Dry weather has made it harder for the fish to move upstream

The move has been approved by the utility Scottish Water and the Scottish Environment Protection Agency because of low river levels, following a request from the River Tweed Commissioners.

The commissioners, who are responsible for fisheries on the Tweed itself and its tributaries, have said they are particularly concerned because a fish counter outside Selkirk recorded less than 100 passing it in September compared with a five year average of more than 1,600.

The operation to increase water volume follows a generally dry summer and autumn.

This is a conservation measure really, we know that these salmon that will spawn now will be returning again in 2008, so we're really doing a bit of future planning Nick Yonge River Tweed Commissioners

Experts estimate that 3,000 Atlantic salmon currently are having difficulty moving up the Ettrick and Yarrow Waters because of an inadequate flow in the rivers.

The extra water will be released gradually over the next three days with the aim of helping the fish leap up a fish ladder at a dam near Selkirk.

Nick Yonge, of the River Tweed Commissioners, said the efforts were a matter of securing the river's future.

"There's an awful lot of fish in Tweed and now the urge to get up into the spawning grounds is increasing," said Mr Yonge.

"We know that they start to spawn in the last week of October and we're getting closer and closer to that date and we need to get them up to spawn.

"This is a conservation measure really, we know that these salmon that will spawn now will be returning again in 2008, so we're really doing a bit of future planning and protecting the return of salmon in five years time."