London river was declared ‘biologically dead’ in 1957; decades later it is filled with sharks and seals
In 1957, parts of London’s famous River Thames were in such bad shape that scientists declared them ‘biologically dead’. The water had been starved of oxygen and decades of sewage and industrial pollution had made it extremely difficult for aquatic life to survive. About 70 years later, the waters are now brimming with sharks, seals, seahorses, eels and many other aquatic organisms. According to a January 2026, health check of the river, published by the Zoological Society of London, its remarkable recovery is continuing.
Animals in Thames
One of the most attention-grabbing headlines about the river this year has been the arrival of sharks. At least five shark species are currently known to use the Thames Estuary, including tope and starry smoothhounds. Scientists believe the estuary may provide important nursery habitat for young sharks.ZSL’s latest figures estimate that the Greater Thames Estuary supported about 2,987 grey seals and 599 harbour seals in 2024. Grey seal numbers have risen sharply since systematic monitoring began in 2013, although the picture for harbour seals is less encouraging. Their population peaked at 796 in 2017 before falling back below its 2013 level. Individual seals can travel surprisingly far upriver too, with sightings in central London and even as far upstream as Richmond.Another success story pointed to in the report is the avocet, a migratory wading bird which had become extinct as a breeding species in Britain by 1842 due to habitat loss. It began making a comeback after World War II, and over the last three decades has seen its population among the tidal Thames more than double
From dead to live
It was not one cleanup or event that transformed the Thames and brought it back to life. Improvements to sewage treatment, tighter pollution controls and decades of conservation work gradually made the river habitable again. The change began in the 1990s, with the passing of regulations relating to the water industry and the treatment of wastewater in urban areas, which imposed a duty on sewage companies to maintain adequate collecting systems and treatment plants. In 1996, the government’s Environment Agency gained oversight of the river.The latest ZSL assessment found more dissolved oxygen in the water and major reductions in pollutants including copper and zinc. “People are often surprised to hear that marine mammals are regularly spotted in central London,” said the ZSL’s Joanna Barker, the European conservation projects manager. “As a top predator, their presence is a good sign that the Thames is getting cleaner and supporting many fish species.”London’s enormous Thames Tideway Tunnel helped further. The 15.5-mile “super sewer” became fully operational in February 2025 and is designed to prevent around 95% of the sewage spills that previously entered the tidal river from London’s overloaded Victorian sewer system. By June 2026, it had already diverted more than 20 million metric tons of sewage.However, the job is far from finished. Nitrate pollution is still increasing, some fish populations remain a concern and the river is getting warmer. ZSL found that summer water temperatures increased by an average of 0.13°C per year between 2007 and 2024.
