N° 7 2021 
impulsive noise monitoring has also been in place in the last few 
years. In the Mediterranean Sea,the Agreement on the Conservation 
of Cetaceans of the Black Sea, Mediterranean Sea and Contiguous 
Atlantic Area (ACCOBAMS; see Maglio et al., 2016) produced an 
inventory of noise-producing human activities over ten years (2005- 
2015), including marine traffic, coastal and offshore construction, 
seismic surveys and military operations. This inventory was used to 
map areas where such activities were carried out and to identify 
noise hotspots. More recently, Merchant et al., (2020) reported 
impulsive noise events between 2015 and 2017 in the North 
Atlantic, Irish Sea and North Sea, showing high concentrations of 
avents over time in some areas. Seismic airguns were the dominant 
(impulsive) sound source in these areas over these three years. 
"he Convention on the Protection of the Marine Environment of 
the Baltic Sea Area!® (HELCOM) and OSPAR Commission have also 
created impulsive noise registers which can be accessed online**. 
"hese investigations are very valuable to improve our understanding 
of the spatial and temporal distribution of noise and to identify 
trends, which is covered in more detail in the next section. 
2.4 Trends in Ocean noise 
Boyd et al., (2008) concluded that studies looking at trends in Ocean 
ambient noise would be highly influential in the management 
af underwater noise. In the context of the MSFD, the European 
Commission refers to anthropogenic continuous low-frequency 
sound in water in one of the two indicators for ‘good environmental 
status’ (see European Commission, 2017). We know that human 
activities generating this type of underwater noise have been 
increasing over the past decades and that this could raise Ocean 
noise levels (Frisk, 2012; Kaplan & Solomon, 2016; Duarte et al, 
2021). Yet even with the strong policy incentive, our understanding 
of trends in the Ocean soundscape remains very limited. 
Long-term data on low-frequency shipping noise is only available 
for the Northeast Pacific Ocean (Andrew et al., 2002; McDonald et 
al., 2006; Andrew et al., 2011; Chapman & Price, 2011) and shows a 
sradual increase in noise levels of approximately 3 dB per decade. 
Recently, follow-up iInvestigations covering the time period 
between 1964 - 1998 showed a 5 dB increase In noise levels at 
frequencies between 63 and 125 Hz related to shipping and a 10 
dB increase in the 16-32 Hz band, which is dominated by baleen 
whale vocalisations rather than anthropogenic sources (Ainslie 
et al., 2021). Ship traffic has been proposed as responsible for 
*he steady rise in ambient noise at low frequencies (10-100 Hz) 
in some Ocean regions (Erbe et al., 2019). However, deciphering 
trends is extremely challenging due to our lack of baseline 
information and the need to monitor over extremely long periods 
in order to document change (Merchant et al., 2016). It is important 
to emphasize that the understanding of temporal trends of 
underwater noise in Europe and beyond requires internationally 
agreed monitoring standards. 
Looking at tomorrow’s Ocean soundscape, we can anticipate 
that some noise-generating activities will increase e.g. offshore 
construction, decommissioning of oil and gas infrastructure, 
and shipping. In addition, new noise-generating activities 
will or may emerge, e.g. deep-sea mining, widespread use of 
autonomous vehicles, and new acoustic communication systems 
(e.g. underwater Wi-Fi) (see Duarte et al., 2021). These need to 
be assessed and managed in the context of the risk assessment 
framework outlined in Figure 1. The 2° World Ocean Assessment 
report (Chapter 20, United Nations, 2021) discusses future trends 
in anthropogenic noise in the marine environment. It concluded 
chat increases in noise are expected in areas such as the Arctic, due 
to the area opening up to shipping, and the Ocean around Africa, 
as investment expands in the region. Finally, global warming will 
change the chemical composition of the Ocean, which might impact 
sound transmission. Although impacts are suspected to be low, this 
issue may need further research (Reeder & Chiu, 2010).. 
The 2020 outbreak of the COVID-19 pandemic and the subsequent 
worldwide lockdowns created a unique scenario for underwater 
noise research. Whilst studies have found that there was a reduction 
in some categories of vessel traffic (March et al., 2021), the impacts 
on underwater soundscapes were less clear (e.g. Leon-Lopez et al., 
2021: Sertlek. 2021). 
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