FOR 18 months it has been a closely guarded secret. Seahorses, the exotic creatures from tropical waters, are alive and well in the Thames estuary.

The fish once thought to pull Neptune’s chariot in some enchanted realm can do the same for Old Father Thames now that the river’s waters are so much cleaner.

The Zoological Society of London discovered the colony in 2006 but decided to keep quiet until legislation could be put in place to protect the seahorses. They live in the river’s estuary, between Essex and Kent.

Scientists have greeted their arrival as an indicator of the increasing purity of the river’s water. Fifty years ago they declared the river biologically dead - killed by the pollution that was the product of industrialisation and urban growth.

Environmentalists, however, will point out that the arrival of the seahorse is also a sign of ecological changes linked to global warning.

Alison Shaw, the Zoological Society of London’s marine and freshwater conservation manager, said: “These amazing creatures have been found in the Thames over the past 18 months during our wildlife monitoring work. It demonstrates that the Thames is becoming a sustainable habitat for a diversity of aquatic life.”

Legislation is coming into force to protect the 6in-long creatures.

The short-snouted seahorses in the Thames are commonly found around Africa and the Mediterranean and only occasionally near the southern coasts of Britain. Their usual habitat is shallow coastal waters rich in weeds and plant life, although they can be found as deep as 100ft.

There are an estimated 30 species of seahorse worldwide but only two are found around Britain. Both the short-snouted and long-snouted seahorse can be found off the south coast; only occasionally have isolated individuals been identified as far north as the Thames estuary.

“They’re rarely found in Great Britain at all,” said Richard Harrington from the Marine Conservation Society. “If they are, then it is usually in the Channel Islands or near Dorset.”

Last year, however, juvenile seahorses of both the short-snouted and long-snouted species were found in the marina at Brighton, East Sussex. This was the first evidence to suggest the fish were actually breeding in British waters.

The Thames discovery illustrates the partial success of attempts to reduce pollution in a river once considered one of the dirtiest in Europe.

The clean-up has already resulted in the return of salmon, which were extinct in the Thames from 1833 until 1974 but are now breeding in Berkshire. Their regeneration has been aided by releases of juvenile fish.

In 2006 a dead otter was found not far from Tower Bridge. Other sightings have confirmed that the mammal species could be staging a comeback.

Conservationists hope wildlife numbers will continue increasing but the Thames still suffers from periodic surges of pollution, especially after rainstorms, which tend to flush sewage from drains into the river.

By 2019 the government plans to have built a 20-mile, £2 billion tunnel to collect such sewage and transport it to a treatment plant in east London.

The lower reaches of the river are often visited by porpoises and dolphins; in 2006 a northern bottlenose whale was watched by crowds of people as it swam upriver as far as Chelsea. It died while being rescued.

Some experts suspect that the decrease in pollutants is only one of the factors behind the appearance of se horses in the Thames. “There has also been a small but definite rise in the temperature of the North Sea and English Channel,” Harrington said, “and this may well be the cause of all the new and exciting species we are seeing.”

The warmer water has also attracted marine species such as the brightly coloured triggerfish to the Channel.

Seahorses are among the few creatures that keep the same mate for life. Couples perform elaborate courtship rituals every morning, which can last up to an hour.

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