Laws giving Scottish ministers powers over English rivers have been branded "constitutionally objectionable".
The whole of the river Tweed would be under Scottish control
New legislation this year will mean the Scottish parliament will control the River Tweed and its tributaries - including those in England.
Legal control is currently shared between Holyrood and Westminster.
Berwick's Lib Dem MP Alan Beith said the move could add to concerns which already exist about Scots MPs being able to vote on English matters.
Under the new legislation, Scottish ministers will have complete control with no legal obligation to consult English politicians.
This means rivers like the Till - a tributary of the Tweed which is entirely in England - will be administered in Edinburgh.
The law will affect decisions like licensing and opening times on rivers.
Mr Beith said it made sense for one body to have complete control of the river system but raised constitutional concerns.
"There is the underlying feeling about Scottish MPs voting on English issues which you get lots of press about," he said.
He said that south of the border there were concerns about the issue.
"I do get a lot of questions about it on the doorsteps from people on this side of the border," he said.
Mr Beith said if something went wrong with the fisheries - in a matter decided solely in Scotland - it could intensify those concerns.
"I think that would make that feeling very much worse," he said.