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Why do whales and dolphins strand on beaches?

People often ask me ‘why’ whales and dolphins do one thing or another.  I’m a whale and dolphin scientist and so it’s my job to know stuff like that.

Over recent days, the world has watched the heart-breaking scenes on two remote New Zealand beaches as almost 500 pilot whales have died after becoming stranded. These tragic deaths came just weeks after 200 pilot whales died on a remote Australian beach. So, the question I am being asked the most is ‘why do whales and dolphins strand en masse like this?’.

I may be a whale and dolphin scientist but even I don’t have a simple answer to that one. Sadly, there are probably as many reasons why as there are strandings themselves, but let’s explore them.

© Brodie Weeding, The Advocate
© Brodie Weeding, The Advocate

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Strong social bonds

Whales and dolphins wash up on beaches all over the world and individual strandings are usually the result of illness or injury where the dolphin or whale cannot navigate or swim properly and they are driven ashore by prevailing winds.

But when it’s multiple individuals – sometimes as many as several hundred – the reason is not always immediately evident and us scientists are still scratching our heads in consternation and sadness as to why these water-dwelling creatures suddenly find themselves on land.

It’s well known that whales and dolphins are very social. The very large whales, such as baleen whales, rarely strand in large groups – although in 2015 there was an unprecedented mass stranding of more than 300 sei whales off the coast of Chile – but the species that do, such as pilot whales, share certain characteristics which mean mass strandings are more common.

Sticking together

They tend to be deep-water dwellers, more likely to strand than other species, probably because they’re not used to the contours of shallow waters. They live in strongly-organised, highly-social groups. They tend to have lead individuals, who the others in the group (some related, some not) follow. A problem for the leader, or even a fellow pod mate, such as sickness, injury or disorientation, may cause the whole group to strand as their strong social bonds means they won’t leave a pod mate alone. Some people have speculated that when humans feel ill, we say ‘I am ill’, but when a pilot whale is ill, it’s more a case of ‘we are ill’. This ‘herding behaviour’ seen in pilot whales for example, is what hunters in the Faroes rely on - drive in a few whales and the others will follow.

Pilot whales strand in Tasmania in 2020 © M. Brasier
Pilot whales strand in Tasmania in 2020 © M. Brasier

Navigational error

Research suggests that whales and dolphins might navigate their way around the ocean using Earth’s magnetic field, and analysis has found that live strandings often occur on shores where lines of equal magnetic force meet the coastline perpendicularly. In other words, in some instances the whales or dolphins are misled by these abnormalities and follow them ashore, with potentially disastrous consequences.

Certain shallow, gently sloping shores, with soft sandy or muddy bottoms, often featuring sandbars that extend out to sea, seem to be particularly prone to live strandings. This is the case in some parts of New Zealand for example, and it seems that whales and dolphins find this kind of shore particularly confusing. It may be that their echolocation abilities – designed for deep-sea living - work less well in areas of soft sediments, and that stranding results primarily from navigational errors. Sudden changes in tidal cycles can also cause individuals who have strayed into shallower waters to suddenly find themselves ‘high and dry’.

Noisy seas

There is a growing concern that more and more strandings are the result of our increasing and often very noisy activities at sea. Human-induced underwater noise, like sonar or pulses from seismic testing can deafen and disorientate whales, interfering with their ability to communicate and navigate, ultimately driving them ashore. Deep sea species living in the open ocean are particularly susceptible to sonar, even from miles away.

I’ve taken part in several rescue efforts and as a responder, mass strandings are incredibly difficult to stomach. Paramount in your mind is the welfare of the individuals who have stranded, innocent beings, surrounded by the cries of their pod mates and suffering immeasurable pain as they slowly become crushed by their own weight.

The more we learn about why these events occur, which is where the crucial work in undertaking post-mortems comes in, the better placed we are to prevent them happening in the future.

To the rescue

In the UK, live strandings are attended by British Divers Marine Life Rescue, supported by volunteers and trained marine mammal medics from WDC and other organisations. In North America, we’re a member of the Marine Mammal Stranding Network and our team acts as first responders to whales, dolphins and other marine mammals in distress along a section of the Massachusetts coastline.

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