Life in the River Thames

Image above: River Thames; photograph by Anna Kunst

Guest blog by Joanne Gilbert

Joanne Gilbert set up WildChiswick during lockdown, researching Chiswick’s wildlife and working out how best to support it. She writes a regular guest blog for The Chiswick Calendar. Here she looks at life in the River Thames, based on a recent report by the London Zoological Society.

Did you know that one of the inhabitants of the Thames travels over six thousand kilometres to visit us? Or that another creature living in the river is thought to be the only species where the male gives birth?

With the current scrutiny of the state of the water quality in the UK’s rivers and the release of State of the Thames report by Zoological Society of London last November, there has been interest in the river and what is living in it.

Image above: River Thames at Strand on the Green; photograph Joanna Raikes

From fresh to salt and dead to alive

The River Thames is the second longest river in the UK, being beaten to the title by the River Severn which is five miles longer.

The source is believed to be at the Thames Head near Kemble in the Cotswolds. As it heads down to the estuary into the North Sea the river contains both salt and fresh water, the salinity increasing as it leaves Teddington lock.

The salinity depends on the state of the tide coming up and the volume of fresh water coming down stream via the tributaries. Interestingly, the fresh water passes over the top of the salt water, so the salinity increases nearer the riverbed, but by the time the river has snaked its way from Teddington to the North Sea, the salinity has equalled out from the surface to the bottom.

Large sections of the river were declared “dead” back in 1957. The river was heavily polluted with sewage and the oxygen levels dropped so low that few species could survive within its waters.

Image above: Fisherman taking part in the annual competition at Tidefest; photograph Anna Kunst

Improvement and things to keep an eye on…

Improvements in water and sewage treatment and a reduction in heavy metals being dumped into the river are just two reasons why, from approximately the 1960s, the river began to be cleaner. Now 70% of London’s water is supplied by reservoirs that collect water from the Thames.

Oxygen concentrations, phosphorus and nitrate levels have been monitored in the Thames over the years. The Zoological Society’s analysis showed that dissolved oxygen concentrations, critical for fish survival, show long-term increases. Phosphorus concentrations have reduced in both the long and short term, showing the effectiveness of improved sewage treatment. However, there has been a long-term increase in nitrate concentrations, which can negatively affect water quality through enrichment.  Too many nitrates can cause algae blooms blocking out the light that can affect the ecosystems within the river.

Water temperature and sea levels continue to rise above historic baselines. This may be caused by global warming and, according to the Zoological Society, will affect the estuary’s wildlife, leading to changes in life-history patterns and species ranges.

Where the levels mentioned above have been monitored in the Thames for a while and therefore trends can be recorded, there has been no such ongoing monitoring of plastic waste in the Thames – until now.

The report gives an annual average of the five most common plastic items found in the river since 2015. The main culprit really amazed me; it is cotton buds!  Why on earth are we putting such a quantity of cotton buds into our waste and it ending up in our rivers?  Crikey!

Look at the downward trend once the Government banned them from being made of plastic. One assumes they are still dumped in this manner despite not being plastic. Stop putting cotton buds down the loo folks!

Image above: Common plastics found per 1msampling area – Credit, Zoological Society of London (ZSL)

A quick snapshot of a few species

Sensational Seahorses

I have always found seahorses intriguing; such beautiful creatures, and I was fascinated to learn they have been found in the Thames.  I always thought they dwelled only in more exotic lands.

Two species are found off the shores of Britain, the Short Snouted and the Spiny. It should not really have been a surprise they were found in the Thames, as they like estuaries and seagrass meadows. However, they are quite sensitive to changes in water temperature and quality, so their appearance is an indicator that the water quality in the Thames is improving.

Seahorses are a fish, in the same class as tuna or salmon. They cannot swim well and use their prehensive tail to hold onto seagrass or seaweed, so they do not get swept away. They have no teeth and suck up plankton and shrimp.

Image above: Seahorse; photograph ZSL

I also read their snouts expand to allow larger prey to be eaten, which left me with quite a comical image in my head! Another interesting fact is that the males give birth. They are believed to be the only creature on earth to do this.

