Field observations carried out during 2013-2014 have established recent noise levels at several sites in the Greater North Sea and Celtic Seas. Detection of trends will require several decades of monitoring. There are presently no threshold values agreed at UK level to determine the risk of adverse ecological impact.


UK target on continuous low frequency sound (ambient noise) in UK seas

There are currently no targets associated with this indicator.

Key pressures and impacts

The main source of anthropogenic ambient noise in the marine environment is from shipping activity (Figure 1). Continuous noise is also generated by other activities such as drilling, the extractive industries, and the operation of renewable energy devices. Continuous noise levels are likely to increase if the level of activity (particularly the volume of shipping) in UK waters increases, and no measures are taken to reduce noise levels from individual sources (for example, ships). The effects of ambient noise on marine life include masking of biological sounds, behavioural responses (including disruption to foraging) and increased physiological stress.

Humpback whales and commercial shipping

Figure 1. Humpback whales and commercial shipping. Credit: Whale Center of New England.

Measures taken to address the impacts

Some activities (such as dredging) are subject to marine licensing, as introduced by part 4 of the Marine and Coastal Access Act 2009 and part 4 of the Marine (Scotland) Act 2010. Potential impacts of noise are considered where relevant. Mitigation measures may include, for example, timing restrictions on noisy activities during periods when marine organisms are at their most vulnerable to disturbance (such as fish spawning seasons or marine mammal breeding seasons). For shipping noise, the UK promotes the use of non-mandatory guidelines for ship quieting technologies, such as those issued by the International Maritime Organisation (2014).

Assessment method

Areas that have been assessed

The assessment covers the Greater North Sea and Celtic Seas. Monitoring locations are shown in Figure 2.

Map of monitoring locations

Figure 2. Map of monitoring locations. Abbreviations refer to monitoring area: Celtic Sea (CS), northern North Sea (NNS), and southern North Sea (SNS). Note that data from site NNS6 were unusable and omitted from further analysis.

Monitoring and assessment methods

Underwater noise was monitored at 11 monitoring locations: 10 locations in the North Sea (9 locations in the Northern North Sea by Marine Scotland, and 1 location in the Southern North Sea by Cefas), and one location in the Celtic Seas (by University of Exeter) during 2013-2014, using autonomous underwater recorders moored to the seabed.

Assessment thresholds

Currently, no specific exposure thresholds have been proposed.

Regional cooperation

The UK coordinates with OSPAR Contracting Parties through the Intersessional Correspondence Group on Noise (ICG-NOISE) and is collaborating with other OSPAR Contracting Parties on two proposals for joint noise monitoring, one covering the Greater North Sea, the other covering the Atlantic Area.

Method Details

Data were collected using autonomous underwater acoustic recorders during 2013 and 2014 under three separate programmes and collated centrally by Cefas for analysis. Southern North Sea data were recorded as part of the EDF Energy New Nuclear Build marine studies programme from February to September 2013, using a Jasco Autonomous Multichannel Acoustic Recorder G3. Northern North Sea deployments formed part of the Marine Scotland East Coast Marine Mammal Acoustic Study and were made in the summers of 2013 and 2014 (August to October and June to September respectively) using Wildlife Acoustics SM2M, while the Celtic Sea deployments were made at the Wave Hub test site and recorded almost continuously during 2013 and 2014, also using the Wildlife Acoustics SM2M.

In total, 13.4 terabytes of acoustic data were collected by the field programmes, constituting over 6 years of monitoring. The data analysis was carried out in several stages using a modified version of PAMGuide software (Merchant and others, 2015), which corrected for the frequency sensitivity of the acoustic recorders. Data were analysed at 1-second resolution in 1/3-octave frequency bands from 50 Hz to 1000 Hz, and those corresponding to the Marine Strategy Framework Directive monitoring frequencies (63 Hz and 125 Hz) were selected for further analysis. Empirical probability distributions of noise levels in these frequency bands were computed using a kernel smoothing density function as described by Merchant and others (2012). Finally, several statistical metrics of these distributions were computed for each region to provide indicative baseline levels: the median, the root mean square level, the mode and the 90th percentile. Metrics for the northern North Sea were aggregated by first computing the metrics for each of the monitoring sites, and then taking the median value in each case.

An analysis was also conducted to assess the statistical power of the monitoring stations (that is the combination of location and data collection regime) to detect significant trends in noise levels, following the methodology described by Weatherhead and others (1998). This analysis estimates the number of years of monitoring required to detect a given magnitude of trend and depends on the variance and autocorrelation of the data.


