Mitigation Methods
Environmental impacts of the operation of active sonar are required to be carried out by US law. Procedures for minimising the impact of sonar are developed in each case where there is significant impact.
The impact of underwater sound can be reduced by limiting the sound exposure received by an animal. The maximum sound exposure level recommended by Southall et al. for cetaceans is 215 dB re 1 μPa2 s for hearing damage. Maximum sound pressure level for behavioural effects is dependent on context (Southall et al.).
A great deal of the legal and media conflict on this issue has to do with questions of who determines what type of mitigation is sufficient. Coastal commissions, for example, were originally thought to only have legal responsibility for beachfront property, and state waters (three miles into sea). Because active sonar is instrumental to ship defense, mitigation measures that may seem sensible to a civilian agency without any military or scientific background can have disastrous effects on training and readiness. Navies therefore often define their own mitigation requirements.
Examples of mitigation measures include:
- not operating at nighttime
- not operating at specific areas of the ocean that are considered sensitive
- slow ramp-up of intensity of signal to give whales a warning
- air cover to search for mammals
- not operating when a mammal is known to be within a certain range
- onboard observers from civilian groups
- using fish-finders to look for whales in the vicinity
- large margins of safety for exposure levels
- not operating when dolphins are bow-riding
- operations at less than full power
- paid teams of veterans to investigate strandings after sonar operation.
Besides the expense of some of the mitigation measures some of them might interfere with operations. For this reason, mitigation requirements for wartime use of naval sonars can differ from either civilian mitigation requirements or from military requirements during exercises or peacetime operations. Prohibition on night time operations may be a huge waste of expensive assets. Ramping up a signal in intensity may have no impact on geophysical operations, but sonar does not work very well if you give the target submarine a warning so that he can do countermeasures. On board civilian observers are used in tuna-boat operations, and in dredging exercises, which are radically different from military operations.
Read more about this topic: Marine Mammals And Sonar
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