Cetacean stranding is a phenomenon when Cetacea (whales and some other members of that order) strand themselves on land, usually on a beach. Beached whales often die due to dehydration, the body collapsing under its own weight, or drowning when high tide covers the blowhole. Several explanations of the stranding have been proposed.
Every year up to 2,000 animals beach themselves. Although the majority of strandings result in death, they pose no threat to any species as a whole. Only about 10 cetacean species frequently display mass beachings, with 10 more rarely doing so.
Body size does not normally affect the frequency, but both the animals' normal habitat and social organization do appear to influence their chances of coming ashore in large numbers. Odontocetes that normally inhabit deep waters and live in large, tightly knit groups are the most susceptible. They include the sperm whale, a few species of pilot and killer whales, a few beaked whales and some oceanic dolphins.
Solitary species naturally do not strand en masse. Cetaceans that spend most of their time in shallow, coastal waters almost never mass strand.
See also: List of sperm whale strandings
Strandings can be grouped into several types. The most obvious distinctions are between single and multiple strandings. The carcasses of deceased cetaceans are likely to float to the surface at some point; during this time, currents or winds may carry them to a coastline. Since thousands of cetaceans die every year, many become stranded posthumously. Most whale carcasses never reach the coast and are scavenged or decomposed enough to sink to the ocean bottom, where the carcass forms the basis of a unique local ecosystem called whale fall. Single live strandings are often the result of illness or injury, which almost inevitably end in death in the absence of human intervention. Multiple strandings in one place are rare and often attract media coverage as well as rescue efforts. Even multiple offshore deaths are unlikely to lead to multiple strandings due to variable winds and currents.
A key factor in many of these cases appears to be the strong social cohesion of toothed whales. If one gets into trouble, its distress calls may prompt the rest of the pod to follow and beach themselves alongside. Many theories, some of them controversial, have been proposed to explain beaching, but the question remains unresolved.
Whales have beached throughout human history, so many strandings can be attributed to natural and environmental factors, such as rough weather, weakness due to old age or infection, difficulty giving birth, hunting too close to shore and navigation errors.
In 2004, scientists at the University of Tasmania linked whale strandings and weather, hypothesizing that when cool Antarctic waters rich in squid and fish flow north, whales follow their prey closer towards land. In some cases predators (such as killer whales) have been known to panic whales, herding them towards the shoreline.
Their echolocation system can have difficulty picking up very gently-sloping coastlines. This theory accounts for mass beaching hot spots such as Ocean Beach, Tasmania and Geographe Bay, Western Australia where the slope is about half a degree (approximately 8 m (26 ft) deep 1 km (0.62 mi) out to sea). The University of Western Australia Bioacoustics group proposes that repeated reflections between the surface and ocean bottom in gently-sloping shallow water may attenuate sound so much that the echo is inaudible to the whales. Stirred up sand as well as long-lived microbubbles formed by rain may further exacerbate the effect.
Some strandings may be caused by larger cetaceans following dolphins and porpoises into shallow coastal waters. The larger animals may habituate to following faster-moving dolphins. If they encounter an adverse combination of tidal flow and seabed topography, the larger species may become trapped.
Sometimes following a dolphin can help a whale escape danger. In 2008, a local dolphin was followed out to open water by two Pygmy sperm whales that had become lost behind a sandbar at Mahia Beach, New Zealand. It may be possible to train dolphins to lead trapped whales out to sea.
Pods of killer whales, predators of dolphins and porpoises, very rarely strand. It may be that heading for shallow waters protects the smaller animals from predators and that killer whales have learned to stay away. Alternatively, killer whales have learned how to operate in shallow waters, particularly in their pursuit of seals. The latter is certainly the case in Península Valdés, Argentina, and the Crozet Islands of the Indian Ocean, where killer whales pursue seals up shelving gravel beaches to the edge of the littoral zone. The pursuing whales are occasionally partially thrust out of the sea by a combination of their own impetus and retreating water and have to wait for the next wave to carry them back to sea.
