Allan Bantick, Scottish Beaver Trial: "This is a landmark day"
A total of 11 beavers have been released into the wild in Argyll as part of a reintroduction programme.
Four more may join the Scottish Beaver Trial being run in Knapdale Forest.
The beavers have been brought to Scotland from Norway and their release marks a return to the UK after a 400-year absence.
The release will be studied to determine whether the trial should be extended and beavers reintroduced across Scotland.
The Scottish Beaver Trial (SBT) is being carried out by the Scottish Wildlife Trust and the Royal Zoological Society of Scotland.
Project manager Simon Jones said the release of the beaver families on Friday "went extremely well".
"They were placed into purpose-built artificial lodges at carefully selected points around the trial site," he said.
"They will now gradually gnaw their way out of the lodge at a pace that is comfortable for them before exploring their new surroundings."
Mr Jones said that following the release, the "real work" of the trial could now begin.
He added: "First and foremost, this is a scientific study of how the beavers cope naturally in the Scottish environment and what effect they have upon it.
"We will be closely tracking the beavers' activities and collecting data over the next five years to help inform the independent scientific monitoring.
"This will help the Scottish Government in making any final decisions on the future of beavers in Knapdale Forest or elsewhere in Scotland."
Not everyone, however, is in favour of the reintroduction scheme.
Last year, the Association of Salmon Fishery Boards said it would be "recklessly irresponsible" to approve new schemes before looking at the impact on fish.
Concerns were also raised by Alan Kettlewhite, a biologist with Argyll Fisheries Trust, ahead of Friday's beaver release.
These charismatic creatures are not only likely to create interest in Scotland from further afield but crucially can play a key role in providing good habitat for a wide range of wetland species
Roseanna Cunningham Environment Minister
"Potentially they can alter the habitats of fish, restricting access to spawning grounds," he said.
"I think the concerns are based on studies in other countries where sometimes dam-building can prevent fish access to their spawning grounds, particularly in dry years where you don't get much rain in the autumn time."
But Allan Bantick, chair of the Scottish Beaver Trial, believes the programme is a step forward in "rebuilding the natural biodiversity of Scotland".
"Our critics worry that beavers might pose a risk to migratory fish numbers, including salmon," he said.
"This has not been found to be the case anywhere else in Europe.
"However, the notion cannot be tested with this trial because there is no Atlantic salmon present in the trial site.
"Our beavers will be released within a designated trial area, which should be large enough to sustain the natural expansion of their population over the next five years."
Scotland's Environment Minister Roseanna Cunningham visited the trial site in Argyll on Friday morning.
She said the release marked "a historic day for conservation".
"These charismatic creatures are not only likely to create interest in Scotland from further afield but crucially can play a key role in providing good habitat for a wide range of wetland species," she said.
The beavers are tagged to help with the monitoring of their progress
"And while a great deal of research has already gone into the reintroduction, this work is far from over.
"Observations and data collection over the next five years will play a crucial role in assessing the long-term future for beavers in the Scottish landscape."
Darren Dobson, from the Carinbaan Hotel near the release site, said he was delighted at the prospect of beavers, and hopes they will prove to be a major tourist attraction.
He said: "Generally speaking it's all positive. I haven't met anyone myself who is negative to the idea.
"It's going to bring more tourists - and this is just one more thing to add to what this area's got."
Scottish Natural Heritage (SNH) will monitor the relationship between beavers and woodland, water plants, river habitat, water levels, otters, dragonflies, damselflies and freshwater fish.
The beavers themselves will also be under close scrutiny, using tracking data.
SNH will co-ordinate the scientific monitoring work with a range of independent bodies, including Oxford University Wildlife Conservation Research Unit and the Argyll Fisheries Trust.
It is contributing £275,000 to the cost of monitoring the trial.
It is claimed the trial will be a major contribution to Scotland's Species Action Framework, which identifies 32 species, including European beaver, as the focus of new management action.
The beavers released on Friday were captured in the Telemark region of Norway in September last year.
They were flown to the UK in November and spent six months in quarantine.
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