After going into labour, the male gives birth to anything from a few to several thousand embryos (depending on the species) that float about in the water like plankton and require no further parental care.

The Short Snouted have been sighted in the Thames around Greenwich. Several individuals have been seen, which makes researchers believe they are living and breeding in the Thames rather than just passing by.

In 2008 both British Sea Horses were protected under the Wildlife and Countryside Act (1981 as amended). They are a ‘Feature of Conservation Importance’ for which Marine Conservation Zones can be designated.  In the Thames there is work underway to establish more seagrass meadows with the hope of conserving the Seahorses within its waters.

Image above: River Thames at Strand on the Green; photograph Joanna Raikes

Sharks out there…

There are three shark species to be found in the Thames, and I am pleased to report that there is little chance of anyone being pulled off their paddle board and used as a snack by any of these species.

One is the Tope which is critically endangered globally and appears on the IUCN Red List of Threatened Species. They can grow up to 190 cm long, live more than 50 years and mature between 10 to 17 years old. They only give birth to pups every two to three years.

They are highly migratory and move towards the poles in the summer to pup and towards the equators in winter, returning to the same spot every three years. Adult Tope feed on cephalopods and pelagic fishes, juveniles prefer to feed on benthic invertebrates (creatures that live in the sediment and have no backbone).

We then have the Starry Smoothhound, commonly referred to as ‘Hound Sharks’ and listed by the IUCN red list as Near Threatened – meaning that their population trend is decreasing.

This is a smaller species of shark growing to about 140cm. They have a splattering of small, bright white spots along their back, which give them their name “starry”. They are the only species of Smoothhound to be found in British waters and can be found at depths of up to 200 metres, feeding almost exclusively on crustaceans.

It appears they are quite promiscuous with one litter having been found to have three different fathers. They are understudied as a shark, so hopefully their presence in the Thames means we can learn a bit more about them.

Finally, we have the Spurdog, also known as the Spiny Dogfish, Cape Shark, or Piked Dogfish. They can inhabit waters up to 900m deep but are more usually found in waters shallower than 200m. They are a schooling shark with gestation periods of up to 24 months and can live for up to 40 years.

They have sharp spurs at the front of each dorsal fin that are believed to be used to fend off predators. These can inflict swelling and pain. Best admired from a distance, I think.

Image above: Glass Eel; photograph ZSL

Just a boring eel? – I don’t think so!

The fact that sharks and seahorses are living in the Thames is very exciting and makes great headlines. However, the creature that really got my attention was the European Eel. Its lifecycle is a pure marvel!

For centuries no one knew where eels spawned. It wasn’t until 1904 that a Danish man, after many years of casting fishing nets to capture young eels (in various stages of growth), found where we still believe they originate today – the Sargasso Sea.

The Sargasso Sea is a vast patch within the Atlantic Ocean named for the free-floating seaweed called sargassum and is located off the coast of America, by the Bermuda Triangle. It has no land border and is the only sea not to do so. It is believed the European eel returns here to spawn and the amazing life cycle begins.

Image above: European Eel lifecycle – credit Thames Rivers Trust

An amazing life cycle

The eggs are laid and eventually hatch into larvae called Leptocephalus – not a nasty disease, but leaf-shaped creatures that are carried toward Europe on the ocean currents.

This journey takes two years and covers over 6,000 km, the longest known migration of any eel species. Once they reach our shores the larvae again change into what are known as glass eels measuring about six inches long.

Before they leave the sea to find a home, they change yet again into elvers.  Their colour changes to brown so they are less likely to be seen and eaten by predators which are numerous ranging from fish, birds, and mammals. Elvers are feisty determined little creatures that will surf tides, cross land, and crawl over obstacles to reach that perfect patch of fresh water to call home.

Once a good home is found containing places to hide and food to eat, the elver will live and grow there, turning into what is known as a yellow eel. Males will live there for up to 12 years and females for up to 20 years and grow up to a metre long.

