The foundation has voiced concerns about the beaver plans
A river foundation in southern Scotland has released a report it says shows beavers could have a "severe negative impact" on salmon and sea trout.
The Tweed Foundation's findings come ahead of plans to reintroduce the mammals in Argyll later this year.
It says claims that beavers pose no threat to migratory fish are "entirely demolished" by its study.
Organisers of the Scottish Beaver Trial say it could help to create new wetland habitats and attract other species.
Four beaver families were brought into the UK last year as part of plans to reintroduce the mammals to Scotland for the first time in 400 years.
They are due to be released in Knapdale, Argyll, on a trial basis in the spring.
The Scottish Beaver Trial is being carried out by the Scottish Wildlife Trust and the Royal Zoological Society of Scotland.
However, the Tweed Commission said it was increasingly concerned by "continual and flawed statements" that beaver dams would have no negative impact on fish.
Clerk Nick Yonge said: "Whilst beavers are zoologically very interesting animals, that interest largely stems from their ability to change the habitat in which they live, from one kind to another, by making dams.
"These dams create instream ponds and flood the land above them.
"Whilst this suits some species, it does not suit our native fish which are mostly migratory and need to move up and down our rivers."
The commission has called for "informed debate" before the Knapdale trial proceeds.
They have also sought assurances that all beavers will be removed at the end of the trial regardless of the outcome.