Times have changed since stretches of the River Thames were declared “biologically dead” in the 1950s. A colony of seahorses was revealed to have made the London waterway its home this week, joining more than 100 species of fish, dolphins, seals, porpoises and the occasional whale spotted in the murky waters in recent years.

The short-snouted seahorses were first confirmed in the Thames 18 months ago, but their presence was kept secret until new laws came into force to protect them from collectors. Alison Shaw of the Zoological Society of London compared their discovery to “finding treasure”.

They have also been used as definitive proof of the astonishing revival of the Thames, from the filthy open sewer of a few decades ago to a gleaming playground for wildlife. But while the water is undeniably cleaner, there may be other reasons for the seahorse riding into the town. Neil Dunlop of the Environment Agency says: “We’re not quite sure why they’re here, but the river water has been clean enough for seahorses for some time.”

After more than a century of cholera outbreaks, inquiries and MPs being forced to hang sheets soaked in lime from the windows of the Houses of Parliament to battle the river’s stink, the battle against Thames pollution began in earnest in the late 1960s. New sewage works ended the regular dumping of raw effluent into the water — a damaging practice that encouraged bacteria to use up all of the water’s essential dissolved oxygen. Tighter regulation on riverside industry helped too, and by 1974 salmon were found in the Thames for the first time in 150 years.

Recent surveys have found bass, flounder and Dover sole, as well as the lamprey, a fat, blotchy eel that was a favoured delicacy of King Henry I. The photogenic porpoises, dolphins and friends have followed the fish.

Shaw says this is all good news, but that the Thames still suffers from severe pollution from time to time, particularly when periods of heavy rain flush the raw sewage of millions of Londoners into the river. For that reason, the Environment Agency says it is reluctant to endorse the “cleanest urban river in Europe” boast for the Thames, coined to celebrate the 1970s clean-up and still used by, among others, the mayor’s office.