Again, what happens next is a mystery. No one knows what mechanism takes place to make an eel decide to return to the sea. As an eel heads back from fresh to sea water it turns into a silver eel, its stomach shrinks so it needs less food on the journey, and its pupils grow big so it can see the way through the murky waters of the deep sea.

It is believed the eels return to the Sargasso Sea to spawn but this spectacle has never been witnessed. It is also thought they die there, but again we are not 100 percent sure. How about that?


Image above: Eel pass; photograph ZSL

European Eels are critically endangered. They have been overfished and suffer from pollution and habitat loss. A recurring theme with our wildlife habitat loss is that of fragmentation. This is something I mention a lot when talking about hedgehogs having their green space accessibility reduced by concrete, building and fences.  This is also a problem for the European Eel.

Research along the Thames and its tributaries has shown that eel numbers leaving the Thames and travelling up the tributaries have reduced by 90% since the 1980’s – truly shocking!  Over 2,000 barriers to eel migration have been listed and every river off the Thames has at least one such barrier such as a weir or a lock.

In the eels’ favour, is its feisty nature. Being willing to cross ground and over obstacles means that organisations such as ZSL and Thames Rivers Trust are now building access passes over these barriers to allow eels to continue their migratory journey. It is thought that about 138.5 hectares of additional habitat have so far been created, which is great news indeed.

Image above: Freddie; photograph Mary Tester

Seals, a top predator in our midst

We can’t really talk about creatures living in the Thames without mentioning the animals we have grown to love and enjoy being able to see.  There are two types of seal that visit the Thames – the harbour and the grey.

Grey seals are the most populous, with approximately 3,500 to be found in the Thames Estuary.  Harbour seals have suffered population plummets due to disease over the years but the population in the Thames has been growing and is now at approximately 900.

Image above: Seals on the Thames; photograph ZSL

The seal numbers are surveyed each year and the trend shows numbers increasing. Seals are now spotted as far upstream as Teddington, and they also haul out and breed along the Greater Thames Estuary.

It cannot yet be said that the increase in population is down to a cleaner Thames as there is not enough data to support it, however there must be a healthy ecosystem to be able to sustain a food chain that will support a top predator such as a seal.

I am sure some people will remember Freddie the seal who lost his life last year after an attack by a dog. Shortly after this Mary Tester, a British Divers Marine Life Rescue medic, set up Thames Seal Watch with other concerned regular users of the river.

Mary has been working with MP Tracey Crouch to provide seals with stronger legal protection and Tracey’s Ten-Minute Rule Bill to protect seals from harassment will be read in parliament on Wednesday 9 February. Let us hope they are given the protection they need.

Images above: People out enjoying the river at Tidefest; photograph Anna Kunst

Recreation on the Thames

The Zoological Society of London’s report gives data on how we recreationally use the Thames. Rowing is the most popular activity on the river with over 5,800 people taking part in rowing activities in a year.

Surveys aimed at measuring use of recreational areas around the Thames showed 43% of respondents visited the paths, 31% parks, 21% beaches and 5% the nature reserves. The Thames is clearly used by a lot of us for sport, education and cognitive health.

With ongoing monitoring and work to improve the state of the Thames, including the new Thames Super Sewer, we should all be able to continue enjoying this “blue space”.  Whilst we enjoy the outdoor pleasures of having this wonderful river on our doorstep, we also need to acknowledge that cleaner water and climate change could continue to invite other creatures into the Thames. We need to show them the respect and distance they require to fulfil their lives too.

Want to know some more?

The State Of The Thames 2021 – Download the full report by the Zoological Society London here.

Thames Catchment Community Eels Project – Thames Rivers Trust website here.

The Seahorse Trust – British Seahorse survey here.

Thames Seal Watch – Sealwatch London website here.

WildChiswick – Learn a bit more about WildChiswick, what we do and why we do it here.

Read more stories on The Chiswick Calendar

See also: Equivalent of 400 Olympic-sized swimming pools of sewage spilled into Thames on each of two days

See also: Chiswick’s Tidefest organiser Martin Salter instrumental in forcing Government’s U-turn on environment bill

See all the latest stories: Chiswick Calendar News & Features

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