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How the Thames went from ‘biologically dead’ to full of seals (and what to do if you spot them)

By Alice French

Sixty years ago the Thames was deemed ‘biologically dead’. Now seals are returning to our city’s river. Alice French hits the water to find out more

Heard of Sammy the seal? Back in 2013 he found viral fame as London’s greediest pup. Videos showed the wiley critter hanging out at Billingsgate Market and munching on fish from porters. Turns out he’d been heading to the fish market every day for 10 years for a salmon snack. Since then, small groups of the grey mammals have also been spotted in the area – and they’re not the only seals in the Thames.

Every year the Zoological Society of London (ZSL) count up just how many mammals London’s waters hold. Last year’s survey estimated we have around 964 harbour (ie common) seals and 1,552 grey seals living in the Thames. For some, this is hard to believe. Just 60 years ago the river was deemed ‘biologically dead’ due to pollution.


‘Just 60 years ago the river was deemed “biologically dead”’

ZSL’s marine biologists are now embarking on their fifth study of seals in the Thames Estuary – and I’ve been invited to come along. I set off on a wet speed boat ride down the River Stour with Anna Cucknell, ZSL’s Estuaries and Wetlands Conservation Manager.

Anna explains that ZSL has records of marine animals in the Thames dating back to the 1200s, but they got driven away as the river became uncontrollably polluted. In fact, back in 1957, there were practically no animals living in the then-murky water (it even smelt rotten, it was so polluted!). Thanks to initiatives such as the Thames Tideway Tunnel – a new sewer put in place to tackle the problem of overflows from London’s Victorian sewers – the river’s been able to re-populate with small fish and flounder. And the lure of such tasty food has encouraged seals to come over to breed.


‘The lure of tasty food has encouraged seals to come over to breed’

‘Now the Thames is the UK’s busiest waterway, with a highly diverse number of marine species,’ Cucknell explains. Even on our short boat ride, I spot upwards of 25 of the grey mammals hauled out on the banks around the Stour, all different shades of grey, brown and rusty-red colours (the latter is actually due to industrial matter, which increases the iron levels in the water, but it’s totally safe according to Anna).

‘We also have the Thames Marine Animal survey, where the public are encouraged to report seal sightings,’ Anna tells me while we watch the seals flop into the water. ‘It helps us to find out the timeframe of when they are in certain places. It’s brought in around 2,000 sightings since it was set up.’ Anna says that often people wrongly assume that seals are in trouble when they spot them hauled on the banks of the Thames, and try to intervene. ‘We had a Facebook video sent to us once, showing a harbour seal caught in a net, being petted, which can be really detrimental to them and not to mention dangerous, as the claws on their flippers can do some real damage.’


‘Anna says: “We had a Facebook video sent to us once, showing a harbour seal caught in a net, being petted”’

Why should we be keeping an eye out for these creatures? Well, Anna says developers need to be in the know so they can avoid disturbing the wildlife when they’re planning new buildings. ‘This is why we are doing our first breeding survey next year,’ she says. ‘So we warn these big companies off haul-out spots that are possibly – although we can’t know for sure – used for pupping. It’s also really important that we protect these animals, simply because they are very charismatic and great wildlife to have!’

London’s most charismatic seals, Sammy and his Billingsgate crew, are a perfect example of why we should treasure these animals. ‘Have I seen Sammy the seal?’ laughs Anna. ‘Who hasn’t? He’s a part of the city! Really, I think he is not just one seal, and there’s probably a few “Sammys”...’

If you’re lucky enough to spot a seal record it in the ZSL Thames Marine Animal Survey

Read more: Days out in London for animal lovers

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