The term stranding refers to an aquatic animal observed in an inappropriate location, for example, an offshore species found inshore. Most often, stranded animals are found on a beach or in shallow water. Observations as far back as Aristotle and illustrations from the Middle Ages show us that marine mammals have been stranding for millennia. There are many causes of strandings, such as disease, ship-strike, injuries, storms, and entanglement. Only very few strandings have been attributed to sound.
In the U.S. alone, about 1,000 cetaceans and 2,500 pinnipeds strand annually. Some animals strand live and are returned to sea. Others die at sea or on shore. Animals may strand singly or in groups. When 3 or more animals strand together in time and place, it is called a mass stranding. Some species, such as pilot whale, mass strand regularly around the world.
Determining the cause of a stranding or death of a stranded animal can be difficult. Dead stranded animals are sometimes necropsied which is a thorough examination of the entire body. Scientists usually have little or no information about the animal’s history or the circumstances that preceded the stranding. A cause of death can be determined in only about half of all stranding cases.
There is consensus that military sonar exercises have contributed to mass strandings of beaked whales. However, it is still not clear if it is simply the sound of the sonar , or other aspects of the military exercises, such as multiple ship maneuvers, that resulted in the strandings.
Mass strandings of beaked whales are rare, with only 136 mass stranding events reported from 1874 to 2004 . Of these, two reported details on the use, timing, and location of sonar in relation to mass strandings. Ten other mass strandings coincided in space and time with naval activity that may have included military sonar. As of 2014, there are five additional documented events of beaked whale stranding in association with military sonar exercises. All these events had three consistent features: (1) the stranding locations were less than 80 km from the 1,000-m depth contour (that is, where deep water occurs near shore); (2) they occurred in areas where beaked whale mass strandings had previously been reported; and (3) all included Cuvier’s beaked whale, a species that does not commonly mass strand.
Although these beaked whale strandings were closely related in time and location to the use of military sonars by many nations, whether the sonar sounds caused the strandings has still not been determined. In five well-documented cases, there is sufficient information about the military exercises and the times and locations of the strandings to determine that multi-ship exercises with sonar contributed to the strandings. These events occurred in Greece (1996), Bahamas (2000), Madeira, Portugal (May 2000), and the Canary Islands (2002 and 2004). The necropsies that were performed (described below) found similar injuries, but none of the animals were found to have acoustic trauma. There are currently few peer-reviewed scientific publications that describe and discuss these strandings. The majority of authoritative information on these strandings can be found in official investigation reports of the events.
Bahamas 2000: Fourteen beaked whales, one spotted dolphin, and two minke whale were reported stranded in the Northern Bahamas Islands on March 15 and 16, 2000. Six beaked whales and 1 spotted dolphin died during this event. The strandings occurred within a 36-hour period and along a 240-km arc following the passage of five U.S. Navy ships taking part in an exercise that also used mid-frequency (1-10 kHz) sonars. The incident has been described in a report issued jointly by the U.S. Navy and the National Marine Fisheries Service, a formal necropsy report by scientists and veterinarians, and a peer-reviewed publication of the necropsy results. The spotted dolphin stranded on the opposite side of the island chain from the beaked whales and was found to be malnourished with evidence of chronic, debilitating disease. It was decided that this animal’s stranding was coincidental and unrelated to the mass stranding event. In contrast, the beaked whales that stranded were all in good condition with no evidence of significant disease. Blood deposits were found in and around the ears in several of the animals, but this was not caused by acoustic trauma. The reports concluded that the animals died from hyperthermia.
Madeira 2000: Three Cuvier’s beaked whales mass stranded near Madeira, Portugal between 10 and 14 May 2000 . A fourth animal was reported floating in the water by a fisherman, but it did not come ashore. A NATO naval exercise off Portugal started just one day prior to the stranding (May 9 to May 14, 2000). The head of one beaked whale was in adequate condition to be examined . It was found to have blood in and around the eyes, ears, and brain. This animal and another that were also examined on site were also found to have lung hemorrhaging.
Canary Islands 2002: In September, 2002, a mass stranding of fourteen beaked whales occurred in the Canary Islands. This stranding began about four hours after the start of a nearby NATO naval exercise involving ships of many nations that were using several types of mid-frequency sonar. The details of the sonar transmissions that occurred are not available. Ten of the stranded animals were found to have gas bubbles and hemorrhages in several organs  .
Canary Islands 2004: Four Cuvier’s beaked whales were found floating nearshore or stranded on the northern Canary Islands between 21 and 26 July 2004. These strandings occurred one week after the NATO military exercise MEDSHARK/Majestic Eagle ’04 off the Atlantic coast of Morocco. The naval exercise involved several warships that used mid-frequency sonar (2-10 kHz) in different areas and on different days, from July 11 to July 15, 2004. Three of the four whales were necropsied (the fourth whale found stranded on July 26 was too decomposed for analysis). The three necropsied whales were all in good condition with no evidence of disease. Microscopic gas bubbles were detected within many tissues, but it was not possible to determine if these were the result of decomposition or another process. Fat emboli, which are not associated with acoustic trauma, were detected in the lungs, kidneys, liver, and lymph nodes of all three animals and probably contributed to the whales’ rapid deaths.
Acoustic resonance: One proposed hypothesis was that hemorrhages occurred because air-filled tissues (such as the lungs and head sinuses) resonated when exposed to the sonars, causing blood vessels nearby to rupture. This hypothesis was ultimately found to be unlikely because for resonance-related motion to cause injury, the tissues would need to move large amplitudes. NOAA held a workshop in 2003 to discuss the effects of acoustic 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 at the received sound levels and frequencies  would have caused tissue movements in the nanometer range, which is far too small to cause injury. Further, key air spaces in the animals examined did not contain hemorrhages. Even more important, because of similarities in lung and sinus structures among toothed whale species, resonance should have damaged most of the species in the vicinity, but only beaked whales stranded.
