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From offshore wind farms to dredging, Dr Federica Pace assesses the impact of industrial noise on the marine environment.
Introduction
At the same time as the government pushes to increase the use of renewable energy sources, planned offshore developments are now subject to more stringent environmental regulations, which seek to safeguard the Good Environmental Status (GES) of our seas.
The EU Marine Strategy Framework Directive (MSFD) requires member states to achieve GES by 2020, so as to maintain biodiversity in their seas. GES is defined in the MSFD as “ the environmental status of marine waters where these provide ecologically diverse and dynamic oceans and seas which are clean, healthy and productive within their intrinsic conditions, and use of the marine environment is at a level that is sustainable, thus safeguarding the potential for uses and activities by current and future generations .” The MSFD lays out 11 criteria (see Box 1) to determine GES and assess the current condition of our seas, and sets targets for each criteria (CEFAS 2012). The UK has transposed the MSFD into domestic law through the Marine Strategy Regulations 2010.
One of the main causes of concern is the introduction of noise into the marine environment, given that numerous marine species including dolphins and porpoises rely on sound as their principal means of communication and navigation in this environment.
The introduction of sound is likely to disrupt the marine environment because sound travels great distances underwater. Therefore, before any development proceeds, it is necessary to gain an understanding of the baseline soundscape. An environmental impact assessment must be carried out and underwater noise measurements be taken.
Underwater noise in the marine environment
Developments such as new harbours, offshore wind farms, installation of electrical cables underwater, dredging, or even the introduction of new ferry routes, will all generate noise at certain stages. For instance, the use of servicing vessels during the construction or operational phases generates continuous noise at low frequencies that overlap with the communication signals of baleen whales. Accordingly, each operation will have its own acoustic signature that must be identified and quantified in order to assess its impacts on species present in the area. While some impacts on marine mammal and fish species have been quantified into specific thresholds that cause hearing loss, either permanently or temporarily, the impacts that lead to changes in behaviour and also wider population impacts are still largely unknown.
The UK government has responded to the MSFD by establishing a noise registry to monitor the anthropogenic noise sources introduced in the marine environment. The extent to which activities have to be monitored as part of this differs depending on the type of sound source, for instance how loud it is predicted to be. Given the current lack of knowledge, this registry is intended to improve our understanding of the current status of underwater noise, to assess how anthropogenic activities modify ambient noise levels over short periods versus long term.
By contrast, the German government has responded to the MSFD with a more precautionary approach, by establishing specific thresholds that cannot be exceeded during certain anthropogenic activities. For example, a sound exposure level limit of 160 dB re 1 µPa 2 s outside a 750 m radius for pile-driving operations (BSH 2013) appears in the licence conditions for the installation of offshore wind farms.
Due to these restrictions and limits, regulation of noise (particularly impulsive noise – see Box 2) in the marine environment will impact heavily upon industry. In particular, pile-driving operations in the oil, gas and renewables industry could be seriously affected by the application of further environmental restrictions, such as imposed timescales within which work has to be completed or not allowing overlap between developments in a particular area. This could result in a significant increase in the overall project cost.
Marine renewables
The marine renewables sector across Europe is under extreme pressure from the relevant government bodies to reduce the cost of energy, which in turn means that production costs need to be minimised. Monitoring noise levels and introducing mitigation measures to reduce noise emissions, where necessary, can substantially increase project costs. Therefore, the targets established by individual governments with regards to this descriptor of the MSFD may impact the overall strategy for renewable developments in that particular state.
Monitoring noise levels associated with the installation of renewable energy devices affects the offshore wind, wave and tidal industries, but in different ways. The main concern for offshore wind is the installation of turbines, which requires extensive periods of pile-driving. These are operations that produce very loud noise, which may cause hearing loss or transient impairment to marine mammals, as well as substantial damage to commercially important fish stocks.
