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Mass strandings: What are they and why do they happen?

31 Mar 2014

Mass stranding events have been known to occur since ancient times. It seems that there are more cases nowadays, primarily due to the increased number of sightings as larger number of people live close to the sea, and possibly due to increased anthropogenic activities at the high seas and near to the shores. Nevertheless, the exact causes of stranding can be attributed to multiple factors.


Mass stranding events (MSEs) can be defined as two or more individuals of the same species beaching at the same time and same location. Scientists found that the majority of the species affected are toothed whale species such as false killer whales, long-finned pilot whales, Atlantic white-sided dolphins and white beaked dolphins. In rare cases sperm whales and killer whales can also strand themselves. It is extremely difficult to save beached individuals since they normally pass away as a result of dehydration or their body can collapse under its own weight.


Physiological and environmental factors, for instance rough waters, exhaustion, disease, parasites, injury, old age, birthing difficulties, hunting too close to the shore, behavioural tendencies to follow the leader, harmful algal blooms and magnetic field anomaly can all lead to stranding events. Additionally, loud noises from underwater earthquakes and volcanos can damage whale’s sinuses and cause disorientation. Scientists have also identified so called “mass stranding hotspots” around the world where strandings seem to be more frequent. For example, at Ocean beach in Tasmania and at Geographe bay in Western Australia the main cause can be the local underwater topography and bathymetry. Here, the gentle sloping continental margin may attenuate or weaken the sound which can lead to difficulty in hearing the acoustical echo. At Cape Cod bay in Massachusetts, scientist highlighted that high frequency of pilot whales strandings may happen because these species try to traverse a channel that has been closed for hundreds of years. They also speculated that these species lack the ability to learn. Nevertheless such hypothesis cannot be applied everywhere.


The most well-known factor that leads to mass stranding is the use of underwater sonar. Beaked whale species are found to be the most sensitive but blue whales, the largest animals, were found to abandon feeding and swim away in response to loud noise. Military sonar used by navies can cause mechanical damage in ear cavities or lead to haemorrhaging. Also, heavy noise can frighten or panic whales, which can lead to sudden surfacing, and subsequently cause decompression sickness. 


Even though, both natural and anthropogenic factors can lead to whale beaching, by reducing human activities at areas where whales frequently migrate and feed can at least help to cut the number of incidents that are triggered by such activities.


By Wanda Bodnar



Image Source: Jamie Dyer British Divers Marine Life Rescue





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