For the first time since records began, an otter has reached the isolated Farne Islands, off Britain's North Sea coast, one of the country's most important sanctuaries for seabirds.

The discovery this week of the species' distinctive tracks on Brownsman, an islet on the outer edge of the little archipelago, has startled naturalists and marks a new high for the recovery of Britain's once-threatened otter population.

"We almost had to rub our eyes with disbelief when we found the tracks," said David Steel, head warden for the National Trust on the Farnes, yesterday. "It is staggering that an otter could survive the perilous journey out to the islands." The otter, which is thought to be still on Brownsman, although it has not yet been seen, will have faced strong currents on a swim, which is three miles in a direct line. Although the sleek, streamlined mammals are at home in the water, the distance is one of the longest British otters are known to have travelled. Steel said: "It can be tricky to reach the islands even on a relatively calm day, and we've recently had force nine gales which makes this journey that much more special. We have no previous records of otters on the Farnes.

"We have no idea how long it might stay but the islands are very rocky and offer it a safe place to live and it should be able to find an abundance of food."

The single otter will not pose a threat to seabird colonies, although wardens are sure that it is still on Brownsman because gulls there have been unusually agitated in the last week. The annual Farnes nesting survey last week reported a rise in the numbers of the six main seabirds, including the guillemot, kittiwake and shag, although puffins have declined.

Paul Chanin, of the Mammal Society, said: "This is a really exciting discovery. We know that otters like living on the coasts where there is food available and will sometimes travel to islands. To find them so far off shore shows not just their strength but that they have a remarkable ability as navigators."

Otters suffered a serious decline in the last century because of pesticide poisoning in rivers and hunting. By the 1960s, they were seldom seen and evidence of their presence was found at only 6% of their holts, or riverside lairs.

A steady revival has raised this figure to 30%and otters have been seen in the Thames at London, and the Aire in central Leeds, although they are still classed as a vulnerable species on the red list of the International Union for the Conservation of Nature.