The installation of a wind farm can take anything from a few days to several months, depending on the size of the development. While the immediate impact of introducing a very loud sound source may be the same in terms of causing hearing damage in an individual animal regardless of duration, a project causing loud noise emissions for a prolonged period can lead to changes in the behaviour of a population of marine mammals, for example, if the pressure is such to cause avoidance of a favourable breeding or feeding habitat. Once installed, however, wind turbines will lead to little noise other than the increased shipping traffic associated with its servicing.
Conversely, concern regarding the impact of wave and tidal devices on marine mammals is greater during the operational phase due to collision risk. In this case, if the developments are too quiet there is a risk that they may not be easily detected by marine mammals, who may end up going through a tidal turbine to chase a fish shoal, with potentially lethal consequences.
For this reason, installation of renewable devices and their operation are often associated with the deployment of acoustic deterrent devices to keep marine mammals outside of an established mitigation zone area. However, while these devices are useful for preventing lethal effects, they inherently introduce even more noise into the marine environment.
Because of these issues, consents for the installation of renewable devices are subject to environmental impact assessments aimed at identifying the risk factors for the species that are likely to be found in the area where the works will take place. Current focus is shifting towards understanding the impacts that these developments will have in combination and across wider areas than those occupied by the proposed development.
Monitoring underwater noise
Monitoring underwater noise can be extremely challenging and the cost of the survey techniques has to be balanced with the added value of gathering this information. In recent years, the technology for monitoring underwater noise has improved incredibly, with several affordable and good quality instruments appearing on the market. This has led to multiple ways in which surveys can be conducted. Instruments are now available that allow the monitoring of sound underwater both in the short and long term, autonomously or from a boat, and also allowing real-time data transmission to a remote location.
The use of passive acoustic recorders makes it possible to collect data from the initial stages of a project. This provides information about the baseline ambient noise before a development is started and about movements of marine mammals in the area before any disturbance is introduced. Such recordings obtained before construction allow changes in ambient noise levels to be detected. Without such a reference point, future measurements cannot be quantitatively assessed and the potential for biological impacts cannot be determined.
Take boat based work as an example. As it inherently introduces noise into the environment, ambient noise measurements may be artificially increased and detections of biological components may be biased. In some cases, the response of some animals may be to stop vocalising or leave the area, meaning that this method of obtaining acoustic data can give an incomplete understanding of the soundscape. In other cases, some animals may be attracted to vessels, meaning that the presence of certain marine mammal species during boat-based surveys may be overestimated. Having baseline data as a reference point can help to overcome these issues.
Without baseline data and an understanding of its natural variations, the creation of regulations and policies regarding underwater noise is likely to be conservative, with regulators adopting a precautionary approach when presented with licensing applications. Such an approach would increase the mitigation measures that a developer is required to put in place to comply with its licence – increasing the overall cost of the project for potentially very little gain.
Passive acoustic monitoring is just one of many survey techniques used to assess the impact of development on marine ecosystems. It requires minimal human intervention and can be used when weather conditions are highly adverse.
Other survey methods include visual surveys of marine fauna from a boat where trained observers record the presence and number of animals seen during previously determined transect routes followed by the vessel. This type of survey is usually accompanied by a towed hydrophone that can detect and record the vocalisations of the animals seen by the observers and also any missed by them. The data can be used to estimate local populations of marine mammals. One major constraint of this type of survey is its weather-dependence, since it requires good weather conditions during daylight periods, and that it is much more expensive and time consuming than passive acoustic monitoring.
Within Europe there is a lack of coordination between member states on how to address GES outside continental waters and there is no consensus on the thresholds that should be applied, nor the monitoring best practice. The Crown Estate has published a review on good practice for measuring underwater noise, giving practical guidance on how to carry out surveys and set up the monitoring equipment. The lack of knowledge exchange could mean equipment that is most fit for purpose may be overlooked by a regulatory body. As yet there is no publication detailing specific guidance for monitoring marine mammals across Europe. Ecologists must rely on guidance from individual governments (where it exists) and the expert knowledge of marine biologists and underwater acousticians.
Published: 04th Mar 2015 in AWE International
