important and often underestimated contributor, especially in coastal zones. Recreational motorboats and yachts collectively create persistent background noise during peak activity periods (DCE, 2023). This contribution is highly seasonal, with the greatest intensity in summer months, when thousands of leisure boats operate daily in popular sailing regions such as the Gulf of Gdańsk, Stockholm Archipelago, and Finnish coast.

Although individual recreational vessels are quieter than commercial ships, their cumulative effect can lead to chronic exposure and be a significant stressor for marine fauna.

Fish

The effect of underwater noise on fish is difficult to define unambiguously, largely because most studies have been conducted under laboratory conditions rather than in natural environments. However, there is increasing concern that anthropogenic noise may interfere with fish’s ability to detect biologically relevant sounds, which are essential for survival ^[2].

Experimental studies indicate that fish may exhibit short-term behavioural changes when exposed to noise, such as altered activity patterns, although some species may habituate over time. Other studies have shown more persistent effects, including reduced hearing sensitivity and increased use of shelters or increased metabolic rates and activity levels. Additionally, noise pollution can negatively affect reproductive success in species that rely on acoustic and visual signals during courtship ^[2].

Harbour Porpoises                                                                                                      

Harbour porpoises (Phocoena phocoena), the only cetaceans permanently inhabiting the Baltic Sea, rely on echolocation as a basic tool for communication, hunting and orientation in space. As the species is considered very vulnerable, even subtle changes in the acoustic environment can disrupt their peace.

Studies have shown that vessel noise can lead to behavioral changes, eg. increased dive duration, changes in travel direction and also temporary or permanent shifts in hearing, acoustic masking of communication signals, and increased stress levels in marine mammals ^{[1, 2]}. When exposed to vessel noise, porpoises may flee away from boats or dive deeper ^[3].

Noise can also interfere with feeding – in areas with heavy vessel traffic, scientists found that the characteristic chronic “buzzing” sounds porpoises produce while hunting was significantly decreasing during peak boat activity, indicating reduced feeding behavior ^[4]. Taking into account that porpoises have a very high metabolism, constant disruptions of their feeding in the areas of intense ship traffic can have significant consequences for their health and survival ^[3].

Seals

The Baltic Sea is home to three seal species: grey seal, harbour seal, and ringed seal. While the effects of chronic vessel noise on seals are less well studied than for porpoises, researchers have shown that also seals, among many marine mammals alter their behaviour in response to underwater noise pollution.

Research on seals and related species and vessel traffic has largely focused on their behaviour after surfacing or in waterside areas, where they are easily observed. Common responses to approaching boats include flushing into the water, increased alertness, and head-raising, although these studies primarily address the presence of vessels rather than underwater noise itself ^[1].

However, studies suggests that underwater noise can cause increased stress levels, temporary hearing impairment and interference with seals communication, as vocalizations may overlap with ship noise or decrease in intensity in the presence of vessels. Similarly, to porpoises, seals may react to underwater noise by changing their diving behavior and also shifts in habitat use, ex. abandoning resting or breeding areas ^[1].

Follow these 5 simple rules:

1. Sail when possible

Whenever wind conditions allow, use sails instead of the engine. Sailing eliminates noise and reduces disturbance to marine animals.

2. Reduce speed

Noise levels generally increase with vessel speed. Slowing down in areas known for wildlife presence can reduce underwater noise as well as fuel consumption.

3. Avoid sensitive areas

If possible, keep distance from known habitats of marine mammals, especially shallow coastal areas where seals feed or rest.

4. Be wildlife-aware

If you observe marine mammals resting on sandbanks or surfacing nearby slow down, avoid sudden changes in speed and keep a respectful distance. Responsible wildlife watching helps minimize stress for animals.

5. Maintain your engine and propeller

Regular maintenance reduces mechanical vibrations and improves efficiency. A well-maintained propeller and engine produce less noise and fewer emissions.