Findings from the 2012 UK Initial Assessment

For the 2012 assessment, there were insufficient data to provide a quantitative assessment of the current status of continuous underwater noise in UK seas due to a lack of available information from monitoring studies.

Latest findings

The monitoring study carried out using field data recorded in 2013 and 2014 provides baseline levels for each of the monitoring locations. Results are summarized in Table 1. Median noise levels in the North Sea were 90.5 dB re 1 µPa in the 63-Hz band, and 93.6 dB re 1 µPa in the 125-Hz band, based on the aggregate median across 10 monitoring locations (see Figure 2). In the Celtic Sea, median levels at the single monitoring location were 82.0 dB re 1 µPa and 83.3 dB re 1 µPa for the 63- and 125-Hz bands, respectively. The median level is the 50th percentile, but higher percentiles of the noise level distribution (such as 90th percentile) will be more responsive to changes in noise levels from passing ships, and so may be a more appropriate metric for the assessment. At present, it is unclear whether the ambient noise levels reported may have negative impacts on marine life at the population or ecosystem scale.

Table 1. Summary metrics of ambient noise levels for UK reporting regions, with data for the North Sea sites represented by the median value among the monitoring sites. Metrics represent all data from each site (that is over both monitoring years where applicable).


Reporting region

1/3 octave band sound pressure level [ dB re 1 µPa ]



63 Hz

125 Hz


Celtic Sea



North Sea




Celtic Sea



North Sea



90th percentile

Celtic Sea



North Sea



Root mean square (RMS) level

Celtic Sea



North Sea



Trend assessment

Power analysis (Table 2) indicated that at least three decades of monitoring would be required to detect statistically significant trends at the monitoring locations in the southern North Sea and Celtic Sea (the duration of the northern North Sea recordings was too brief for this analysis). As such, there were no discernible trends in the Indicator.

Table 2. Power analysis results showing magnitude of changes predicted to be detectable over the duration of Marine Strategy Framework Directive monitoring cycle (6 years), and the duration of monitoring required to detect trends of 3 dB/decade and 1 dB/decade. Results are presented for the southern North Sea (SNS) and Celtic Sea (CS) sites only (see Fig. 2), and for the 125-Hz frequency band. “dB” is here used as shorthand for units of dB re 1 µPa. C.I.: confidence interval.



Total % change detectable over 6 years

Total dB change detectable over 6 years

Years to detect 3 dB/decade trend (95% C.I.)

Years to detect 1 dB/decade trend (95% C.I.)





13.9 (7.6, 25.5)

29.0 (15.8, 53.1)




14.6 (7.7, 27.6)

30.3 (16.0, 57.5)

90th %ile



19.7 (10.7, 36.6)

41.1 (22.2, 76.0)

RMS level



22.0 (13.9, 34.9)

45.8 (28.9, 72.7)





14.0 (9.2, 21.5)

29.2 (19.0, 44.7)




16.7 (9.4, 29.8)

34.8 (19.5, 62.1)

90th %ile



16.3 (8.5, 31.0)

33.8 (17.7, 64.5)

RMS level



24.9 (12.5, 49.5)

51.8 (26.0, 103.0)

Multi-decadal measurements of ambient noise levels are only available for the Northeast Pacific, which indicate rises of approximately 1 dB re 1 µPa per decade between the 1960s and 1990s (Andrew and others, 2002), based on the median noise level at 100 Hz. The power analysis (Table 2) indicates that to detect a trend of this magnitude would require 30.3 years, 95% confidence interval (16.0, 57.5), of continuous monitoring at the southern North Sea monitoring location, and 34.8 years, 95% confidence interval (19.5, 62.1), at the Celtic Sea location, based on the median noise level in the 125-Hz band.


Baseline noise levels have been recorded for a number of sites around UK waters in accordance with the Indicator, which will serve as a benchmark to assess future noise levels. However, the analysis revealed that trend detection for this Indicator will require several decades of monitoring, which is incompatible with the 6-year assessment cycle of the UK Marine Strategy.

Knowledge gaps

The key knowledge gap is to understand if and how levels of manmade noise lead to effects at the population and ecosystem scales, and how to quantify the risk of impact at these scales. While there is clear and mounting evidence for negative effects of manmade noise on individual animals (and effects on animal interactions), it is inherently challenging to isolate the effect of manmade noise on a particular animal population or ecosystem from other potential sources of variability.


