More than 100 seal pups born in the Thames 60 years after the river was declared 'biologically dead'

Jessica Carpani
Experts have counted over 100 seal pups on the river’s shores.  - ZSL

In 1957 the River Thames was so heavily polluted it was declared “biologically dead” by the Natural History Museum leaving hope for marine life lost.  

But now, more than 60 years after the museum's dismal proclamation, experts have counted over 100 seal pups on the river’s shores. 

Scientists from the Zoological Society of London (ZSL), an international conservation charity, were "thrilled" to discover 138 pups on the sandbanks and creeks of the river in what is the first ever comprehensive count of the offspring. 

Using a light aircraft, they were able to capture images during pupping season of the harbour seals, also known as the common seal.  

The count is evidence that the London river’s ecosystem has come back from the brink after the National History Museum said its low oxygen levels and polluted waters would mean nothing could survive in its murky tides. 

The pioneering research took place in 2018, with the data released for the first time today. It saw scientists carefully analyse hundreds of photos taken as part of UK-wide seal monitoring initiative.

Explaining the decision to count using photographs, ZSL said it was “much easier, and so more accurate, to count the seals in photos instead of the constantly moving, playful creatures”.

Conservation Biologist, Thea Cox said: “We were thrilled to count 138 pups born in a single season"   Credit: ZSL

The figures are good news for the harbour seal population which was decimated by two local disease outbreaks of Phocine Distemper Virus (PDV) in 1988 and 2002. 

Conservation Biologist, Thea Cox said: “We were thrilled to count 138 pups born in a single season. 

“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.”

Project Manager, Anna Cucknell, who leads ZSL’s Thames conservation said: “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. 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.”

While the Thames is home to both harbour seals and grey seals, only the former breed there.

Thames seal population estimates have been conducted annually by ZSL since 2013. The most recent results, from 2017, recorded 1,104 harbour seals and 2,406 grey seals across the estuary.

While population surveys indicate that numbers of pups are rising it is not yet known if this is due to resident seals having pups or from adults migrating from other regions where colonies are known to be dwindling.  

Found throughout the estuary and river, including in central London, the breeding survey allowed researchers to better understand the seals in the Thames and the reasons behind their changing numbers.

As top predators, the harbour seals are a strong indicator of a complex estuarine environment. 

A researcher undertaking a seal survey Credit: ZSL

Other species are also thriving in the river’s ecosystem as Cucknell added that it is “an essential nursery habitat and home to many animals”. 

These include more than 100 species of fish, including two species of shark, short-snouted seahorses and the Critically Endangered European eel.

Last year the river even hosted a rare beluga whale named Benny for three months. While initially feared to be stuck, experts later found he was happily feeding on the plentiful fish in the stretch of the river by Gravesend.

But, experts warn that marine life remains threatened.

Last year, the RSPCA told the Telegraph that it was the worst year for seals encountering “horrifying” plastic-related injuries. The animal charity reported that six grey seals required urgent care in Norfolk after becoming trapped in plastic rubbish.

In a bid to prevent similar incidents in the Thames, ZSL has spearheaded the #OneLess campaign, which aims to free the capital from single use plastics.

The charity have also set up a citizen-science led Outfall Safari to tackle pollution from misconnected sewerage pipes across London, where the public can systematically survey the outfalls in rivers to identify pollution and notify the relevant authorities.