Interim report of the HELCOM CORESET project

8. Link to anthropogenic pressures
Impulsive sound inputs in the Baltic are directly linked with offshore construction of wind farms, use of
different types of Sonar, Depth Sounder and Fish Finder, explosions mainly for clearing of old ammunition
and the use of Acoustic harassment devises for fi shing and scaring of marine mammals during construction
periods of wind turbines. Shipping as well as dredging of sand and gravel and the operation of
offshore wind farms are related to the introduction of continuous sounds introduction.
9. Pressure(s) that the indicator refl ect
Anthropogenic underwater noise in the baltic environment.
10. Spatial considerations
Sound travels in water about fi ve time faster than in air and absorption is less compared to air. Due
to t relatively good transmission underwater, sound acts at considerable spatial scales leading to vast
impacted areas (e.g. >1000km² during pile-driving of a wind park) avoided by porpoises for a longer
time. Noise of different sources can add up and may lead to synergies. Transmission varies with frequency:
low frequency signals typically travel further whereas higher frequencies attenuate more rapidly,
therefore fewer individuals might be exposed. Due to the low salinity of the Baltic a considerably lower
absorption can be observed compared to other sea areas, which means that the radiated acoustic power
travels over broader distances, especially at higher frequencies.
11. Temporal considerations
Although each single sound may attenuate rather quickly under water, the total amount appears to
increase constantly. Persistence of sound is very variable – ships on passage generate continuous sound
whereas explosions are very short-term. From late spring to early autumn, harbour porpoises give birth
and tend to very young calves that depend completely on the guidance of their mothers. This relationship
appears to be particularly sensitive to disturbance by underwater noise. On days with pile driving
activities in Nysted 20-60 percent less grey and harbour seals seeked their resting places than on usual
days. A negative correlation was shown between main shipping lanes in the Baltic and harbour porpoise
abundance.
12. Current monitoring
So far, no monitoring of underwater noise appears to be in place in the Baltic Sea beside of the obligations
for construction and operation of offshore wind farms.
13. Proposed or perceived target setting approach with a short justifi cation.
The introduction of impulsive and continuous sounds should be measured and modelled in order to
predict for the cumulative impacts on marine life. Data from aerial and ship-based abundance surveys of
marine mammals need to be used for habitat modelling. The sound sources that exceed thresholds that
are likely to entail signifi cant impact on marine animals have to be defi ned and these levels need to be
defi ned in a precautionary way. Maximum sound exposure levels at a certain distance from the sound
source for pile driving activities are already in force in Germany, for example, and could become compulsory
in all EU waters. Overall allowable noise budgets per area and time need to be defi ned. The total
amount of energy introduced into any area over a standardized time should be generally limited and permitted
on a case by case basis with binding mitigation measures in place.
To establish the species specifi c impact as a function of the distribution of noise over time and space the
above mentioned steps can be used to create a (threshold) factor as an indicator for the impact of noise.
For species of concern, it is therefore necessary to develop a three-dimensional (propagation) model that
takes account of the duration of noise events. The result of this modeling process would be a map with a
grid of related impacts based on the Sound Exposure Level (SEL). The benefi t of this approach is the possibility
of defi ning acceptable levels based on scientifi c estimates.
High-frequency sounds, e.g. from depth sounders, fi sh fi nders and other SONAR should be limited,
especially in shallow coastal areas, to the minimum. The level of underwater noise below 300 Hz is dominated
by noise inputs from ships. This noise is broadband and has a maximum level of 50 Hz. It can defi -
nitely be correlated with the propeller of a ship. Ship quietening measures need to be identifi ed with the
view on how to infl uence different operational parameters of the ship (e.g. fuel consumption).
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