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FEATURE ARTICLE – Underwater noise

The latest piece in our series of features looks at the world of underwater noise and the associated impacts and assessment.
Published: 4 July 2016

​​​​​​​​​Nathan Merchant, Cefas

For many marine animals, the ocean is an 'acoustic habitat'. Sound can penetrate dark or murky seas and travels much further than light underwater. Many marine organisms have evolved ways of using sound for communication, navigation, and detection of predators and prey. Baleen whales warble low-frequency calls over tens or hundreds of kilometres; in the mating season, aggregations of fish grunt and croak in chorus; dolphins click at high frequencies to echolocate fish; snapping shrimp make loud pistol shots with an oversized claw –­ this sonic wonderland is far from the 'silent world' of Jacques Cousteau.

In the modern era, humans have also learned to use sound underwater. From ship-borne depth sounders to underwater communication systems and seabed mapping to antisubmarine warfare, acoustic signals underpin the sensor technologies we use to measure, detect, and navigate in the oceans. Many human activities also generate sound as an unwanted by-product: the slow rumble of ship traffic, the repetitive thump of pile driving, the occasional boom of an explosion. How do marine animals react to these noisy newcomers? Could this affect marine ecosystems? If so, how might we monitor manmade sound and assess its possible impact? And what might we do about it?

The evidence

Mass strandings of cetaceans linked to military sonar activity are the most dramatic example of our acoustic impact on the oceans - and first raised the issue in the public consciousness. However, subtler effects of man-made sound on marine animals are increasingly being discovered1, and at lower sound levels, which implies that these effects are more widespread and potentially more detrimental to populations.

Behavioural and physiological reactions to man-made sound have now been observed in marine mammal, fish, and invertebrate species. For example, harbour porpoises have​ been displaced from pile driving of offshore wind turbines by more than 20 kilometres,2 lab experiments with shore crabs have found ship noise playback to disrupt feeding behaviour3, and European seabass have shown stress responses when exposed to in situ pile driving4.

Researchers are now looking beyond the impact on individual animals to consider how noise might affect the wider ecosystem, through effects on interactions between animals, effects on species which perform ecosystem functions, and the consequences of individual disturbance at the population level. Studies have demonstrated the reduced ability of fish to avoid predators in the presence of motorboat noise5, reduced nutrient cycling activity in langoustine exposed to noise6, and a probable low risk of population-level effects from disturbance by boat traffic and dredging activities based on modelling of a bottlenose dolphin population7.

The data deficit

Policy-makers and environmental managers are starting to respond to the risk of impact from underwater noise, notably through the European Marine Strategy Framework Directive (MSFD), which aims to achieve good environmental status (GES) in European seas by 2020. The MSFD includes two Indicators of underwater noise under Descriptor 11: Indicator 11.1 for impulsive noise (e.g. pile driving, explosions), and Indicator 11.2 for continuous noise (mainly shipping). However, policy-makers are constrained by a lack of long-term field measurements in European waters, which impedes the assessment of trends and current status of noise levels. Efforts are underway to establish regional joint monitoring programmes under the regional seas conventions (such as OSPAR, HELCOM) which will provide data from field measurements on an ongoing basis.

Regional assessment and the ICES noise register
One aspect of joint monitoring is to record and analyse the distribution of impulsive noise activity occurring in European seas, under Indicator 11.1 of the MSFD. This indicator gives an overview of the cumulative impulsive noise activity in a given region. Within OSPAR (Northeast Atlantic), countries are now sharing data on activities, which is usually provided through the regulatory consenting process (e.g. for pile driving activities, or seismic surveys for oil and gas exploration). This data is being gathered centrally into the Impulsive Nois​e Register, which has been created and is being maintained by ICES, and is now also being used by the HELCOM countries (Baltic Sea).

