Sonar termination as a cause of mass cetacean strandings in Geographe Bay, south-western Australia

Geographe Bay, south-western Australia has been host to several past mass live cetacean (whale and dolphin) strandings. It is noticed that the majority of stranded whales tend to be healthy, toothed cetaceans (Odontoceti) which employ echolocation as a method of navigation. This paper explores a bioacoustic mechanism known as sonar termination as a major factor in the occurrence of these strandings in Geographe Bay. Sonar termination occurs when a navigational echolocation click projected towards the coast critically attenuates to a point where it is not detectable. The paper proposes two mechanisms contributing to sonar termination: first, the presence of a gently sloping shore and second, the presence of continuously created stagnant micro sized bubbles (microbubbles). By depicting a wedge shaped coastline as a perfect flat reflector the attenuative effect of multiple reflections and resident microbubbles in a coastal water column on a cetacean echolocation signal is calculated, and a limiting distance that a cetacean may be able to detect the presence of a shoreline is determined from these results. A brief review of the most recent mass strandings at Dunsborough (03/04/05, 02/06/05) is presented and the plausibility of the bioacoustic mechanism’s role in the strandings is investigated.