The groups are asking the Court of Session in Edinburgh for a judicial review of a government decision earlier this year relating to a salmon farm.

Louise Batchelor reports: "The groups are fighting for the species' survival"

If they win they say it could lead to much stricter controls - if they lose they plan to seek new legislation from the Scottish Parliament.

Parasitic sea lice, which the groups say are being spread from fish farms to passing salmon and sea trout, are being blamed for drastic reductions in wild stocks.

The BBC's Liz Mackean: "The decline has affected all Scottish rivers"

Earlier this year, while Secretary of State for Scotland, Donald Dewar approved the extension of a salmon farm in Easter Ross, saying the potential affect of sea lice on wild fish was irrelevant to the application.

It is this principle which is being contested in the application for judicial review at the Court of Session.

Proof 'beyond doubt'

The Scottish Executive, which is now responsible for fish farming and fisheries issues, refused to comment ahead of the hearing.

But the northern director of the Scottish Environment Protection Agency, Professor Donald Mackay, is on record as saying: "The case that damage to stocks of sea trout and wild salmon by sea lice associated with caged fish farming ... should now be accepted as proof beyond reasonable doubt."

Lice burrow into the salmon's skin

Fisheries biologist Alan Kettlewhite has been monitoring the situation for the West Coast Fisheries Trust.

"Scientific opinion is that 15 lice will cause fish to die. We've been finding fish with more than 200 lice on them."

In sea water, the lice dig into a salmon's skin and feed off its blood. The only way to get rid of them is for the fish to "turn tail".

Mr Kettlewhite added: "They return to fresh water to get rid of the lice because the lice can't survive in that environment."

Decline accelerated

However, if the fish are unable to stay in the sea, they cannot feed and grow big enough to breed.

Other reasons for the decline of wild stocks have been cited as over-fishing and global warming. But the fisheries groups are convinced sea lice are the main cause.

River catches have declined

Colin Innes, an environmental lawyer and chairman of the Salmon and Sea Trout Association, said: "Over the past 10 years that decline has accelerated and that has coincided with the expansion of the fish farming industry in Scotland.

"We believe one of the main factors has been the sea lice produced by farms affecting wild fish."

Jane Wright, chairwoman of the Sea Trout Group on Loch Fyne in Argyll, said: "The four main rivers used to have an annual catch of about 1,700 fish, that includes rod and line fishing and a netting station. Now it's reduced to about 45 fish."

She warned that the total loss of wild species would be "a huge, huge disaster for this country".