Cetacean stranding
Cetacean stranding, commonly known as beaching, is a phenomenon in which whales and dolphins strand themselves on land, usually on a beach. Beached whales often die due to dehydration, collapsing under their own weight, or drowning when high tide covers the blowhole. Cetacean stranding has occurred since before recorded history.
Several explanations for why cetaceans strand themselves have been proposed, including changes in water temperatures, peculiarities of whales' echolocation in certain surroundings, and geomagnetic disturbances, but none have so far been universally accepted as a definitive reason for the behavior. However, a link between the mass beaching of beaked whales and use of mid-frequency active sonar has been found.
Whales that die due to stranding can subsequently decay and bloat to the point where they can explode, causing gas and their internal organs to fly out.
Species
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 ten cetacean species frequently display mass beachings, with ten more rarely doing so.All frequently involved species are toothed whales, rather than baleen whales. These species share some characteristics which may explain why they beach.
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. This includes the sperm whale, oceanic dolphins, usually pilot and Orcas, and a few beaked whale species. The most common species to strand in the United Kingdom is the harbour porpoise; the common dolphin is second-most common, and after that long-finned pilot whales.
Solitary species naturally do not strand en masse. Cetaceans that spend most of their time in shallow, coastal waters almost never mass strand.
Causes
Strandings can be grouped into several types. The most obvious distinction is between single and multiple strandings. Many theories, some of them controversial, have been proposed to explain beaching, but the question remains unresolved.;Natural deaths at sea: 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. Offshore deaths of multiple whales are unlikely to lead to multiple strandings, since winds and currents are variable and will scatter a group of corpses. Most carcasses never reach the coast, and are scavenged, or decompose enough to sink to the ocean bottom, where the carcass forms the basis of a unique local ecosystem called a whale fall.
;Individual strandings: Single live strandings are often the result of individual illness or injury; in the absence of human intervention these almost inevitably end in death.
;Multiple strandings: Multiple strandings in one place are rare, and often attract media coverage as well as rescue efforts. The strong social cohesion of toothed whale pods appears to be a key factor in many cases of multiple stranding: If one gets into trouble, its distress calls may prompt the rest of the pod to follow and beach themselves alongside.
Environmental
s have beached throughout human history, with evidence of humans salvaging from stranded sperm whales in southern Spain during the Upper Magdalenian era some 14,000 years before the present. Some 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, or 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 have been known to panic other 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. 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.
A 2017 study by scientists from Germany's University of Kiel suggests that large geomagnetic disruptions of the Earth's magnetic field, brought on through solar storms, could be another cause for whale beachings. The authors hypothesize that whales navigate using the Earth's magnetic field by detecting differences in the field's strength to find their way. The solar storms cause anomalies in the field, which may disturb the whales' ability to navigate, sending them into shallow waters where they get trapped. The study is based on the mass beachings of 29 sperm whales along the coasts of Germany, the Netherlands, the UK and France in 2016.
"Follow-me" strandings
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 lead a whale out of 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.
Orcas' intentional, temporary strandings
Pods of killer whales – predators of dolphins and porpoises – very rarely strand. It might be that killer whales have learned to stay away from shallow waters, and that heading to the shallows offers the smaller animals some protection from predators. However, killer whales in Península Valdés, Argentina, and the Crozet Islands in the Indian Ocean have learned how to operate in shallow waters, particularly in their pursuit of seals. The killer whales regularly demonstrate their competence by chasing seals up shelving gravel beaches, up to the edge of the water. 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 re-float them and carry them back to sea.File:Orcas in Punta Norte Valdes Peninsula - panoramio.jpg|thumb|A killer whale hunting sea lions at Valdes Peninsula, Argentina, by deliberately stranding itself
In Argentina, killer whales are known to hunt on the shore by intentionally beaching themselves and then lunging at nearby seals before riding the next wave safely back into deeper waters. This was first observed in the early 1970s, then hundreds times more since within this pod. This behavior seems to be taught from one generation to the next, evidenced by older individuals nudging juveniles towards the shore, and can sometimes also be a play activity.
Sonar
There is evidence that active sonar leads to beaching. On some occasions cetaceans 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 cetaceans. In contrast, some who strand themselves due to seemingly natural causes are usually healthy prior to beaching:Direct injury
The large and rapid pressure changes made by loud sonar can cause hemorrhaging. Evidence emerged after 17 cetaceans were 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 cetologist, 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 dolphins, tearing tissue around the ears and brain. Apparently not all species are affected by sonar.Injury at a vulnerable moment
Another means by which sonar could be hurting cetaceans 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 cetaceans 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, forming bubbles, which are responsible for decompression sickness.
Diving patterns of Cuvier's beaked whales
The overwhelming majority of the cetaceans involved in sonar-associated beachings are Cuvier's beaked whales. Individuals of this species strand frequently, but mass strandings are rare.Cuvier's beaked whales are an open-ocean species that rarely approach the shore, making them difficult to study in the wild. 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. Fernández et al. in a 2013 letter to Nature 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 spatially separated from the next stranding. At the time of the incident, there was no connection made with active sonar; A. Frantzis, the marine biologist investigating the incident, made the connection to sonar because he discovered a notice to mariners concerning the test. His report was published in March 1998.
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. Data tags have shown that Cuvier's dive considerably deeper than previously thought, and are in fact the deepest-diving species of marine mammal yet known.
At shallow depths Cuvier's stop vocalizing, either because of fear of predators, or because they don't need vocalization to track each other at shallow depths, where they have light adequate to see each other.
Their surfacing behavior is highly unusual, because they exert considerable physical effort to surface by a controlled ascent, rather than passively floating to the surface as sperm whales do. Every deep dive is followed by three or four shallow dives. 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. This behavior suggests that the Cuvier's are in a vulnerable state after a deep dive – presumably on the verge of decompression sickness – and require time and perhaps the shallower dives to recover.