There is evidence that active sonar leads to beaching. On some occasions whales have stranded shortly after military sonar was active in the area, suggesting a link. Theories describing how sonar may cause whale deaths have also been advanced after necropsies found internal injuries in stranded whales. In contrast, whales stranded due to seemingly natural causes are usually healthy prior to beaching:
The low frequency active sonar (LFA sonar) used by the military to detect submarines is the loudest sound ever put into the seas. Yet the U.S. Navy is planning to deploy LFA sonar across 80 percent of the world ocean. At an amplitude of two hundred forty decibels, it is loud enough to kill whales and dolphins and already causing mass strandings and deaths in areas where U.S. and/or NATO forces are conducting exercises.—Julia Whitty, The Fragile Edge
The large and rapid pressure changes made by loud sonar can cause hemorrhaging. Evidence emerged after 17 cetaceans hauled out in the Bahamas in March 2000 following a United States Navy sonar exercise. The Navy accepted blame agreeing that the dead whales experienced acoustically-induced hemorrhages around the ears. The resulting disorientation probably led to the stranding. Ken Balcomb, a whale zoologist, specializes in the killer whale populations that inhabit the Strait of Juan de Fuca between Washington and Vancouver Island. He investigated these beachings and argues that the powerful sonar pulses resonated with airspaces in the whales, tearing tissue around the ears and brain. Apparently not all species are affected by sonar.
Another means by which sonar could be hurting whales is a form of decompression sickness. This was first raised by necrological examinations of 14 beaked whales stranded in the Canary Islands. The stranding happened on 24 September 2002, close to the operating area of Neo Tapon (an international naval exercise) about four hours after the activation of mid-frequency sonar. The team of scientists found acute tissue damage from gas-bubble lesions, which are indicative of decompression sickness. The precise mechanism of how sonar causes bubble formation is not known. It could be due to whales panicking and surfacing too rapidly in an attempt to escape the sonar pulses. There is also a theoretical basis by which sonar vibrations can cause supersaturated gas to nucleate to form bubbles (cavitation).
The overwhelming majority of the whales involved in sonar-associated beachings are Cuvier's Beaked Whales (Ziphius cavirostrus). This species strands frequently, but mass strandings are rare. They are so difficult to study in the wild that prior to the interest raised by the sonar controversy, most of the information about them came from stranded animals. The first to publish research linking beachings with naval activity were Simmonds and Lopez-Jurado in 1991. They noted that over the past decade there had been a number of mass strandings of beaked whales in the Canary Islands, and each time the Spanish Navy was conducting exercises. Conversely, there were no mass strandings at other times. They did not propose a theory for the strandings. A letter to Nature by Fernández et al. in 2013 reported that there had been no further mass strandings in that area following a 2004 ban by the Spanish government on military exercises in that region.
In May 1996 there was another mass stranding in West Peloponnese, Greece. At the time it was noted as "atypical" both because mass strandings of beaked whales are rare, and also because the stranded whales were spread over such a long stretch of coast with each individual whale spacially separated from the next stranding. At the time of the incident there was no connection made with active sonar, the marine biologist investigating the incident, Dr. Frantzis, made the connection to sonar because of a Notice to Mariners he discovered about the test. His scientific correspondence in Nature titled "Does acoustic testing strand whales?" was published in March 1998.
Dr. Peter Tyack, of Woods Hole Oceanographic Institute, has been researching noise's effects on marine mammals since the 1970s. He has led much of the recent research on beaked whales (and Cuvier's beaked whales in particular). Data tags have shown that Cuvier's dive considerably deeper than previously thought, and are in fact the deepest diving species of marine mammal. Their surfacing behavior is highly unusual because they exert considerable physical effort to surface in a controlled ascent, rather than simply floating to the surface like sperm whales. Deep dives are followed by three or four shallow dives. Vocalization stops at shallow depths, because of fear of predators or because they don't need vocalization to stay together at depths where there is sufficient light to see each other. The elaborate dive patterns are assumed to be necessary to control the diffusion of gases in the bloodstream. No data show a beaked whale making an uncontrolled ascent or failing to do successive shallow dives.
The whales may interpret the unfamiliar sound of sonar as a predator and change its behavior in a dangerous way. This last theory would make mitigation particularly difficult since the sound levels themselves are not physically damaging, but only cause fear. The damage mechanism would not be the sound.
If a whale is beached near an inhabited locality, the rotting carcass can pose a nuisance as well as a health risk. Such very large corpses are difficult to move. The whales are often towed back out to sea away from shipping lanes, letting them to decompose naturally, or they are towed out to sea and blown up with explosives. Government-sanctioned explosions have occurred in South Africa, Iceland, and Australia. If the carcass is older it is buried.
A beached whale carcass should not be consumed. In 2002, fourteen Alaskans ate muktuk (whale blubber) from a beached whale, and eight of them developed botulism, two requiring mechanical ventilation. This is a possibility for any meat taken from an unpreserved carcass.
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|Wikimedia Commons has media related to Beached whales.|
- Protecting Whales from Dangerous Sonar (Natural Resources Defense Council)
- Does Sonar Harm Whales? (Tasmanian whale stranding October 2005: Epoch Times)
- Sensors and sensibility: navies factor mammals into sonar use Jane's Navy International, 25 August 2006
- Whale Strandings Skegness East Coast UK, February 2006