It has taken 30 years, but the otter's comeback is now complete. After becoming extinct across most of England in the Fifties and Sixties, one of Britain's best-loved animals has now returned to every English county, the Environment Agency announced yesterday.

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The slow but steady recolonisation of its former haunts has been rounded off with the reappearance of otters in Kent, the last county to have been without them, the agency said.

The otter's return represents a happy ending to one of the worst episodes in modern British wildlife history: the sudden disappearance of one of our most widespread and charismatic mammals.

The process began around 1956 and was almost certainly caused by the introduction of powerful organochlorine pesticides such as aldrin and dieldrin. Residues of these chemicals were washed into the rivers where otters lived, poisoning them.

As wild otters are hard to spot – their presence is usually detected by their spraints, or droppings – it was several years before the scale of their disappearance began to dawn on people, but by then they had been wiped out over vast areas of lowland England.

Despite the banning of organochlorine pesticides in the mid-Sixties, otters continued to decline, and their population reached a low point by the end of the 1970s, when they had effectively vanished from everywhere except the West Country and parts of Northern England (although good numbers remained in Wales and Scotland).

The first national otter survey, carried out between 1977 and 1979, detected the presence of otters in just over 5 per cent of the 2,940 sites surveyed; all the sites were known to have held the animals previously.

But then a comeback gradually began. Helped by a substantial clean-up of England's rivers, which brought back fish to many once-polluted watercourses, and by legal protection, otters began to spread back eastwards into England from their strongholds in Devon and in areas of the Welsh borders, such as the Wye Valley.

By the time of the fourth otter survey, carried out between 2000 and 2002, more than 36 per cent of the sites examined showed otter traces; and when the fifth survey was carried out, between 2009 and 2010, the figure had risen to nearly 60 per cent, with otters back in every English county except Kent. Now wildlife experts at the Environment Agency have confirmed that there are at least two otters in Kent, which have built their holts on the River Medway and the River Eden.

"The recovery of otters from near-extinction shows how far we've come in controlling pollution and improving water quality," said Alastair Driver, the Environment Agency's National Conservation Manager. "Rivers in England are the healthiest for over 20 years, and otters, salmon and other wildlife are returning to many rivers for the first time since the industrial revolution.

"The fact that otters are now returning to Kent is the final piece in the jigsaw for otter recovery in England and is a symbol of great success for everybody involved in otter conservation."

Otters are at the top of the food chain, and are therefore an important indicator of river health. The clean-up means that they are now inhabiting once-polluted rivers running through cities – something which would have been unthinkable before the population crash – and they have been detected in places such as Stoke-on-Trent, Reading, Exeter and Leeds, as well as in more likely urban centres, such as Winchester.