On September 21st 2020, 470 pilot whales were stranded on Ocean Beach, Tasmania, in the largest mass whale or dolphin stranding event recorded in Australian history. Exactly two years later, a further 230 pilot whales were found stranded on
the same beach. Two weeks later, in October 2022, 477 pilot whales died stranded on Chatham Island, New Zealand. These tragic, high-profile stranding events make headlines, but why do they happen?
Strandings happen globally and are becoming more and more common. Between 1990 and 2014, the UK Cetacean Strandings Investigation Programme (CSIP) reported over 12,000 cetacean strandings; in 2018 alone, there were 7,320 marine mammal
strandings in the United States; and the New Zealand Department of Conservation attends to an average of 85 strandings every year.
Cetacean strandings occur when dolphins, whales and porpoises strand themselves on land, typically beaches, or the carcass of cetaceans that died at sea reach land. | Abdinor Salad / Unsplash
Cetaceans (whales, dolphins and porpoises) and pinnipeds (seals and sea lions) are known to beach in response to extreme weather, confusing coastal topography and other natural causes such as sickness, injury and old age.
For example, in Chile, the severity of coastal storms coincided with higher numbers of stranded South American sea lion pups. Whilst in Massachusetts, disease was found to be the leading cause of death among 314 marine mammals stranded
between 2000 and 2006.
Human activities are also responsible for stranding animals, through ship strikes, by-catch (species unintentionally caught up in fishing gear) and pollution. In the Mediterranean Sea, one in five stranded whales showed evidence of ship
strikes and of the 12,000 stranded cetaceans reported by CSIP, almost 20% of those necropsied died through by-catch.
‘Human activities are also responsible for stranding animals, through ship strikes, by-catch (species unintentionally caught up in fishing gear) and pollution.’
Noise pollution has more recently been linked to cetacean strandings. Between 1960 and 2004, 30% of beaked whale strandings were associated with naval bases or military sonar activities.
In 2002, 14 beaked whales washed up in the Canary Islands during NATO military sonar tests. These whales showed signs of decompression sickness, suggesting an interruption to their typical deep dives. By ascending from their dive too
quickly, gas bubbles formed in their tissues and the animals likely became sick or disorientated, causing them to strand.
Whilst it is extremely challenging to prove decompression sickness as the cause of death for stranded cetaceans, largely due to the delay in reporting strandings, there is a growing trend between military activity and beached animals.
Delays in reporting strandings can reduce the amount of information available due to the effects of decomposition, but even cetacean bones can be used to provide information on species identity, genetics and life history. | Eric
Kilby / Flickr
What defines a mass stranding?
Mass strandings involve anywhere from two to hundreds of cetaceans, as seen recently in Australia and New Zealand. Species with particularly strong social bonds, such as pilot whales, strand in large groups more frequently.
Their instincts drive the group together even if an individual is injured, sick or an especially poor navigator, as is believed to be the case with the recent mass stranding in New Zealand. The gentle sloping of the sandy beaches around
Chatham Island could have potentially caused errors in their navigation systems.
‘Species with particularly strong social bonds, such as pilot whales, strand in large groups more frequently.’
As with many other cetacean species, pilot whales rely on echolocation to negotiate their surroundings, which can become impaired in shallower waters. Once the tide recedes, the animals become beached and their chances of survival are very
low, even if help is at hand.
How can strandings be used to help conservation?
Whilst stranding events can be devastating to species survival and ecosystem health, they can be instrumental in monitoring marine mammal populations. Necropsies not only provide information on how an organism died, but also on its life
history; answering questions on where individuals have been, what they eat, their age, sex and their number of pregnancies. These answers are particularly valuable for rare and elusive species where little is known about their populations.
Additionally, strandings can be used as an indicator of population health, abundance and distribution, with larger populations expected to have higher numbers of stranded individuals.
Many cetacean species are currently under threat from human activities, and further research and protective measures are needed to ensure their survival. | Bart / Unsplash
For example, the UK saw a rise in humpback strandings following the recovery of their populations as a result of the whaling ban. Necropsy results can also indicate significant threats facing populations, as seen in 2019 when a juvenile
sperm whale was found beached in Scotland with nearly 100 kilograms of plastic in its gastrointestinal tract.
An improved understanding of population health and the threats facing them can lead to more effective and targeted conservation policies. Important habitats can be included in marine protected areas (MPAs: zones where there are limits to
human activity) and improved waste management can reduce the amount of plastic reaching the ocean.
‘Strandings can also be used as an indicator of population health, abundance and distribution.’
To date, there are around 20,000 MPAs worldwide, covering roughly 1% of our oceans, yet only 600 of these areas protect whale and dolphin habitats. 900 marine species have been recorded interacting with plastic pollution, and recent
research has shown blue whales can ingest up to 10 million microplastic pieces every day.
As populations continue to struggle, there is a need to provide further, more stringent protection to minimise our destructive impact on marine organisms.
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