ocean noise

The ocean is a loud place to be: waves crashing, rain hitting the surface, ice cracking or marine life calling to each other, these are just some of the natural noises you find there. Add to that far more powerful sources of noise: ships, wind turbines, sonar or drilling and it can become deafening. With a steady increase in activity on the ocean, an increase of about 3 dB (decibels) per decade has been measured.

Noise has been recognized by the World Health Organization as a global pollutant since 2011 but until recently little attention was paid to the ocean soundscape. Not only are some sounds powerful enough to rival a fighter jet, they also spread: while sound travels in air at 343 metres a second, in water this increases to 1,480 metres a second, dependent on the sound emitted and the chemical composition of the water. The lower the frequency, the further the sound can travel, the higher the acidity of the water, the less low-frequency sound can be absorbed. The fact that sound travels so efficiently in water, capable of covering thousands of kilometres without significant loss of energy, means that noise created off the coast of Australia by, for example hydrocarbon exploration, can be heard in the Caribbean. Similarly the noise made by the propellers of a container ship or the sonar sound from a submarine radiates out around it for thousands of kilometres, with an enormous and inescapable impact on marine life, not just in the vicinity but also many kilometres away. 

You can listen to a variety of sounds in the DOSITS audio gallery. The Monterey Bay Aquarium Research Institute’s Soundscape Listening Room also gives access to both a live stream and a library of sounds, both recorded using a hydrophone (underwater microphone) off central California which continuously records sounds within and outside the range of human hearing. The sounds fall into three categories: biophony (sounds of marine life), geophony (sounds of the earth), and anthrophony (sounds of human activities).

What effect does it have?

The ocean is often a dark and turgid place to live, already from a depth of 200 metres there is little to no light at all. Instead of sight, marine life often depends on sound to communicate, feed, navigate and reproduce, to determine the size, shape, speed, distance and direction of objects underwater, to establish location and whether food, danger or a potential mate is near. While research about the hearing capacity of the majority of marine life is limited, it is however known that in addition to mammals like dolphins, whales or porpoises, fish and invertebrates also use sound for communication. 

As their natural soundscape is masked, marine life reacts by changing pitch, noise level or even repeating calls when ocean noise gets too high. Anthropogenic noise interrupts the ability to navigate and communicate, increases stress levels, interferes with feeding, breeding and nursing, reduces the natural response to predators, causes developmental delays, slows growth rates, reduces reproduction rates, increases temporary hearing trauma or causes permanent hearing loss through internal bleeding and causes mass strandings and death.

Where does it come from?

Shipping

Since the middle of the 20th century the number of ships has grown exponentially, doubling every decade, and with this development the ocean is getting louder and louder. When ships are underway enormous amounts of sound energy are created by propeller cavitation (the popping of bubbles from the propeller) and, to a lesser extent, by the engine machinery on board. Likened in volume and intensity to the noise from a pneumatic drill, the sound radiates steadily out from the vessel at a low frequency ranging from 20 to 100,000 Hertz, the same frequencies used by many whales and other marine mammals.

Listen to a merchant ship

Over time migratory patterns have been seen to change, indicating that marine mammals try to avoid busy shipping routes. Research into the effects of shipping noise on whale populations shows a clear reduction in numbers in the busy shipping areas mainly concentrated in the northern hemisphere, while lower shipping rates in the southern hemisphere are reflected by higher numbers of marine mammals. Events such as the 9/11 attacks or the COVID-19 pandemic have seen periods with a vast reduction in marine traffic and consequently ocean noise. In these periods of limited activity marine biologists have measured lower stress levels in marine life, and novel behaviour such as whales napping, socializing and feeding in otherwise busy shipping lanes.

Efforts are being taken to design quieter ships. In the meantime a simple reduction of speed (so-called slow-steaming) could also have an important effect on the noise ships create (with the added benefit of cutting emissions and reducing ship strikes at the same time). Rerouting is another simple step: by moving shipping lanes, feeding and breeding areas of marine mammals could be avoided at particular times. The IMO has been looking into routing measures to protect whales and other cetaceans from noise during breeding seasons by keeping ships away from specified areas.

Hydrocarbon exploration

Since the 1950s offshore exploration of gas and oil fields has boomed, currently 30% of annual global oil and gas production comes from offshore drilling. Hydrocarbon exploration research usually involves setting off regular explosions from seismic air guns towed behind survey ships to see how the acoustic energy is reflected by the seabed in order to detect the presence of fossil fuels. About 30 to 40 guns are set off simultaneously at low frequencies every 10 to 20 seconds over periods of weeks or even months. The effect on marine life can be devastating, as seen by strandings of whales in areas where seismic exploration is underway.

Listen to a seismic air gun

Alternatives to seismic surveys, where sound waves are spread over longer periods of time, are in the early stages of development. In the meantime, and until more research is conducted into the effect of sound generated by oil and gas exploration and production activity on marine life, effective mitigation strategies can be implemented such as systems which detect the presence of marine mammals near seismic operations allowing operators to temporarily shut down operations to prevent any over exposure.

