Beaked whales, sonar and the “bubble hypothesis”

Abstract

Although spatio-temporal links exist between some deployments of active mid-frequency sonar and beaked whale mass strandings, the underlying mechanism(s) remain a topic of debate. On theoretical grounds, acoustically mediated in vivo bubble formation in marine mammals exposed to high level anthropogenic sound sources has been proposed as a potential mechanism. More recently, pathological findings consistent with systemic gas and fat embolism and associated lesions observed in severe decompression sickness have been reported in 10 beaked whales (of three species) that mass stranded in the Canary Islands in 2002 during naval sonar operations. Acute and chronic gas embolic lesions have also been demonstrated in single UK-stranded cetaceans, with the highest prevalence in deep-diving species such as beaked whales and Risso’s dolphins (Grampus griseus). These findings demonstrate that cetaceans can experience in vivo gas bubble/emboli development, possibly through decompression-related off-gassing of nitrogen supersaturated tissue or embolization of intestinal gas. Emerging beaked whale dive profile data shows a typical combination deep dives (>100 m) and short surface intervals, with deep foraging dives having considerably slower ascent than descent rates., These dive profiles are predicted to contribute to higher levels of nitrogen supersaturation than would occur in cetacean species with typically shallow dives (< 20 m), longer surface intervals between dives, or more rapid ascent and descent rates (> 2 m/s). It is hypothesized that behavioural disruption of normal beaked whale dive profiles could occur at received levels of sound significantly lower than that causing direct tissue damage, and may precipitate potentially fatal bubble formation driven by excessive tissue nitrogen supersaturation on surfacing. Alternatively, it may be necessary for an external stimulus, such as acoustic exposure, to induce destabilization of pre-existing bubble nuclei. Confirmation of in vivo nitrogen gas bubble formation in diving cetaceans, and the characterisation of acoustic signal types and levels necessary to trigger bubble formation via adverse behavioural responses or the destabilization of existing bubble nuclei, are future research priorities.