Up to four beaver families will be released at lochs in Argyll
Ministers are facing demands to halt new trial introductions of the European beaver in Scotland.
The Association of Salmon Fishery Boards said it would be "recklessly irresponsible" to approve new schemes before looking at the impact on fish.
It has outlined its concerns in a letter to the environment minister, Mike Russell.
The Scottish Government has given the go-ahead for up to four beaver families to be released in Argyll next year.
Under the scheme, the beavers will be captured in Norway this autumn and put into quarantine for six months.
Three or four families will then be released in spring 2009, with five loch sites being considered in Argyll.
It will be the first time in more than 400 years that there have been beaver families in Scotland.
The scheme will also be the first ever formal reintroduction of a native mammal into the wild in the UK.
The Association of Salmon Fishery Boards wrote to the environment minister in March opposing the release scheme.
In a new letter, chairman Hugh Campbell Adamson said the original points raised had not been "addressed or satisfied".
He also cited research which said that beavers damming rivers could have a negative effect on migrating salmon.
He said: "It is vital to assess impacts on fish, to ensure that a modelling exercise is undertaken to identify the typical areas in Scotland where beaver-built dams are predicted.
"This must be undertaken before any consideration is given to expanding the concept of the existing trial to other areas."
Environment Minister Mike Russell said: "The European beaver will return to Knapdale in Argyll next spring on a trial basis, which is an exciting development for wildlife enthusiasts all over Scotland and beyond.
"The Association of Salmon Fisheries Boards are among a number of interested stakeholders who the project may impact upon, and I will of course be happy to discuss these issues with the ASFB and to ensure they are involved as the trial goes ahead.
"The impact of the beavers on Knapdale, as well as how the animals have adapted, will be closely scrutinised before any further reintroduction takes place."