Similar surveys under way in England and Scotland suggest they are recovering there too. Welsh otters are spreading to western England, moving back to rivers abandoned years ago.
Three surveyors from the Vincent Wildlife Trust spent seven months in 1991 trudging along 400 miles of Welsh river bank. They visited more than 1,100 sites which had been surveyed in 1977 and 1984. Their findings were published yesterday.
The two most obvious otter signs are their droppings or spraints - black streaks sometimes deposited on a little mound of sand built by the animal - and their paw prints. The trust found markings at 53 per cent of sites, compared to 38 per cent in 1984 and 20 per cent in 1977.
Dr Elizabeth Andrews, a zoologist and one of the surveyors, said it was impossible to estimate the otters' population or how fast their numbers were growing. They are secretive, nocturnal mammals which can travel over 20 miles in one night. She only saw one during the entire survey.
'It's a bit like hunting for a needle in a haystack - you need to know what to look for,' she said. 'They are particularly fond of leaving droppings under bridges, which makes it all the harder for the surveyors.'
More evidence of recovery comes from the growing number of sightings reported by the public. But the increase in otter traces has not been universal.
There was no sign of otters on Anglesey. The number of sites with otter markings remained the same in the upper Severn and its tributaries, and it had fallen in Snowdonia from the 1984 level. But the increase was indisputable across most of the Principality.
Dr Andrews believes , acid rain and extensive conifer plantations could be one reason why otter numbers are not recovering along the upper Severn. Much of the lakes, streams and rivers of upland Wales is known to have been acidified by air pollution. Conifer trees exacerbate the damage by trapping the acid moisture on their needles, and the acidified rivers might not harbour sufficient fish for otters.
In Snowdonia, extensive clearing of bankside shrubs and trees could have left otters without well-sheltered sleeping places, leaving the land drained and providing less water for the rivers.
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