Revived river boasts seals, sea horses and one piranha

In 1957, scientists declared the river Thames biologically dead, killed by the pollution that was a consequence of industrialisation and urban growth. Animal life, they said, could no longer survive in any measurable form in waters so bereft of oxygen.

But even as the Thames reached its nadir, things began to change. The industries of the South-east and the docks themselves declined during the 60s and 70s, reducing the amount of river traffic and pollution. At the same time, better methods of sewage treatment were introduced and power stations cleaned up their act. Public awareness increased. And the water began to recover.

Despite its muddy appearance, due to the silt stirred up by the tides, the Thames is now a much purer river. It is estimated that 122 species of fish can now be found in the river, which has become an important nursery for both bass and sole. Several species of seal, together with dolphins and porpoises, have been seen and populations have been found of oddities such as sea horses, American crayfish and Chinese crabs.

Esther Carmen, regional fisheries officer for the Environment Agency, said: "The importance of the Thames as a home for fish has grown with water quality. And it will get even better as the quality of water improves." The agency monitors water quality constantly, ready to pump in fresh oxygen if levels decrease.

SEALS

A survey by the Zoological Society of London (ZSL) showed that, in the 12 months to last July, there were 41 sightings of common seals, a total of 41 individuals, and 19 sightings of grey seals, totalling 30 separate animals. As well as being regularly spotted in the estuary areas, seals have been seen around Canary Wharf, Millwall Docks and by the Thames Barrier. In the past, they have been seen as far upstream as Richmond.

DOLPHINS AND PORPOISES

Whales are not the only cetaceans to be found in the Thames. The ZSL survey recorded 19 sightings of harbour porpoises, totalling 62 individuals. Although many were in the outer estuary, a group of three was recorded in December 2004 near Vauxhall Bridge. About 18 bottle-nosed dolphins were seen in the estuary during the same 12 month period.

FISH

According to the Environment Agency, 122 species of fish have been recorded, with dace, eel, smelt and flounder being numerous. But, more crucially, the entire Thames, from Teddington eastwards, has been designated as an important nursery for both sole and bass, the young fish migrating there from their breeding grounds.

Fish used to warmer waters, such as anchovy and red mullet, rarely found farther north than Cornwall, are now also caught. Attempts to reintroduce breeding salmon have so far failed.

CURIOUS CREATURES

Although sea horses are not unknown in British waters, the discovery of a short-snouted sea horse, by a fisherman off Leigh-on-Sea in June 2004, was hailed as confirmation the Thames was truly a clean waterway. Notoriously sensitive to dirty water, it was the first found since 1976. Cleaner water, however, is not believed to lie behind the discovery of a single piranha fish, which appeared to land out of the sky on an Environment Agency boat in February 2004. The fish, shoals of which can kill a man in seconds, is not thought to have made the journey alone from its normal habitat of the Amazon basin, but was illegally abandoned and picked up by a seagull.

OTHER SPECIES

The improved water quality has also allowed invasive species to thrive. Chinese mitten crabs are first believed to have escaped from the holds of docked ships in the 1930s, but grew rapidly in numbers during the 1990s, even causing riverbank erosion in some places. American crayfish, believed to have been dumped by a restaurant in the 1960s, are now abundant and are threatening native crayfish.