Sounding out the secrets of the sea
Box 4 | Disturbing beaching events
Could the extensive use of sonar for scientific or military purposes have effects on the health of sea animals? One well documented case in 2002 involved the use of medium frequency sonar by the US military and the beaching of beaked whales. Examination of the whales showed internal haemorrhaging and gas bubbles present in organs, evidence for tissue damage similar to that observed in divers with ‘the bends’. The study suggested that the observations were consistent with the effects of exposure to sonar and called for a limit to future military activities. Exposure to sublethal levels of sound that cause hearing impairment would also eventually be lethal to animals because they are unable to locate food or avoid predators.
The use of explosives under water can also be extremely harmful to sea animals. Explosives produce shock waves of very high intensity sound that can cause physical damage to the whole bodies of sea animals, not just their hearing, and either kill them or leave them severely injured.
Other beaching events
Not all beachings coincide with the use of sonar by the military. In many cases the reason for beaching is not identified. Could this be due to confusion induced by competing sonar signals or is there some other explanation?
Some animals that beach themselves are simply in poor health, suffering from ear infections for example, providing a clear link with their perception of sound and the stranding. It has been suggested that other beachings are the result of anomalies in ocean temperature or salinity in the region that make the land edge ‘invisible’ near smooth shallow beaches. The effect of chemical contamination on the ability of animals to hear sounds is also being investigated.
Australian researchers are looking for patterns in the beaching behaviour. They have found that seasonal fluctuations in the water temperature and nutrients can affect the abundance and location of prey. Peaks in the abundance of prey can be used to predict beaching events in Tasmania. Some areas experience more beachings than others, suggesting that localised features in the sea floor or shoreline increase the likelihood of events. When the presence of these features are combined with the behaviour of animals that have strong social bonds, distressing beaching events occur.
Boxes
Box 1. Comparison of the properties of sound in air and water
Box 2. Measurement of sound levels
Box 3. Use of sonar in the sea
Related sites
Bends may be culprit in whale strandings (News in Science, 9 October 2003, Australian Broadcasting Corporation)
Whale strandings (The Science Show, 6 November 2004, Australian Broadcasting Corporation)
A whale strandings database and network is developing (Ecos, Issue 123, 2005, page 7)
The fatal shore (School of Physics, University of Western Australia)
Posted August 2007.