Marine mammals are known to strand, and unfortunately many stranded animals die. Strandings occur worldwide. Approximately 40,000 stranded marine mammals were reported in the United States alone by the National Marine Fisheries Service stranding network over the decade 1990-2000, with an average of 3,600 strandings per year. Investigating the cause of a stranding is very difficult, because scientists must try to reconstruct what happened after the event, have little or no history on the individual animals, and, as a result, can generally draw firm conclusions in only about half of all strandings.

One controversial issue is the extent to which there is a relationship between the use of military sonar and the stranding of cetaceans, particularly beaked whales. Beaked whale strandings are relatively rare, with 17 beaked whale strandings reported in the U.S. in 1999 and 5 strandings in 2000. Strandings of more than one beaked whale at the same time are very uncommon. Stranding events involving multiple beaked whales have been reported that coincided closely in time and space with military activities using sonar. In three well-documented cases, there is sufficient information about the sonar operations, the times and locations of the strandings, and the injuries to the animals to associate the strandings with sonar use. These events occurred in Greece (1996), the Bahamas (2000), and the Canary Islands (2002). There are currently only limited scientific publications describing and discussing these strandings, and most of these publications have not undergone independent scientific review. Although these strandings are closely related in time and space to the operation of military sonars, the mechanism by which the sonars might have caused the strandings is still a mystery.

Locations of the three best documented beaked whale strandings that coincided with military activities involving the use of sonars. Two minke whales also stranded during the incident in the Bahamas in 2000.

There are a number of hypotheses that attempt to explain the observed stranding data coincident with the use of sonar. A hypothesis is a tentative explanation proposed by a scientist. The data are then carefully examined to determine whether or not the hypothesis is in fact consistent with observations. A scientific theory, on the other hand, is a generally accepted explanation that is supported by a solid body of evidence.

One hypothesis that has been proposed to explain the internal hemorrhaging that was observed in the Bahamas stranding is that tissue damage can occur when resonance from loud sounds causes air- or fluid-filled organs (such as the lungs or swim bladder) to vibrate at very large amplitudes. As the organs vibrate, blood vessels and tissues of the organs might become damaged. NOAA held a workshop in 2003 to discuss resonance in cetaceans (for more information see Report of the Workshop on Acoustic Resonance as a Source of Tissue Trauma in Cetaceans). The workshop concluded that acoustic resonance was not likely the cause of the injuries in the Bahamas stranding for several reasons. One reason was that the mid-frequency (1-10 kHz) sonars did not operate at the lung's resonant frequencies.

A hypothesis that has been proposed to explain the gas bubbles and tissue damage observed in the stranding in the Canary Islands is that they were consistent with decompression sickness. The scientists suggested that beaked whales might have changed their diving pattern in response to the sounds and come to the sea surface faster than normal, causing bubbles to form in the tissues. This hypothesis is still being debated and more research is needed to develop conclusive answers. The gas bubbles and tissue damage that have been observed could have resulted from many causes, some that are not related to sound. A recent report has found degeneration in the bones of sperm whales specimens obtained over the last 111 years. The scientists hypothesize that the degeneration is due to bubble formation associated with decompression sickness that is unrelated to sound. These hypotheses about decompression sickness, bubble growth, and degeneration in the bones of sperm whales have not been tested, and they should not be used as scientifically accepted explanations until they are. They are basically ideas that scientists are now testing and may or may not be correct.

Another hypothesis to explain the cause of the tissue damage is that sound causes bubbles to form or grow in tissues that are supersaturated with nitrogen. One way this could happen is through a process called rectified diffusion. In this hypothesis, sounds cause small bubbles that normally exist in the blood and tissues to grow larger. It is unlikely that this process caused the tissue damage observed in the Bahamas stranding because the sound exposures were too short. However, if sound caused bubbles to form or grow, they would continue to grow by static diffusion as long as the tissues remained supersaturated, which could resulting in tissue damage. Whether or not this hypothesis is plausible for marine mammals is still being debated.

Much more scientific research is needed to understand why there is a relationship in time and location between the beaked whale strandings and the use of multiple, mid-frequency sonars in nearshore areas. At present, it is uncertain whether stranding events are limited to beaked whales and near shore areas. Science is an evolving process and future work may help us further understand what we are observing.

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