Sound is continuously present in the underwater environment, and is produced naturally for example by wind, waves, ice, and thunder storms, as well as by animals. Human activities cause additional sounds which may have a polluting effect. These are typically by-products of marine activities and infrastructure, such as shipping, bridges, or underwater construction work, but are also spread deliberately by the use of echo-sounders, sonars and seismic airguns, for example. HELCOM has developed monitoring of underwater sound, and agreed that underwater sound should not have negative impact on marine life in the Baltic Sea.
Sound waves propagate over long ranges in water and their impact may occur far from the sources, across national boundaries. Two categories of sound are identified: continuous and impulsive.
Continuous sound from a source can be constant, fluctuating, or slowly varying over a long time interval. Various human activities may generate continuous sound. Examples of activities which influence the local sound environment include bridges, offshore wind turbines, shipping and boating. One concern is that human generated continuous sound may mask animals’ communication and signals used for orientation.
Impulsive sound is characterised by short duration and a fast pulse rise time. The sound associated with piling, underwater explosions or airgun signals used in seismic surveying are examples of impulsive sound. This type of sound can displace animals, and scare them away from significant areas for feeding, calving and other social interactions, as well as cause temporary or permanent hearing loss if no mitigation measures are applied.
There is a variation in how well animals hear different frequencies, and therefore different species will perceive different parts of the soundscape in different ways. For example, fish hear low frequencies better than marine mammals, and porpoises hear higher frequencies better than seals. The sound produced from shipping occur at frequencies which overlap with the hearing range of several species, including fish and marine mammals (Figure 4.4.1).
A good environmental status with respect to underwater sound requires that the level and distribution of both continuous and impulsive sounds should not cause negative impacts on marine life (HELCOM 2013a). At this time, such levels have not been defined for sound sensitive species in the Baltic Sea.
Continuous low frequency anthropogenic sound
Continuous sound in the Baltic Sea was monitored in a comprehensive study using automated hydrophone loggers in 2014 by the project Baltic Sea Information on the Acoustic Soundscape (BIAS). The data were used to develop modelled soundscape maps (Figure 4.4.2), which show the spatial and temporal distribution of continuous sound in different frequency bands across the Baltic Sea (1/3 octave bands of 63, 125 and 2000 Hz). The lower frequency bands assessed are mostly related to ship induced sound, and the higher frequency bands are measured due to their ecological relevance. Areas with high sound levels are identified particularly along major shipping routes, and within these, the highest prevalence is seen in the southernmost areas.
Monitoring of ambient sound is carried out by several countries on a temporary basis, and a regional programme for monitoring continuous underwater sound is under development.
Impulsive sounds may cause displacement as well as physical damage to marine animals, unless mitigation measures are successfully applied.
The occurrence of activities associated with loud impulsive sounds, such as hydro-acoustic measurements, underwater explosions and pile driving, can (since 2015) be logged in a regional registry established by HELCOM and OSPAR and hosted by ICES (2018). Countries have agreed to register these activities, and reports on sound-generating activities have so far been supplied by six countries during the period 2011–2016 (Table 4.4.1). In the future the registry will provide a quantitative view of activities that generate impulsive sound and their distribution in the Baltic Sea to support future status assessments.
Information from the registry will also support evaluation of possible impacts on species and decisions on mitigation strategies to be applied when conducting impulsive sound generating activities.
|Country||Impact pile driving||Sonar or acoustic deterrents||Airgun arrays||Explosions||Generic explicitly impulsive source|
|Estonia||0 (2012-2016)||NR||0 (2012-2016)||90 (2012)|
Across the Baltic Sea there is strong temporal and spatial variability in sound levels, but there is still considerable uncertainty regarding to what extent marine species may be impacted.
Harbour porpoise and seals are likely to be especially affected by human generated sound of specific frequencies and levels (Kastelein et al., 20110). They have very good underwater hearing abilities and rely on sound for their orientation, communication and foraging. Harbour porpoise also uses echolocation to find prey. Many Baltic fish species hear and produce sound at low frequencies. For example cod uses sound to communicate and to perceive their environment. For most species, including fish, diving birds and the majority of Baltic invertebrates, little is known about what role sound plays, even though it is likely that it is essential in at least some part of their life cycle and that they could be affected by high sound levels.
For the first time in the HELCOM assessment, spatial information of the sound distribution in the Baltic Sea has been compared with maps of key areas for sound-sensitive species. The overlap (Figure 4.4.3) gives indication of the risks from sound generating activities to different species. Spawning areas for cod and recruitment and foraging areas for harbour porpoise are examples of areas with elevated risk of impact.
A changing sound environment
There is no data to show how sound levels have changed over time in the Baltic Sea. Looking ahead, at least some of the human activities which may generate underwater sound are likely to increase, such as off-shore construction work, energy installations and shipping, as well as dredging and leisure boating. Depending on the scale of such expansions, as well as technical developments in maritime activities, it is likely that both the level of sound and its character will change over time. There is still limited knowledge about how marine animals may react to or be affected by human induced underwater sound. With respect to areas, species and seasons involving high risks, pre-emptive mitigation measures and the implementation of sound reduction solutions are foreseen to play an important role in counteracting and reducing impacts, as well as maritime spatial planning.