138 Seal Pups Born In The River Thames, 60 Years After It Was Declared Biologically Dead


Katy Evans

Katy is Managing Editor at IFLScience where she oversees editorial content from News articles to Features, and even occasionally writes some.

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Seal pups spotted in the Thames estuary. ZSL/Jonathan Kemeys

In the 1950s, Britain’s famous River Thames was so polluted it was declared “biologically dead”. Now, 60 years later, it is playing nursery to over 130 seal pups, the first pup-count by the Zoological Society of London (ZSL) has revealed.  

Last summer – the pupping season for seals – saw the first official comprehensive pup-count of seals in the Thames, which winds its way from Gloucestershire, through Oxford and London, to the estuary, where it meets the North Sea. Carried out through analysis of photographs taken of resting seals from a light aircraft so as not to disturb them, ZSL is excited to reveal 138 pups were born last year.


“We were thrilled to count 138 pups born in a single season,” said conservation biologist Thea Cox in a statement. “The seals would not be able to pup here at all without a reliable food source, so this demonstrates that the Thames ecosystem is thriving and shows just how far we have come since the river was declared biologically dead in the 1950s.” 

Photographing the seals from above means not disturbing them. It's a lot easier to count seals that aren't moving! ZSL/Thea Cox

The Thames is host to both harbour seals and grey seals, which ZSL has been carrying out population estimates of since 2013. The most recent records, from 2017, saw 1,104 harbour seals and 2,406 grey seals across the estuary, confirming that seal populations in the Thames are on the rise. However, conservationists weren’t sure if this was due to resident seals having pups, or the addition of adult seals from other possibly failing colonies joining the Thames crew. This prompted them to carry out the first comprehensive breeding survey, affectionately known as the “pup-count,” of harbour seals. 

Look ma, I can fly! Graham Mee SE RSPB

Why just harbour seals?

“Incredibly, harbour seal pups can swim within hours of birth which means they are well adapted to grow up in tidal estuaries, like the Thames. By the time the tide comes in they can swim away on it,” Anna Cucknell, project manager, and leader of ZSL’s Thames conservation, explained. “Grey seals, on the other hand, take longer to be comfortable in the water, so breed elsewhere and come to the Thames later to feed.” 


So, why is this so exciting?

In 1957, a Port of London Authority report declared no fish had been seen in the river in the 64 kilometers (40 miles) between Richmond in west London and Tilbury in Essex between 1920-1956. Large stretches of the river were without oxygen, without which nothing could live, rendering it biologically dead. Now, it hosts not only a rising population of seals, but also over 120 species of fish, two species of shark, short-snouted seahorses, endangered European eels, porpoises, dolphins, and even the occasional whale.

If you spot any of these marine mammals in and around the Thames, you can take part in ZSL's Thames Marine Mammal Conservation project. You can report a sighting, which will get added to the Marine Mammal Survey Map (where you can also check out other people's sightings, and follow seals wearing tracking devices) and even help count and ID seals from ZSL's various camera traps. Combining citizen science projects with official surveys like the pup-count will ensure a better understanding of seal numbers and behaviors in the UK. 

Project manager Anna Cucknell might just have the dream job. ZSL