Oceans of noise - Whale and Dolphin Conservation Society

Pierson et al. (1998) indicated a more flexible exclusion zone based on received sound pressurelevels is needed.Exposure levelsA panel has been convened in the US (see for example, HESS 1997) to set a maximum soundlevel to which a cetacean should be exposed. Such levels are commonly used for mitigationpurposes in military and industry activities. This is not such an easy task for whale watching orAHDs/ADDs where source levels and sound characteristics may vary dramatically.However, some consider that there are insufficient data to accurately determine acoustic exposureguidelines for any marine mammal (Ketten 1998).Stop-work proceduresOnce an animal has entered a designated area of protection, in some instances, operations are shutdown. This is particularly so with seismic activities and military applications involving activesonar and explosives. Once the animal moves out of the protected area, after a designated time,operations are ramped up to normal operating levels.Ramp-upIt has been previously assumed that ramp-up or ‘soft start’ would alert marine animals withsufficient time that they would move away from the noise source before physical damageoccurred. However, some studies show that animals may become habituated to persistent noiseand may remain in the vicinity when repeated exposure could cause physical damage (HESS1997). It is possible that ramping up a high energy sound source could be harmful (Pierson et al.year unknown), as animals might be attracted to the source by initially weak sounds and thusexposed to potentially harmful levels as sound intensity increases.While ramp-up must be considered as a minimum standard in the absence of other methods, itcannot be assumed that these measures are reducing harm to animals in the area, as it is notknown that they do actually move away from the source (Pierson et al. 1998). Studies are beingconducted to test the effectiveness of ramp-up (MMS 2001).Bubble screeningThis method of mitigation has been shown to reduce sound levels at the source (Wursig andEvans year unknown). A curtain of air bubbles consumes some of the sound energy produced andthen propagated from the source. Further investigations are needed into this potential source ofmitigation before its effectiveness can be assessed.Use of explosivesExplosives may be used in some instances to deter marine mammals, and particularly seals, froman area. This may occur before the use of larger explosives, for example, to decommission an oilrig or for military purposes. As well as concerns about the introduction of an additional source ofnoise into the marine environment, consideration should be given to the impulsive nature ofexplosives, which may have a physical impact (see for example, Lewis 1996).78