Britain's most popular wild animal is now found throughout England and on parts of every river catchment. The number of sites where its spraints - droppings - are found has quadrupled in 14 years.
Welsh and Scottish surveys show the otter is thriving there too, but it was in England that the fish-eating mammal suffered its greatest decline, which is why it appears on a list of 116 endangered animal and plant species which are having rescue plans designed for them. The main causes of the otter's decline were hunting with hounds and poisoning by pesticides. By the mid 1960s it had vanished from most of the country.
The survey, organised by the Vincent Wildlife Trust, took field biologist Rob Strachan two-and-a-half years. He walked along 1,200 miles of rivers and streams in England, visiting 3,188 sites in all checking 600 metres of bank at each. He was searching for, and sniffing, their droppings. The only easy way of distinguishing them from the foul-smelling mink spraints is to use one's nose. "It's like jasmine tea and new mown hay, a sweet smell with just a touch of fish," he said.
Droppings are the only reliable, easily detectable sign of the elusive mammal's presence and they are a good indicator of their population density, whereas ``mink spraints smell quite different - foul and pungent".
The strongly territorial otters often deposit spraints in prominent places, such as flat round stone projecting from the water, to make their presence clear. They also build little mud or sand heaps and leave droppings on top. The females are thought to employ spraints to signal their readiness to mate.
Otter surveys in England, Scotland and Wales have been run by the Vincent Wildlife Trust, a wildlife conservation charity founded 20 years ago by Vincent Weir, a businessman.
This was the trust's first English survey; two previous ones were carried out by the Government's Nature Conservancy Council in the late 1970s and the mid 1980s. For all three surveys the country was divided into 50 kilometre (31 mile) squares, with the same stretches of riverbank in half of these squares examined during each study.
In the first, otter spraints were found at 6 per cent of sites. That rose to nearly 10 per cent in the second, and 23 per cent in the latest survey. In the 1970s, no otter signs were found in 11 of the 32 large squares, but now they are present in every one.
But the report points out that in the Midlands, central-southern and south-eastern England otter numbers are still very low, running into dozens rather than hundreds. The total United Kingdom population is estimated at about 7,500, with up to 1,000 of those living on Shetland where they feed in the sea as well as in rivers.
As a top carnivore with a restricted habitat, Britain's otter population probably never amounted to more than a few tens of thousands. Its decline began in earnest in the 1830s with the invention of the gin trap and more efficient rifles and the growth of gamekeeping.
There was a short-lived otter recovery during the First World War, but then otter hunting with hounds became popular. The final, most rapid decline began in the 1950s with the widespread use of pesticides which either killed the otters or rendered them infertile.
Recovery began as soon as the persecution and poisoning ended. The survey report says the release of 80 captive-bred otters into the wild in East Anglia, southern England and North Yorkshire has played an important part in re-establishing the animal in these areas.
Decline and Recovery of the Otter in England, by Rob Strachan and Don Jefferies, available at the end of May from the Vincent Wildlife Trust, 10 Lovat Lane, London EC3R 8DT; pounds 8.Reuse content