Decompression Sickness: Another hypothesis that has been proposed to explain the gas bubbles and tissue damage observed in the strandings in the Canary Islands is that the animals experienced decompression sickness (DCS)  . 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 their tissues. Some tests have been carried out to determine the probability that this hypothesis is correct. Data from bottlenose dolphin show that rapidly diving animals may accumulate nitrogen in their muscles. The same model of nitrogen accumulation was applied to the diving behavior of the northern bottlenose whale, a beaked whale, and a blue whale, a baleen whale . Their results suggested that in long dives, supersaturation levels as much as 300% might occur. In addition, other studies suggest that if animals stay at the surface longer than normal or otherwise change their diving behavior, they might increase their risk for decompression sickness    . However, a study of a trained bottlenose dolphin that completed 10-12 dives to depths of 30, 50, 70, or 100 m showed no indication of gas bubbles or elevated blood nitrogen levels, not supporting the hypothesis that nitrogen accumulates during repetitive dives . This hypothesis is still being debated and more research is needed to evaluate it.
The hypotheses about bubbles in the bodies being related to DCS overlook the fact that bubbles form fairly rapidly in any animal that has recently dived and dies, so bubbles in a body are not sufficient evidence to diagnose DCS. DCS is a complex syndrome with many components that result in the injury or death of the diver. Bubbles are just part, but not all, of that mechanism. The evidence in the stranded animals is not consistent with DCS’s characteristic set of symptoms and its known mechanisms for injury. In many cases the bubbles reported in the whales are too large or are found in the wrong organs for DCS. Even more important, none of these hypotheses explains a key element of the strandings, which are why beaked whales are the one group that strands
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 whale specimens collected over the last 111 years . Scientists hypothesized that this degeneration is due to bubble formation associated with DCS unrelated to sound exposure. However, these scientists did not consider the possibility that diseases like arthritis or infections may have caused the abnormal appearance in any of these bones. Thus, the decompression sickness hypothesis has not been fully tested as an explanation for observations in the bones of sperm whales, and is not considered a fully explored or accepted explanation of mass strandings.
Diffusion: Another hypothesis put forth to explain the cause of tissue damage is that sound causes bubbles to form or expand in tissues that are supersaturated with nitrogen. One way this could happen is through a process called rectified diffusion . In this case, sound causes small bubbles, which 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 levels of sound exposure required to produce rectified diffusion experimentally are greater than were possible from the sonars by several orders of magnitude. This does not eliminate the possibility that static diffusion occurred, but to date, this mechanism has not been explored or determined to be sufficient to produce tissue damage in marine mammals   .
Environmental Conditions: Research into the acoustic sources and transmission of sound in areas in which the Greece, Bahamas, Madeira, and Canary Island strandings occurred showed three common characteristics . First, in each location, there is deep water close to land, such as submarine canyons. Second, the sources transmitted series of sound pulses at depths shallower than 10 m (33 ft) while moving at speeds of 2.6 m/s (5.1 kts) or more. Finally, since sound speed is dependent on water depth and temperature, some of the transmitted sound remained near the surface and decreased in level more slowly than would be the case under other conditions. Whether these acoustic characteristics influenced the probability that beaked whales detected the sounds, increased the effects of the sounds through greater propagation, or were not relevant remains unclear. The fact that there are common and relatively unique ocean characteristics in the areas of these five stranding events means that they may be worth considering, and that avoiding exercises in areas that are similar may help reduce the risk of stranding from future naval sonar activity.
Behavioral Response: Another hypothesis suggests that the strandings may have more to do with a number of reactions and sensitivities of this group of animals, such as disturbances in their foraging areas that cause unique behavioral reactions, rather than a direct physical cause related to sound-induced injury from sonar specifically.
Scientists are investigating this hypothesis with a series of studies of the behavioral responses of beaked whales to the playback of certain sounds. The first two phases took place at the Navy’s Atlantic Undersea Test and Evaluation Center (AUTEC) Range off Andros Islands, Bahamas, and the third phase was in the Mediterranean Sea. The AUTEC Range includes several hydrophone on the seafloor that can detect vocalizing animals. Scientists also went out in small vessels to attach tags to animals in order to record their dives and movements. A total of 16 acoustic tags were attached to individuals of four cetacean species over the three studies. Significant advances in understanding basic diving and vocal behavior were made and nine controlled exposures were conducted using simulated military, mid-frequency sonar sounds, killer whale calls, and “control” noise. During the AUTEC studies, researchers found that when tagged Blainville’s beaked whales were exposed to all three sounds during deep foraging dives, they stopped echolocatingand slowly ascended while moving away from the sound source . The whales reacted to the killer whale calls at much lower sound levels than they did for the sonar and control noise. However, they did not react to the anthropogenic sounds at the same level as they did for sounds of potentially lethal predators.
The next phase is a five year project off southern California. Additional studies are needed to identify whether these initial observations are generally applicable in other circumstances, and to extend studies to previously untested species such as large baleen whales, seals, and sea lions.
The content on DOSITS is based on well understood scientific principles, peer-reviewed literature, and high quality sources of scientific data. Independent experts who specialize in underwater acoustics have reviewed the material in this section.
- Baleen Whales
- Beaked Whales
- Controlled Exposure Experiments
- Scientific Method
- Tagging Studies
- Toothed Whales
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