Wind farms

With increasing interest in renewable energy, offshore wind farms are growing in popularity. When in operation, wind farms emit a low frequency sound (usually below 700 Hz) from the spinning blades, the vibrations are transmitted down the main shaft into its foundation and the surrounding water. In addition to the operational phase of a wind farm (usually about 20 to 25 years) noise pollution is a factor in the exploratory phase where seismic surveys may be used to locate suitable sites, the construction phase, which may include drilling, and the eventual decommissioning of the farm.

Listen to a wind turbine

Naval sonar systems

The sonar systems used by naval submarines send deafening sound waves across hundreds of kilometres to detect objects in their path: one low-frequency active sonar loudspeaker alone can be compared to the sound of a twin-engine fighter jet at take-off. Naval sonar can injure or kill whales by causing hearing loss, haemorrhages or by driving them rapidly to the surface or to shore. Mass strandings of dolphins and minke whales have been documented following military exercises using sonar.

Listen to a sonar test

Navies could adopt similar strategies to that used by the hyrdrocarbon industry when scheduling exercises. The attitude of navies to mitigation however differs around the world. The European Parliament agreed to a moratorium on sonar exercises in 2004 while other navies, such as that of the United States of America, place national security exercises above any environmental consideration.

What are we doing about it?

The good news is that all of these sources of marine noise pollution can be reduced, the necessary technology or at the very least mitigation strategies already exist, it is merely a question of the stakeholders involved being willing to implement them. What is more, unlike with other forms of pollution, as ocean noise is prevented or at least reduced, it disappears completely, allowing marine life to recover straight away.

One essential element is the availability of both research on the impact of ocean noise on marine life and information on the migratory patterns of marine life. Establishing the migratory patterns in the area concerned should be part of the research carried out before any shipping routes, seismic studies or naval exercises are planned. In this respect, information about marine mammals needs to be readily available in order to inform such decisions. Advances in satellite technology will help: SPACEWHALE II is one such example, tracking whales by satellite.

The Marine Environment Protection Committee (MPEC) of the IMO has had marine traffic noise on its agenda since 2004. In 2008 MEPC agreed to develop non-mandatory technical guidelines to minimize the introduction of incidental noise from commercial shipping operations into the marine environment to reduce potential impacts on marine life. Guidelines were approved in 2014 and updated in 2023 looking into ship design, construction, modifications, and operation, as mere guidelines these are not however enforceable, leaving their adoption to the discretion of ship builders.

The concept of re-routeing shipping at particularly sensitive times of breeding could be taken onboard for hydrocarbon exploration and naval exercises. Migratory patterns should be part of the research carried out before any seismic studies or naval exercises are planned. In this respect, information about marine mammals needs to be readily available in order to inform such decisions.

The Secretariat to the Convention on Biological Diversity has been working on the issue since it was raised at COP10 in 2010. Decision XII/22 taken at COP12 suggested three approaches: 

1. Mitigating and managing anthropogenic underwater noise through the use of spatio-temporal management of activities, relying on sufficiently detailed temporal and spatial knowledge of species or population distribution patterns combined with the ability to avoid generating noise in the area at those times; 

2. Conducting impact assessments, where appropriate, for activities that may have significant adverse impacts on noise-sensitive species, and carrying out monitoring, where appropriate; and 

3. Engaging industry and other relevant sectors, including the naval and mining sectors, when developing guidelines in order to increase their ownership and participation in the implementation of the guidelines.

The parties to the CBD are currently reviewing a draft report on Anthropogenic Underwater Noise: Impacts on Marine and Coastal Biodiversity and Habitats, and Mitigation and Management Measures, which concludes that "Protecting marine life from this growing threat will require more effective control of the activities producing sound, which depends on a combination of greater understanding of the impacts and also increased awareness of the issue by decision makers, on a global, regional and national scale, to implement adequate regulatory and management measures."

Anthropogenic ocean noise has been highlighted as one of the priority threats in the Strategic Plan of the IWC Conservation Committee. In 2018, the Commission agreed on a Resolution that recognized the increasing concern over ocean noise, and clarified next steps in order to better understand and manage the threat.

According to their website, the Global Alliance for Managing Ocean Noise (GAMeON) is an international partnership of scientists, managers, policymakers, and industry representatives looking to transform ocean noise management, developing solutions for managing anthropogenic ocean noise with three key actionable goals:

• Scan horizons to proactively identify emerging concerns and solutions;

• Map existing and emerging knowledge on ocean noise, technology, and policy;

• Create inclusive dialogues and networks to collaboratively solve ocean noise issues globally.

One example of a national database is the German MarinEARs (Marine Explorer and Registry of Sound) which provides an open souce database on underwater noise events including their location, date and duration. The database can be used to support Environmental Impact Assessments in the framework of approval procedures for offshore wind farms or Maritime Spatial Planning.