HM Government (2009) 'Marine and Coastal Access Act' (viewed on 1 October 2018)

HM Government (2010) 'Marine (Scotland) Act' (viewed on 1 October 2018)

International Maritime Organization (2014) 'Guidelines for the reduction of underwater noise from commercial shipping to address adverse impacts on marine life' MEPC.1/Circ.833. Maritime Environment Protection Committee (MEPC). (viewed on 1 October 2018; requires creation of free account for access)

Merchant ND, Blondel P, Dakin DT and Dorocicz J (2012) 'Averaging underwater noise levels for environmental assessment of shipping' Journal of the Acoustical Society of America 132, EL343–EL349. (viewed on 1 October 2018)

Merchant ND, Fristrup KM, Johnson MP, Tyack PL, Witt MJ, Blondel P and Parks SE (2015) 'Measuring acoustic habitats' Methods in Ecology and Evolution 6, 257–265. (viewed on 1 October 2018)

Weatherhead EC, Reinsel GC, Tiao GC, Meng X-L, Choi D, Cheang W-K, Keller, T and others (1998) 'Factors affecting the detection of trends: Statistical considerations and applications to environmental data' Journal of Geophysical Research 103, 17149. (viewed 1 October 2018)


Assessment metadata
Assessment TypeUK Marine Strategy Framework Directive Indicator Assessment

Descriptor 11. Introduction of energy, including underwater noise


Continuous low frequency sound (ambient noise)

Point of contact
Metadata dateWednesday, August 1, 2018
TitleContinuous low frequency sound (ambient noise) in UK seas
Resource abstract

Baseline noise levels have been recorded at 10 sites in the Greater North Sea and one site in the Celtic Seas, in accordance with the Indicator. These will serve as a benchmark to assess future noise levels.


Work underpinning this D11 assessment of the Marine Strategy Framework Directive can be found here in:

Merchant ND, Brookes KL, Faulkner RC, Bicknell, AW, Godley BJ, Will MJ (2016) ‘Underwater noise levels in UK waters’ Nature Scientific Reports, 6: 36942 (viewed on 12 January 2019)

Conditions applying to access and use

© Crown copyright, licenced under the Open Government Licence (OGL).

Assessment Lineage

Data was sourced from Cefas, Marine Scotland Science, and the University of Exeter.

Dataset metadata

Data are summary metrics of sound levels recorded in 2013 and 2014 (Table S2) at the 11 sites in the UK Greater North Sea (10 sites) and UK Celtic Seas (1 site). Data are reported for the required one-third-octave frequency bands centred at 63 Hz and 125 Hz, and additionally at 250 Hz and 500 Hz.

Details of the monitoring programme and data analysis are provided in the accompanying peer-reviewed journal article:

Merchant ND, Brookes KL, Faulkner RC, Bicknell, AW, Godley BJ, Will MJ (2016) ‘Underwater noise levels in UK waters' Nature Scientific Reports, 6: 36942 (viewed on 12 January 2019)



Links to datasets identifiers

Dataset DOI

Marine Scotland. 2018. Contaminant and biological effect data to support MSFD Descriptor 8 1999-2015 by CSEMP Region. DOI: 10.7489/12111-1

The Metadata are “data about the content, quality, condition, and other characteristics of data” (FGDC Content Standard for Digital Geospatial Metadata Workbook, Ver 2.0, May 1, 2000).

Metadata definitions

Assessment Lineage - description of data sets and method used to obtain the results of the assessment

Dataset – The datasets included in the assessment should be accessible, and reflect the exact copies or versions of the data used in the assessment. This means that if extracts from existing data were modified, filtered, or otherwise altered, then the modified data should be separately accessible, and described by metadata (acknowledging the originators of the raw data).

Dataset metadata – information on the data sources and characteristics of data sets used in the assessment (MEDIN and INSPIRE compliance).

Digital Object Identifier (DOI) – a persistent identifier to provide a link to a dataset (or other resource) on digital networks. Please note that persistent identifiers can be created/minted, even if a dataset is not directly available online.

Indicator assessment metadata – data and information about the content, quality, condition, and other characteristics of an indicator assessment.

MEDIN discovery metadata - a list of standardized information that accompanies a marine dataset and allows other people to find out what the dataset contains, where it was collected and how they can get hold of it.

Recommended reference for this indicator assessment

Nathan Merchant2 2018. Continuous low frequency sound (ambient noise) in UK seas. UK Marine Online Assessment Tool, available at:

1Centre for Environment, Fisheries and Aquaculture Science