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 5per 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