Since the River Thames was declared “biologically dead” more than 60-years ago, scientists have conducted the first complete ‘health check’. 

The Zoological Society of London (ZSL) conducted the survey and discovered some unexpected species in the river, including seals, seahorses, and eels.

The River Thames is also home to various species of shark, such as tope, starry smooth hound and spurdog.    

Despite the abundance of wildlife, the new results also reveal climate change has caused a 0.34°F (0.19°C) summer temperature rise in the Thames since 2007, as well as a water level increase.  

The River Thames was so polluted it was declared 'biologically dead' by the Natural History Museum in 1957. Now, a new report shows species living in the Thames include seahorses, eels, seals and even sharks, including tope, starry smooth hound and spurdog

The River Thames became so polluted in 1957 that it was officially declared “biologically dead” by the Natural History Museum. A new study has shown that the Thames is home to many species, including starry smooth hounds, seahorses and seals.


The Natural History Museum declared that the Thames had been declared “biologically dead” by them in 1957. 

The estuary’s water had become again so polluted, that only a few species could survive.

Since then, investment in infrastructure has led to improvements in the water quality of the Tidal Thames (from Teddington Weir onwards), says Zoological Society of London.

This has resulted in a diverse habitat that provides many benefits for people and wildlife.

Today the Tidal River supports more than 115 species fish and nearly 600 hectares saltmarsh. This is a vital habitat for an array of wildlife. 

The Natural History Museum declared the Thames ‘biologically dead” in 1957 because of its pollution. ZSL has released the first State of the Thames Report. It identifies the river’s environmental health, trends and provides a comprehensive overview. 

‘This report looks at what changed in those 60 years since the Thames was almost devoid of life,’ said Dr Andrew Terry, director of conservation and policy at ZSL, in the report’s foreword. 

“We will highlight the positive changes in habitats and species, as well the pressure reductions.  

For the report, experts used 17 different indicators to assess the health of the Thames’ natural environment, including water temperature, dissolved oxygen and the presence of fish, birds and marine mammals. 

They focused on samples in the Tidal Thames – the part of the Thames that is subject to tides – from Teddington Weir in west London onwards towards the Thames Estuary.   

Results show an increase in a range of bird species, marine mammals and natural habitats such as carbon-capturing saltmarsh – a crucial habitat for a range of wildlife. 

The Thames, according to the report is an ‘complex habitat’ that contains large numbers of oysters as well as dead shells. These serve as refuge for many organisms like juvenile fish, crabs, and molluscs. 

Although the decline in fish species has occurred since the beginning of 1990s, more research is required to understand the reasons.   

Pictured, the common smooth hound shark (Mustelus Mustelus), to be found swimming in the Thames

Pictured: The common smooth hound shark Mustelus Mustelus (commonly known as the “Mustelus”), which can be found in the Thames. 

Concerningly water levels are rising ever since the Tidal River began monitoring in 1911. 

Silvertown in east London has seen an average sea level rise of 0.16 inches (4.26mm per year) since 1990.  

The Thames is also at high risk from sewage, which spills from a defective sewage  system that was installed during the Victorian era.  

London’s future sewage system is the Thames Tideway Tunnel. It will be completed in four years and it promises to increase water quality. 

‘This report comes at a critical time and highlights the urgent need for the Thames Tideway Tunnel, known as London’s new super sewer,’ said Liz Wood-Griffiths at Tideway, the company building the tunnel. 

Pictured, building work underway for the Thames Tideway tunnel, a super sewer underneath the River Thames, London

Pictured: Building work is underway on the Thames Tideway tunnel. This super-sewer will be located under London’s River Thames.

“The sewer will capture over 95% of the sewage that flows into the River through London’s Victorian sewers. It is expected to complete its construction in 2025. 

“It will have an important impact on water quality and make it a healthier environment for wildlife to thrive”   

Phosphorus concentrations in rivers all over the UK increased rapidly between 1950 and the late 1980s, primarily because of nutrient-rich sewage, the results reveal, although phosphorus concentrations have decreased overall since the 1990s. 

From 1990 through 2020, we found that the average concentration of phosphorus was lower in most cases than previously thought. 

But, there is a significant increase in the nitrate content over time, which has negative effects on water quality, and can be harmful to wildlife. 

The Thames is also at high risk from sewage, which spills from a defective sewage system that was installed during the Victorian era (stock image)

Also, the Thames is vulnerable to sewage (stock image).

ZSL states that many other chemicals are of concern and aren’t being monitored regularly, potentially causing harm to the Thames wildlife. 

Trends in the short term show water quality improving with positive and long-term growth in dissolved oxygen concentrations (DO) between 2007 and 2020.

It is crucial to understand DO concentrations as they can have a significant impact on relationships among key species living in rivers and fish. 

Dr Terry stated that ZSL works towards creating a more wild and diverse Britain, which is rich in wildlife. 

“This is a continuation of the efforts to lower pressures and restore species, as well as engaging with communities regarding the positive effects of a recovering Thames.  


Seal numbers in the Thames have fallen by 12 per cent since 2019, a 2021 report from the Zoological Society of London (ZSL) shows.

There were 797 harbour seals and 2,866 grey seals detected this year – a combined total of 3,663, according to the charity’s annual seal population survey.

Grey seal (Halichoerus grypus). Grey seals have more obviously contrasting pale bellies and darker grey backs, with larger more irregularly shaped spots and blotches

Grey seal (Halichoerus grypus). Grey seals (Halichoerus grypus) have more clearly contrasting grey bellies and darker gray backs. These animals also tend to be larger and more irregularly shaped with more spots and blotches.

This is down 12 per cent from the 932 harbour seals and 3,243 grey seals during the last survey in 2019 – a combined total of 4,175.

Despite this fall, the charity describes the Thames as ‘a key ecological hotspot’ for the two seal species, which are top predators in the iconic river.  

The numbers mark an amazing turnaround from 1957, when the river was so polluted it was declared ‘biologically dead’ by the Natural History Museum. 

Read more: Seal numbers in the Thames have fallen by 12%, report reveals