Sound is continuously present in the underwater environment, and is produced naturally by wind, waves, ice, and thunder, 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 eco-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 such activities are among others bridges, offshore wind turbines, shipping and boating which also influence on the local sound environment. 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, as they are scared away from the area, and can also cause temporary or permanent hearing loss if no mitigation measures are applied.
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 levels in the Baltic Sea were measured 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 4.4.1), 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 are typical of ship induced sound, and the higher frequency bands are measured due to their ecological relevance.
The maps identify areas with different levels of continuous sound and at the same time they show the statistically calculated temporal distribution of sound levels at these areas. Continued monitoring 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 large scale displacement as well as physical damage to marine animals. In some cases mitigation measures may help to lower the damage.
The occurrence of activities associated with loud impulsive sounds, such as sonar events, airguns and underwater explosions and pile driving, can (since 2015) be logged in a regional registry established by HELCOM and OSPAR and hosted by ICES. Countries have agreed to register these activities, and reports on sound-generating activities have so far been supplied by five countries during the period 2013–2016. Denmark has delivered data on pile driving for 2015 (12 events). Sweden has reported sonar events (90), airguns (31) and underwater explosions (35) in 2015 and Germany pile driving events in 2013 (95) and 2014 (67). Germany had no registered impulsive events in 2015 to be reported according to the reporting guidance (JRC 2014). Lithuania has reported explosions in 2013 (8) and 2016 (12). 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.
Across the Baltic Sea there is strong temporal and spatial variability in sound levels, but as yet it is not clear how much marine species are impacted.
Harbour porpoise and seals are species that are likely to be especially affected by human generated sound. 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 (Figure 4.4.2). 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 (Figure 4.4.1) 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, such as off-shore construction work, energy installations and shipping, as well as dredging and leisure boating are likely to increase. Depending on these developments as well as technical improvements, it is likely that both the level of sound and its character will change over time. Pre-emptive mitigation measures and the implementation of sound reduction solutions are foreseen to play an important role in counteracting and reducing the impact of sound in areas where elevated sound levels are found to impose a risk to sound-sensitive species. Further, maritime spatial planning can help to minimize risks.