Impact of anthropogenic noise on the marine environment – status of knowledge and management

Anthropogenic noise in the marine environment is sound generated by human activities that comes in addition to natural sounds. Increased human activity has led to elevation of the noise levels in our oceans over the last 2–3 decades. Almost all marine organisms use sound to orientate, to find food, to avoid being eaten and to communicate. Elevated noise levels could therefore lead to a degradation of the marine environment and habitats. An increased focus on this, particularly the last 10–15 years, has resulted in research on how different noise sources affect the marine ecosystem. The concern over the anthropogenic noise issue has also revealed a need for regulation of noise pollution. EU and US noise management measures are more advanced than in Norway, as there is currently no overall Norwegian plan for how noise pollution should be regulated. This report summarizes the knowledge on how anthropogenic noise affects the marine environment. We also summarize the status of how the science community interacts with the regulating authorities, and we discuss new technological trends in underwater acoustics that might change the soundscape in the future. The contents of the chapters are summarized below. Direct injury to marine mammals: Powerful noise sources like seismic air guns, naval sonar and detonations can cause direct injury to marine mammals. The hearing organ is particularly vulnerable. However, hearing impairment resulting from noise exposure will be limited to the area very close to the source. Risk of hearing impairment depends on source level, frequency content of the sound, and hearing ability of the exposed species. For a particular noise source and species, both the maximum sound pressure level and the accumulated sound exposure level (acoustic energy) are important to consider. Hearing specialists that can hear high frequency sounds, like porpoises and dolphins, seem to be the species with the lowest threshold values for hearing impairment. Direct injury to fish: Injuries can occur if fish are close to a powerful sound source. Detonations have the greatest potential to cause injuries but are normally spread out in time and space. Hearing damage in fish is temporary, but injuries to the swim bladder or internal organ could be fatal. Injuries due to sound is unlikely to have any consequences at the population level. Behavioral responses of marine mammals: Behavioral responses to sound in marine mammals are well documented. However, the studies on sonar effects outnumber the studies on seismic effects despite the fact that seismic activity is much more common in Norwegian waters. The threshold of response and severity of responses varies between species and the biological context. Beaked whales, porpoises, minke whales and killer whales are relatively responsive species, whereas humpback whale, pilot whales and sperm whales are less responsive species. Typical responses are avoidance of the exposed area, or alteration or interruption of biological activities such as feeding, nursing, resting or communication. We have limited knowledge of how such individual responses might accumulate to cause impacts at population levels, but it cannot be excluded that use of navy sonar or seismic air guns can lead to population-level effects. This will depend on the extent of the exposure, and on where and when such noise sources are used in relation to the density of and the biological context of sensitive species. Behavioral responses of fish: Most sources of human-induced underwater noise are within the frequency range where fish have optimal hearing. Noise, and in particular continuous noise, can affect behaviors such as feeding, reproduction and antipredator behavior, which could lead to population-level effects. There is, however, a need for more research to quantify such population-level effects. Effect on seabirds: The knowledge about effects of noise on seabirds is limited, but such effects cannot be excluded. Since many seabirds are red-listed species, the level of knowledge should be increased. Underwater noise might affect the ability of diving birds to hunt for prey and avoid predators. Noise (e.g. seismic) might also effect the behavior of the prey. Behavioral responses of the birds or their prey might increase the cost of feeding, and ultimately effect nesting success. Effects on invertebrates: There is limited knowledge on the effects of noise on invertebrates, but existing studies suggest that noise can affect activity and hearing. It is unclear which levels of noise are required to cause physical damage. Technological trends in underwater acoustics: New technologies using underwater acoustics are constantly being developed. Many of the new methods for using air guns, acoustic vibrators and continuous sonars have lower peak pressure than conventional methods, but the accumulated energy is often the same. Energy is spread out over time to create a more or less continuous sound. This results in lower sound levels affecting a smaller area, but more continuous noise might increase masking of acoustic information for animals in the exposed area. More research is needed on continuous sound sources. Management advice on the use of seismic sources: Before a seismic survey can be executed in Norwegian waters, a notification has to be sent to the Norwegian Petroleum Directorate. The Institute of Marine Research (IMR) and the Fishery Directorate (FD) are asked to give advice on potential impact of the planned seismic survey on the marine ecosystem and on potential impact on fisheries, respectively. The Office of the Auditor General of Norway recently published a report pointing to the need for the environmental authorities to have a stronger role in regulating seismic explorations, and that management of noise should be more knowledge-based. So far, Norwegian authorities have mostly focused on the potential impact of seismic exploration on fisheries, and environmental concerns, including impact on marine mammals, have been largely ignored. The seismic activity in Norwegian waters is still high, and have moved further north to areas with higher density of marine species. New species will therefore be exposed to seismic exploration, and habitats where biological activities are highly seasonal will be exposed. Seismic noise is an additional stressor on top of climate change and other anthropogenic disturbances. Management advice on sonar use: The Norwegian Navy has implemented instructions for use of active sonar in Norwegian waters based on research and advice from the Norwegian Defence Research Establishment (FFI). The instructions are to be followed by all Norwegian and foreign naval ships operating in Norwegian waters. A decision-aid tool SONATE has been developed to assist the Navy in planning and execution of sonar exercises to minimize risk to the environment. However, there is no regulation of the use of fishery sonars or sonars used for research purposes in Norway. Management advice on underwater detonations: No clear procedure for permitting and risk assessment is established for underwater detonations. Detonations happen at sea when war remnants and dumped munition are demolished, during construction work or as part of naval exercises. Both FFI and IMR contribute with advice to Norwegian regulating authorities on matters related to detonations close to or under the sea. Contrary to the procedures established to give scientific advice on the use of naval sonar and seismic, the advisory work is much more random and ad hoc when it comes to detonations. Summary and recommendations: The last chapter of the report summarizes the current status of knowledge and based on this our recommendations regarding regulatory actions, as well as research needs that will make the management of underwater noise more knowledge-based in the future.