In the Seventies, they were on the verge of extinction. But they're back.
I looked out of the kitchen window one morning and saw a large animal eating one of my fish. I thought it was an otter, but although I've lived in Wales all my life, I've never seen one and wanted to be sure."

Farmer Emyr Breeze says he was more surprised by the animal's audacity than anything, but one of his neighbours suffered far worse. "Otters raided our pond that was full of koi carp which we were growing for sale," she explains.

In both cases, the farmers called in Geoff Lyles, who runs Otters in Wales - a conservation project funded by Welsh Water, the National Rivers Authority and the Countryside Council for Wales. He confirmed otters were responsible, but in spite of their losses both farmers were delighted. "Geoff told me I'd lose all my carp to the otter, then he'd clear out the eels and then start on the trout," says Mr Breeze. "In fact the otter just took the one 16in carp and didn't come back. To be honest, I'm disappointed I haven't seen him since."

Mr Lyles is one of the most important figures in the current resurgence of this water-loving weasel across not only Wales but England, too. His role is to gather data on the animals, as well as act as an otter public relations officer, talking to farmers and owners of fishing rights about potential damage. The job is an important one. Remarkably little is known about the elusive otter, which 20 years ago was on the verge of extinction. Today they enjoy total protection and are on the increase although no one really knows how many there are.

Mr Lyles and fellow scientists monitor otter movements from tracks and droppings: "If there are otters present, there will be droppings - or spraints - every 50 to 100 metres," he explains. These tarry black dabs of excrement, filled with fish scales and bones, are instantly recognisable by the smell - a sweet musty odour. "If there are no spraints in a few hundred yard of bank, you can be pretty sure there are no otters. Also, look for a terrier-sized footprint with five-toes (dogs and foxes have four)."

Otters are easy to confuse with their close relative, the invading American mink. "An otter is much bigger and definitely brown, not black," says Mr Lyles. "And an otter's footprints are irregular, while a mink's are smaller and the toes spread out evenly, like a star."

In the water, otters are more aquatic than mink, capable of holding their breath for three minutes and travelling 450 yards underwater. When hunting, otters typically stay in the water, making short 30-second dives before reappearing. Mink frequently leave the water to dive in from a rock or the bank.

Although the mink are now far more common than their cousins (the Joint Nature Conservation Committee puts the population at 110,000 compared with 7,350 otters), the picture is slowly improving: "When I arrived in Wales in 1980, they were doing badly, restricted to the upland areas of the main rivers: the Usk, Wye and Severn," says Mr Lyles. The problems were due largely to dieldrin, an agricultural pesticide used extensively in the Fifties and Sixties. When this worked its way into the water system, it was particularly bad news for otters, which rely heavily on eels for food. These are very fatty - and dieldrin is fat soluble: "Otters were eating contaminated eels that had already concentrated the poison," he explains.

By the mid-Seventies otters were in serious danger across the whole of Britain - and all but extinct in England (even today there are only six rivers with otters in the South East). In Wales they were reduced to the least polluted upland tributaries.

Even so, our population is far healthier than the Continent's. Once widespread across Europe, the animals are now extinct in seven countries and reduced to a couple of strongholds in southern France and Portugal.

So while Graham Roberts, Otter Project Officer in South East Britain, says there are probably only about 20 animals in the whole of his region, elsewhere the picture is much better. There are healthy populations in Ireland, Scotland, the West Country and, of course, Wales. Indeed, now the use of dieldrin has ended, they are making a come-back.

"There are now otters on just about every major river in Wales, although distribution is very patchy," says Mr Lyles. The healthiest numbers are to be found on the upper parts of the Wye.

"Wales is vital to recovery," explains Mary-Rose Lane, rivers and wetlands officer for the Devon Wildlife Trust and responsible for a rare English stronghold. "It is a good unpolluted base for otters to expand into the Midlands," she says.

Naturally, helping otters is not restricted to data collection. The animals can be actively encouraged by providing undisturbed undergrowth and shelters - bitches need at least an acre of thick cover to rear cubs. Riverside walkers - and particularly their dogs - are a problem.

In spite of this, otters have begun to spread eastwards, recolonising areas of England - the Midlands in particular. Last year, after an interval of 40 years, animals were seen in Oxfordshire, after travelling from the Severn over the Cotswolds into tributaries of the Thames.

So might Welsh otters be seen in London? "It's not impossible, but there would have to be a major revolution in water quality, habitat, disturbance and food supply first," says Ms Lane.