The River Thames' first biological "health check" in 64 years has found sharks are living within the UK's most famous waterway.
The surprising discovery means it the river has a vital role to play in the survival of the Selachimorpha species.
The Zoological Society of London's 'Greater Thames Shark Project' found tope, starry smooth hound and spurdog sharks among the 100-plus species inhabiting the ancient river.
Marine biologists believe sharks gravitate to the Greater Thames Estuary to birth and rear their young.
To date, few details have been gleaned on the numbers of sharks in the Thames but future versions of the State of the Thames Report will seek to shed light on this, as well behavioural characteristics such as how long the sharks stay there and whether their pups return in later life.
Although the big fish found in the Thames are more Shark Tale than the leviathans depicted in Hollywood horrors such as Jaws, the knowledge that they are in the water might add a frisson of jeopardy to those fond of having a dip in the Jurassic-era river.
In 2011, Britain's Got Talent judge David Walliams swam a 140-mile stretch of the River Thames in eight days. Had the aqua-loving comedian known at the time that a shiver of sharks lay beneath, one might assume he would have swum with even more urgency.
Future of fish in the Thames looks bleak
High levels of pollution in parts of the Thames led scientists in 1957 to declare stretches of it “biologically dead” but there has been a marked improvement since, the report said.
Water quality has “exhibited some promising improvements”, with reduced phosphorus concentrations – a change attributed to improved sewage treatment works to reduce harmful levels of nutrients entering the water.
But there has been a long-term increase in nitrate concentrations and the report said “the influences of climate change are clearly impacting the tidal Thames, as both water temperature and sea levels continue to rise above historic baselines”.
On average, summer temperatures in the upper tidal Thames have been increasing by 0.19°C per year and those behind the report said this would “undoubtedly affect the estuary’s wildlife, leading to changes in life-history patterns and species ranges”.
When it comes to wildlife including seals, there have been “improving short-term trends identified for natural habitats, birds and marine mammals”, although the number of fish species showed “a slight decline”, with more research needed to determine the cause, the report said.
Between 2016 and 2020, 17,770 single-use plastic bottles were counted and removed at sites along the tidal Thames, almost half of which were water bottles, the report said.
Some plastics found in the river, including cotton buds and wet wipes, came from sewage overflowing into the estuary, the report said, which not only threatens the ecosystem but “also has a detrimental impact on the perception of the Thames as being ‘dirty'”.
The presence of a juvenile short-snouted seahorse, found in 2017 at Greenwich, “indicates that the tidal Thames is a recovering estuarine ecosystem”, the report said.
Evidence has also been gathered on the importance of the river “as a breeding ground and nursery habitat for fish”, including smelt and European sea bass.
Alison Debney, conservation programme lead for wetland ecosystem recovery at the Zoological Society London, said the report has enabled the team to examine how far the Thames has come on its journey to recovery since it was declared biologically dead.
“The Thames estuary and its associated ‘blue carbon’ habitats are critically important in our fight to mitigate climate change and build a strong and resilient future for nature and people," she said.
“This report has enabled us to really look at how far the Thames has come on its journey to recovery since it was declared biologically dead and in some cases set baselines to build from in the future.”
James Brand, from the Environment Agency, said the organisation was “committed to reaching net-zero by 2030 and improving water quality in our rivers”.
“The Thames Estuary 2100 Plan advocates adapting to rising sea levels by thinking holistically about our riversides, incorporating new wildlife habitat and other environmental improvements as we work in partnership to upgrade flood defences,” he said.
While pollution levels in the Thames have shown signs of improvement, this does not appear to have translated into more fish with the number in tidal areas declining since the 1990s.
Scientists propound global warming, plastics and higher levels of nitrate as likely causes, and, with the report launched as Cop26 rumbles on Glasgow, they exhort the need to act now.