LOST SOULS
BBC Wildlife|October 2021
In May, a minke whale got lost in the Thames, capturing the nation's interest. But why do whales swim up rivers? And how can they be helped?
Jo Caird

The arrival of a whale in a city river never fails to capture the imagination. It’s hard to articulate why: there’s something otherworldly, almost magical, about these unknowable creatures from the deep sea making a sojourn into our busy, urban lives.

Whatever the reason for our fascination, when whales swim up rivers – whether it’s the Thames, the Trent or the Clyde, their appearance always draws a crowd. Take the juvenile minke that turned up in the Thames at Teddington Lock in May. Hundreds of onlookers lined the riverbanks, hoping to catch a glimpse of the stricken creature, while news helicopters overhead reported on its every move – and those of the rescue teams trying to save its life.

Around 1,000 cetaceans – that’s whales, dolphins and porpoises – become stranded in the UK each year, according to the UK Cetacean Strandings Investigation Programme (CSIP). Funded by the British government and its counterparts in Scotland and Wales, the programme’s scientists investigate 100 to 150 of these events annually, conducting post-mortems that provide data on causes of death, disease, diet and the general health of cetaceans in UK waters.

Strandings in rivers make up a tiny proportion of the total – less than 1 per cent, estimates cetologist Rob Deaville, CSIP’s project manager – with the overwhelming majority of cetaceans who get stuck high and dry doing so on the coast.

So what’s behind the highly unusual scenario of whales coming into rivers? “As they’re entering the abnormal habitat of the southern North Sea – and there may be an underlying issue there, we’re still investigating that – they’re hugging the coastline,” explains Deaville. “The whale doesn’t realise when it enters an estuary system, and it keeps going, and then unfortunately ends up in the river system. Unless it goes back out the other way, it’s going to be in trouble.”

The primary challenge for these animals is lack of food. While some cetaceans – species such as harbour porpoises, for example – are able to hunt in shallow water, whales simply cannot access their preferred prey in even our deepest river. Northern bottlenose whales might dive to depths of more than 1,400 metres to find deep-water squid, their favourite food, while an adult humpback can consume up to 1,360kg of plankton a day. These species – others known to have stranded in the UK include long-finned pilot whales and orcas – have an extremely wide distribution around the world, often migrating between warmer and cooler waters to feed and breed. River systems are a totally alien environment for them.

There’s also the issue of injuries sustained by bumping into boats or fixed structures such as weirs, and the general stress of being in unfamiliar, noisy, urban surroundings. Unsurprising, therefore, that the pod of Northern bottlenose whales that turned up in the River Clyde in 2020 and the humpback that was spotted in the Thames in 2019 didn’t fare well. All died, despite attempts to encourage them to return to deeper water.

Not that it’s necessarily the river environment itself that’s to blame. The whales that end up in rivers are often found to be malnourished and exhausted, conditions that likely pre-date – or even cause – the navigational errors that led them there. These species are vulnerable to all manner of threats, from manmade pressures such as entanglement in fishing gear, to natural difficulties like unweaned calves becoming separated from their mothers.

That’s what happened with the Thames minke whale in the spring, believes Deaville. “They’re weaned quite young as a species – around 5–6 months, when they’re around 4.5m [in length] – so that animal may well have been just around weaning age,” he says. “It had nothing in its stomach so it was pretty thin, and we think that was the primary factor in its death.”

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