A possible mechanism for acoustic triggering of decompression sickness symptoms in deep-diving marine mammals

An interest in plausible mechanisms for significant acoustic impact on some species of marine mammals at receive levels significantly below that currently anticipated to cause direct physical trauma has arisen in response to questions of how the operation of sonars may have contributed to mass beaching events of beaked whales. Resonance in cavities and other specific structures was at one time proposed as a mechanism, but after some scrutiny this now appears unlikely. Rectified diffusion was posed as another candidate, but has been demonstrated to be significant only at relatively high pressure levels, exceeding receive levels anticipated in observed beaching circumstances. We examine an alternative proposition; that pre-existing micro-bubbles that are normally stabilized and which do not normally permit gas exchange across their walls can be acoustically activated so that continued growth is supported through static diffusion from super-saturated tissues in the absence of an acoustic field. The proposed mechanism would explain why micro bubbles (believed to be normally present in mammalian tissues) do not grow and cause decompression sickness (DCS) in healthy deep divers with super-saturated tissues, why these micro bubbles do not collapse under the Laplace pressure exerted by surface tension in unsaturated tissues, and why long-duration, deep diving cetaceans such as beaked whales appear to be particularly vulnerable to anthropogenic acoustic exposures. Numerical results for bubble growth modelled according to the treatments of Crum and Mao under tissue super-saturations of 200-300% (an appropriate range for deep-diving marine mammals on surfacing) show that if micro-bubble gas exchange could be activated acoustically, even by only a very brief exposure, this would result in subsequent bubble growth by static gas diffusion so that within 10 minutes their size would be sufficient to cause symptoms of decompression sickness (DCS)