Otters give us hope - they show us how we can right our wrongs against nature. A quarter of a century ago the shy, handsome mammal had vanished from the great bulk of lowland Britain and clung on only in the remoter parts of the country such as Scotland, the Welsh borders and south- west England. Absence made our hearts grow stronger; through films and books masses of people came to cherish their playfulness and intelligence and the months of care they lavished on their young. Too bad that this magnificent fish-eater seemed headed for extinction in Britain just as we learned to love it.

Now we know they are returning across the country. Britain is the only place in Western Europe where otters appear to be making a strong recovery. They are still very rare across swathes of the UK, but you can be pretty sure that otters visit every river catchment in Britain. They move along rivers flowing through big towns and cities, mostly at night, and it is quite conceivable that one can be sleeping, snugly concealed, during the day while dozens of walkers pass by within a few yards. If the current rate of progress continues then within a decade or two anyone who wants to see a wild otter - and has the necessary patience - may succeed on a river near their home. The otter went into serious decline early in the last century, with the invention of the gin trap and more efficient rifles and the growth of gamekeeping. There was a short-lived recovery during the Great War. But then otter hunting with hounds became popular, and in one year hunt records show that 434 were killed for "sport" in England and Wales.

The most devastating decline of all began in the 1950s with the widespread use of long-lasting pesticides used to coat cereal seeds and in sheep dips. These poisons build up in fatty and oily tissues, and they reached high levels in eels - the oily fish which are the otters' favourite. These destructive chemicals were banned in sheep dips in 1966 and a much wider ban came in 15 years later. At last the species began to recover, pushing into abandoned areas from its strongholds in the West Country, Wales and Scotland. A gradual reduction in the river pollution caused by industry, agriculture and sewage treatment works also helped.

We know about this strong comeback from a series of surveys by Government agencies and voluntary groups over the past 20 years, some of which the Wildlife Trusts have participated in. The surveyors walk the riverbanks, looking for droppings or spraints - the more there are then, broadly speaking, the higher the population density. Spraints have a strange and not at all unpleasant smell, rather like jasmine tea. I found this hard to believe until I tried sniffing one.

The Wildlife Trusts have 13 otters and rivers projects around Britain. Between 1990 and 1995 volunteers and officers of the trusts built over 300 artificial holts - otter shelters. Most were made of piles of logs placed on the bankside, but more than 100 underground ones have been installed. The trusts have also been surveying rivers to find out what scrub and trees are there to provide cover for the creatures, and liaising with over 1,000 landowners to plant on denuded banks, fence off cattle and sheep which graze away the waterside shrubbery and restore natural features and vegetation to straight-sided, "canalised" streams and rivers. Allison Crofts, a Wildlife Trusts conservation officer specialising in otters, said: "They are recovering, but they're still not present in three quarters of the places where we think they should be. They still need our help.''

The return of the otter looks like being very good news for the water vole, which had gone into recent and rapid decline.

One reason for the plight of this little mammal - "Ratty'' in Kenneth Grahame's The Wind in the Willows - is that it is preyed on by mink, which escaped from fur farms in the 1950s and now breed in the wild. Surveys have found that where otter numbers have grown fastest, populations of the much smaller and more numerous mink have dwindled. It appears the larger water mammal frightens off or attacks and kills the mink.

Otters are still very few and far between in south-eastern England and the Midlands. The total UK population is estimated to be 7,000, with up to 1,000 living on Shetland where they feed in the sea as well as in rivers and freshwater lochs.