The OSPAR Intersessional Correspondence Group on Noise (ICG-NOISE) is currently working closely with the ICES data team to produce the 2017 OSPAR Intermediate Assessment for impulsive noise. This is the first regional assessment of its kind, and will give policy-makers and regulators a regional overview of cumulative impulsive noise activity in the Northeast Atlantic, including the noise source type (e.g. pile driver, explosion) and intensity. The 2017 Intermediate Assessment will serve as a 'roof report' to inform the subsequent 2018 MSFD assessments of EU Member States within the OSPAR region.​

Towards assessing impact

The present MSFD Indicators for noise are pressure indicators, and are designed to assess the amount of noise-generating activity in EU waters. A necessary counterpart to these pressure indicators are impact indicators, which indicate the risk of impact to the marine ecosystem. Impact indicators help to prioritiz​e management action by identifying where there is a high risk of impact on particular species or ecosystem​ components. The pressure indicators then show which noise-generating activities are leading to this risk and facilitate targeted mitigation. The development of impact indicators within the MSFD is now being considered, and there are substantial challenges to address, particularly around which species or taxonomic groups to focus on and the lack of evidence for clear relationships (dose-response curves) between noise levels and certain animal responses (e.g. marine mammal behavioural responses8).​

A calmer outlook

Moves to better regulate underwater noise both within and beyond Europe have stimulated innovation in technologies to limit noise pollution. The International Maritime Organisation (IMO) has issued guidelines on the reduction of noise radiated from ships, which has encouraged new research and development into ship-quieting technologies, while German regulation of pile driving noise has hastened the development of bubble curtains, which create an acoustic barrier to reduce noise output at source. These new technologies and regulatory requirements raise the possibility of long-term reductions in underwater noise pollution and the prospect of a more tranquil world beneath the waves.

​ 

References

1. Williams, R. et al. Impacts of anthropogenic noise on marine life: publication patterns, new discoveries, and future directions in research and management. Ocean Coast. Manag. 115, 17–24 (2015).

2. Tougaard, J., Carstensen, J., Teilmann, J., Skov, H. & Rasmussen, P. Pile driving zone of responsiveness extends beyond 20 km for harbor porpoises (Phocoena phocoena (L.)). J. Acoust. Soc. Am. 126, 11–14 (2009).

3. Wale, M. A., Simpson, S. D. & Radford, A. N. Noise negatively affects foraging and antipredator behaviour in shore crabs. Anim. Behav. 86, 111–118 (2013).

4. Debusschere, E. et al. Acoustic stress responses in juvenile sea bass Dicentrarchus labrax induced by offshore pile driving. Environ. Pollut. 208, 747–57 (2016).

5. Simpson, S. D. et al. Anthropogenic noise increases fish mortality by predation. Nat. Commun. 7, (2016).

6. Solan, M. et al. Anthropogenic sources of underwater sound can modify how sediment-dwelling invertebrates mediate ecosystem properties. Sci. Rep. 6, 20540 (2016).

7. Pirotta, E. et al. Predicting the effects of human developments on individual dolphins to understand potential long-term population consequences. Proc. R. Soc. London B Biol. Sci. 282, (2015).

8. Ellison, W. T., Southall, B. L., Clark, C. W. & Frankel, A. S. A New Context-Based Approach to Assess Marine Mammal Behavioral Responses to Anthropogenic Sounds. Conserv. Biol. 26, 21–28 (2012).

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Big bubble curtain (BBC) in operation around a pile driving site. The BBC acts as an acoustic barrier, reducing the amount of noise emitted into the surrounding environment. Photo: Trianel GmbH / Lang.

Nathan Merchant​ leads the Noise and Bioacoustics team at Cefas, and is leading the OSPAR Intermediate Assessment for underwater noise. He is a scientific advisor on underwater noise to the UK Government, and is a member of TG NOISE, which advises EU Member States on the implementation of MSFD Descriptor 11. He took his PhD at the University of Bath, UK, focusing on acoustic impacts of human activities on marine life, and latterly carried out bioacoustics research in the Parks Lab at Syracuse University, USA.
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FEATURE ARTICLE – Underwater noise

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