Robyn Grace/EPA

Danielle Andrew 24 Feb 2017, 21:20

The ConversationAround 600 pilot whales recently became stranded on a New Zealand beach, around 400 of which died before volunteers could refloat them back into the sea. Sadly, this kind of mass whale stranding has occurred since human records began, and happens somewhere in the world on a regular basis.

At the end of 2015, 337 sei whales died in a fjord in Chile after the largest ever beaching of whales of this species. Mass strandings can also occur in northern Europe. In February 2016, 29 sperm whales were found stranded on the coasts of Germany, the Netherlands, eastern England, and northern France, a record for this species in the North Sea.

Why do these creatures, which are masters of living in a totally aquatic environment, enter an inhospitable land environment where inevitably some, if not all, will die?

Mass strandings almost invariably involve oceanic species of whales. Long-finned and short-finned pilot whales tend to be the most frequent casualties. Other species typically are false killer whales, melon-headed whales, Cuvier’s beaked whales and sperm whales. All of these normally live in waters over 1,000 metres deep and are very social, forming cohesive groups that in some cases may number hundreds of animals.

Although it’s tempting to automatically blame whale strandings on human activity, the fact that deep-dwelling species of whales most often get stranded, and in the same locations, indicates that in many cases natural causes are more likely to be to blame. Mass strandings of these oceanic species tend to be in very shallow areas with gently sloping, often sandy, seabeds. In those situations, it is no surprise that these animals, which are used to swimming in deep waters, can get into difficulties and even if re-floated will often re-strand.

The echolocation they use to aid navigation also does not work well in such environments. So it is quite possible that the majority of such strandings are simply due to navigational error, for example when whales have followed a valuable prey resource into unfamiliar and dangerous territory. This may have been the cause of the mass stranding of sperm whales in the North Sea, some of which had recently digested oceanic squid in their stomachs.

The ratio of strandings to sightings for sperm whales in the North Sea is significantly higher south of the Dogger Bank where shallow, often sandy, environments prevail. And the same goes for Farewell Spit, Golden Bay in the South Island of New Zealand, where the recent pilot whale stranding took place and where similar incidents have occurred several times in the last few years.

Both areas have seen a number of mass strandings of those particular species in the past. In the southern North Sea, there are records of mass strandings of sperm whales dating back at least to 1577.

However, mass strandings aren’t only caused by whales getting lost or misjudging the depth of the water. One or more individuals may be diseased and, as they become weaker, they seek shallower waters so they can more easily come to the surface to breathe. Once their bodies come to rest on a hard surface for any extended period, there is a greatly increased chance that their chest walls will be compressed and their internal organs damaged.

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