The River Thames weaving through central London as seen from the air

The city of London has grown up alongside the Thames, using it for sewage, recreation and transport over the years. Image © Csaba Peterdi/Shutterstock

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Sharks, seahorses and seals found in River Thames' first full health check

Sharks, seahorses and smelt are among the residents of the River Thames revealed in a new report.

While conditions in the river are generally improving, climate change and nitrate pollution still pose a threat to its recovery.

The Thames is getting healthier, researchers have suggested, but progress is being hampered by increased pollution and climate change.

Research published by the Zoological Society of London (ZSL) found that oxygen levels in the river are increasing, while certain pollutants like phosphorous are on the decline as sewage treatment is improved.

However, sea level rises in the tidal Thames threaten valuable habitats along the waterway, and the number of fish in the river has fallen for reasons that are as of yet unexplained.

Dr Andrew Terry, the director of conservation and policy at ZSL, says, 'This report looks at what changed in those 60 years since the Thames was almost devoid of life. We highlight some of the reductions in pressures and improvements in key species and habitats, and set benchmarks for indicators on how the river is used. 

'However, we also note some key elements of pollution increasing and evidence of climate change having an impact. A healthy Thames is vital in mitigating some of the impacts of climate change. 

'As we increasingly recognise the intrinsic and economic value of nature's services to humans, we hope to see investment in the continued restoration of the river.' 

Dead fish washed up by a river

The river was declared 'toxic' by a Museum scientist in the 1950s. Image © Nikolay Gyngazov/Shutterstock

The 'toxic' Thames

The River Thames has been a site of human activity for thousands of years. The Museum's collections feature human remains dating back to the Bronze Age which have been found during work on the waterway over the past two centuries.

As London has grown, humanity activity has taken a toll on nature. Chemicals and waste have been dumped into the water, which in 1858 led to 'The Great Stink' where hot weather exacerbated their smell.

This led to the construction of large sewer systems, which are still in use today. While these moved the problem from the heart of the capital, waste was still being dumped into the river.

By the 1950s, parts of the river were declared 'biologically dead'. Museum scientist A.C Wheeler noted in 1957 that 'polluted water either prevents or deters fish from ascending the river,' adding that some parts of the river were 'toxic' due to industrial waste.

This research, along with that of many others, led to the construction of treatment works and sewer improvements along the Thames which started the river's recovery. Since 2003, ZSL has monitored the health of the tidal river between Teddington and the Isle of Sheppey.

A smooth hound shark lying in shallow water on a beach

Fish such as the smooth hound were caught using seine nets and identified before being returned to the river. Image © Alessandro De Maddalena/Shutterstock

Muddying the waters

The report found causes for optimism, including levels of dissolved oxygen in the river. While oxygen levels fluctuate naturally, human activities can cause it to decline sharply and stop fish from being able to breathe.

The most common way this occurs is through the discharge of sewage into the river. The microorganisms involved in breaking down the waste use up oxygen, which can in turn cause more plants and animals to die.

The scientists found that while periods of low oxygen continued, they were becoming less severe. 

It was a similar situation for phosphorous, which enters the river after draining from agricultural land and as sewage. All the rivers monitored in the report showed declines in the amount of the nutrient.

The declines are believed to result of new sewage treatment works and improvements to existing ones, which prevent as much sewage entering into the water. A new 'super-sewer', the Thames Tideway, is also due to be complete by 2025 and it is hoped it will reduce the amount of sewage released into the river during heavy storms.

The positive trends in these compounds have been a boon for nature. The populations of the 92 species of birds on the Thames are increasing. Wading birds, for instance, have almost doubled their populations. Meanwhile, seal populations are also continuing to increase while invertebrates remain stable.

However, it was a different story for fish. The researchers found a long-term decline in species, though they were stable in the short term. 

Two avocets wading in the morning light

Populations of wading birds like the avocet have doubled since the 1990s. Image © Rudmer Zwerver/Shutterstock

This is concerning because of the 115 species that live in the Thames, 20 of them use it as a nursery, including European seabass and the smooth hound shark. While the decline may be the result of pressure on fish outside the river, the researchers intend to continue monitoring other potential causes.

These include nitrate levels rising, which can cause eutrophication that kills fish, as well as an overall deterioration in preserved habitats. Saltmarsh, which supports fish, birds and insects, has declined by almost half since the nineteenth century, though there is some evidence it is gradually returning.

This return, however, could be stopped by climate change as rising river levels encroach into valuable habitats. A survey site in Silvertown in east London found that the Thames rose by around 4.3mm a year for the past 30 years on average, significantly above the overall trend of around 2mm a year since 1911.

The water is also getting warmer. The upper Thames is warming by around 0.2°C a year in summer, which leaves the water less able to hold the oxygen life needs to survive. 

Overall, the researchers found that the Thames has improved significantly, but that work must begin to meet the challenges of the future. The authors intend to study changes in plastic and chemical pollution in the water, as well as monitoring factors such as the connectivity of the river and the creation of new habitats.

They hope that this will allow the Thames to continue its recovery, providing a home for both nature and humanity.