Beaked whales, sonar and the “bubble hypothesis”

Abstract

Spatio-temporal links between some deployments of active mid -frequency sonar and beaked whale mass strandings are now widely accepted to be indicative of some cause (sonar) and effect (stranding); however, the underlying mechanism(s) remain a topic of intense scientific debate. Among potential mechanisms considered for these stranding events, theoretical mechanisms for acoustically mediated in vivo bubble formation in marine mammals exposed to high intensity anthropogenic sound sources (e.g. naval sonar) have been proposed. More recently, pathological findings consistent with in vivo bubble formation and tissue trauma observed following severe decompression sickness have been reported in three beaked whale species (comprising 10 necropsied individuals) that mass stranded in the Canary Islands in 2002 contemporaneously with naval sonar use. Bubble formation associated with acute and chronic tissue injury has also been demonstrated in some individually-stranded cetaceans in the UK, although the definitive cause of these bubbles has not been established. These pathological findings demonstrate that cetaceans can experience gas bubble development, possibly through induced off-gassing of nitrogen supersaturated tissue. Emerging data from beaked whale dive profiles suggest that these species exhibit a combination of slow ascent rates and short surface intervals. It has been hypothesized that behavioural disruption of normal beaked whale dive profiles (e.g. accelerated ascent combined with extended surface interval), could occur at received levels of noise significantly lower than those needed to directly damage tissues, and may precipitate a potentially fatal degree of nitrogen bubble formation in tissues. Alternatively, it may be necessary for an external stimulus, such as acoustic exposure, to induce destabilization of pre-existing bubble nuclei. The confirmation of whether in vivo nitrogen gas bubble formation occurs in diving cetaceans and whether it can serve as a mechanism in sonar-induced beaked whale mass strandings are future research priorities. Research topics should identify acoustic signal types and levels necessary to trigger an adverse behavioural response or cause the destabilization of existing bubble nuclei.