Some human activities contribute noise to the world ocean. Activities that produce sound include transportation, construction, defense, geophysical exploration, and oceanographic research. Federal laws such as the Endangered Species Act (ESA), Marine Mammal Protection Act (MMPA), and National Environmental Policy Act (NEPA) that aim to protect animals from harassment (which includes impact from sound sources) have motivated the development of mitigation techniques and alternative technologies. The goal is to reduce the impacts of anthropogenic (man-made) sound on marine mammals, although the extent to which many of these measures is effective has not been determined. There has been a great deal of research effort devoted in the last decade to increasing our knowledge of hearing sensitivities amongst all marine animals. This new information may ultimately assist in mitigation efforts.
A variety of approaches have been used to reduce the effects of anthropogenic sound:
- Avoidance of marine mammal habitats.
- Detection of animals and modification of the sound-producing activities.
- Modification or removal of the sound source.
- Ramp-up of the sound signal intensity.
- Sound screening.
One way to mitigate the effects of anthropogenic sound is to avoid areas with high concentrations of marine mammals that might be affected. This requires that one knows where high concentrations of marine mammals occur. For example, the image below shows the concentration of Cuvier's beaked whales in the eastern tropical Pacific. This information can be valuable in mitigating potential effects on these cetaceans.
This mitigation approach requires detecting animals in the area of human activities so that the impacting activity can be halted or modified to avoid affecting the animals. For example, the oil industry uses visual and acoustic monitoring to detect marine mammals in the vicinity of ships conducting airgun surveys to locate oil reserves. Visual detection either by boat or aircraft is common and most useful for species that spend a good portion of time at the surface. Acoustic detection techniques can identify animals in the area, but out of sight of the visual observers, by passively listening for sounds the animals make. Using an array of hyrdophones even allows for the calculation of the location of the animals producing the sounds. This method is useful for highly vocal species. Active acoustic detection is another approach that can locate and track marine animals underwater, even if they are not vocalizing.
A number of techniques can be used to reduce the impacts when it is not possible to eliminate the sound source completely. For example, it may be possible to change the frequency or amplitude of the sound source. Hearing sensitivities are known for many small cetaceans and pinnipeds (see Hearing Sensitivity Studies).
In some cases it may be possible to reduce noise that is generated as a by-product of other activities. It is conceivable that ship noise could be reduced using some of the techniques that the navies of the world use to quiet their ships, for example. Shipping noise dominates low-frequency background noise in many parts of the world. (For more information see What are common underwater sounds?).
Gradually increasing the sound source level ("ramp-up") is another approach that is often used. The notion is that animals can detect and respond to low levels of sound before it reaches levels that could potentially be harmful. In addition, ramping up of sound levels may reduce the chance that the animals will be startled by a sharp change in the noise field. For ramp-up to be effective the animals must respond appropriately to low-sound levels by moving away from the sound source. This seems reasonable, but there have not been any studies to show that this is the case.
When changing the signal produced by the sound source is not possible, other mitigation techniques can be employed. Impacts due to stationary sound sources such as pile-driving or marine explosives can be reduced by the use of bubble screens or barriers that dampen sound transmission. In the waters of western Hong Kong a bubble screen was used to dampen the sound of pile driving in hump-backed dolphin habitat. The bubble curtain was successful in reducing the sound by 3-5 dB at a range of a kilometer.
- Barlow, J. and Gisiner, R. 2006, "Mitigation, monitoring, and assessing the effects of anthropogenic sound on beaked whales." Journal of Cetacean Management and Research.
- Ferguson, M.C., Barlow, J., Reilly, S.B., and Gerrodette, T. 2006, "Predicting Cuvier's (Ziphius cavirostris) and Mesoplodon beaked whale densities as functions of the environment in the eastern tropical Pacific Ocean." Journal of Cetacean Research and Management. 7(3):287–299.
- Mellinger, D.K. and Barlow, J. 2003, "Future directions for acoustic marine mammal surveys: stock assessment and habitat use." NOAA OAR Special Report, NOAA/PMEL Contribution 2557, available from NOAA Pacific Environmental Laboratory. 37pp.
- Greene, Jr., C.R., McLennan, M.W., Norman, R.G., McDonald, T.L. and Richardson, J.W. 2003, "Using DIFAR sensors to locate calling bowhead whales and monitor their migration." 15th Biennial Conference on the Biology of Marine Mammals, Greensboro, NC, 14-19 December, 2003.
- Miller, J.H. and Potter, D.C. 2001, "Active High Frequency Phased-Array Sonar for Whale Shipstrike Avoidance: Target Strength Measurements." Proceedings of the IEEE/MTS Oceans 2001: 2104-2107.
- Würsig, B., Greene, C.R. and Jefferson, T.A. 2000, "Development of an air bubble curtain to reduce underwater noise of percussive piling." Marine Environmental Research 49: 79-93.