Back in May, I spent two highly enjoyable but ultimately fruitless days trying to spot an otter with Kevin O'Hara, who works for the Northumberland Wildlife Trust and, having spent most of his career with them, knows more about the nation's favourite wild mammal than almost anyone else in the country. Sadly, we failed to spot a single one, although I learned a lot about them and we did see reassuring quantities of otter poo. Or spraints, as they are technically known.

The reason for the story was that Britain's fifth national otter survey was due out later in the summer. Having been driven – by pollution and pesticides, mostly – almost to extinction in England, the otter has made an extraordinary comeback in the last decade or so. At the time of the first national survey, in 1977-79, just 5.8% of 3,327 riverbank and wetland sites examined showed any evidence of otters at all; experts reckoned there may have been only dozens left. The fifth survey was expected to confirm their resurgence.

It did. The survey found that now 58% of those same 3,327 sites showed recent signs of otter activity – a tenfold increase in just 30 years, and a remarkable 22% surge since the last survey in 2002. "Kent and East Sussex are now the only counties in England without significant otter populations," says the Environment Agency's Andrew Crawford, the survey's author. "There are very few south of a line running from Bognor, north to Hemel Hempstead then across to Chelmsford. But their numbers are spreading towards the south-east and it should only be a matter of time – a decade or so, maybe – before they reach Kent."

Besides making it an offence to intentionally kill otters, two key factors have allowed the species to re-establish itself so successfully. The first is the banning of organochlorine pesticides such as dieldrin and aldrin, which caused blindness, immune-system collapse and mass-breeding failure. The second important change has been the vast improvement in the quality of the water in Britain's waterways. Some £30bn has been spent on improving sewage treatment since 1990, and according to the Environment Agency, the waterways are now in a better state than at any time since the start of the Industrial Revolution.

The Agency sees few new threats on the horizon, Crawford says, and otter-related efforts around the country are now focused mainly on habitat improvement (planting trees, fencing off stretches of riverbank, digging out ponds) and on building neat little otter-ledges under bridges so the 150-200 killed on roads every year won't have to cross them in the first place.

"Otters don't like high flows under a bridge, so they run across the roads instead," he says. "It's one of the main dangers they face." That, and the irate anglers and fishery owners who, with otters newly resurgent, have started complaining about the impact on their prize carp.