CHEMICALS used to shake parasitic lice from farmed salmon are affecting Scotland's waters, the country's environmental watchdog has found.

The body has now unveiled proposals for a tougher regime that "will strengthen the regulation of the sector".

The changes could see some facilities forced to relocate to deeper waters.

Terry A'Hearn, chief executive of the Stirling-based watchdog, said: "Scottish salmon farm medicine is significantly impacting local marine environments, which increases the now substantial weight of scientific evidence that the existing approaches do not adequately protect marine life."

Almost 190,000 tonnes of Atlantic salmon were produced in Scotland last year as the sector reached a record high.

Producers hope to increase this further as part of ambitious aquaculture growth plans drawn up by industry leaders keen to catch up with top-producing nations Norway and Chile.

More than 200 facilities are already in operation, with most sited along the west coast and others around Orkney and Shetland.

But a Holyrood committee said expansion plans could "cause irrecoverable damage to the environment" without major improvements and campaigners have petitioned the Scottish Government for change on the grounds of animal welfare and environmental concerns.

In recent weeks footage emerged of farmed fish swimming in cages despite substantial damage caused by sea lice, which eat through skin and can kill their hosts.

The parasites are a major cause of stock losses for the sector.

And images have also surfaced of massive pits filled with dead fish on the sands of North Uist.

Salmon & Trout Conservation Scotland (S&TCS), which counts Prince Charles as patron, is amongst organisations to blame commercial fish farming for a drop in wild populations.

The survival rate of salmon during their marine phase has fallen from around 25% to 5% over the last 40 years.

Earlier this year the Scottish Government committed around £700,000 to "help address the range of pressures related to the decline of Scottish wild salmon stocks".

Sepa says its deep dive into the impact of aquaculture is one of the largest and most comprehensive ever undertaken.

Hundreds of samples taken from around eight fish farms were analysed.

Tests found sea lice medicines Emamectin Benzoate (EmBz) and Teflubenzuron (Tef), last used in 2013, in 98% and 46% of samples respectively.

The agency says residues were "more widely spread in the environment around fish farms than had previously been found", adding: "The research concluded that the impacts of individual farms may not be contained to the vicinity of individual farms."

A seven-week consultation on changes – which include controls on EmBz and new siting rules that could lead to fewer fish farms in shallower, slow-flowing waters and more farms in deeper and faster-flowing waters – will now take place.

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A'Hearn said: "Sepa is clear that our job is to make sure environmental standards protect the marine environment for the people of Scotland and we make sure the industry meets those. That's unequivocally our focus.

"Consequently across the last 16 months we've done more science, more analysis and more listening than ever before. Whilst we're seeing innovation in the sector, we've concluded that Scottish salmon farm medicine is significantly impacting local marine environments which increases the now substantial weight of scientific evidence that the existing approaches do not adequately protect marine life."

Echoing the words of a Scottish Parliamentary committee, he went on: "We agree that 'the status quo is not an option', which is why we're announcing firm, evidence-based proposals for a revised regime that will strengthen the regulation of the sector."

Julie Hesketh-Laird, chief executive of the Scottish Salmon Producers Organisation, said: "We share Sepa's vision of an innovative, sustainable salmon industry underpinned by clear and accurate regulation. This report will remove many of the barriers preventing the development of more modern facilities further from the shore and we look forward to Sepa's support as the industry makes this change.

"The discovery of residues is important information but it should be remembered that salmon farmers were operating to Sepa guidelines throughout the past five years."

However, Scottish Greens environment spokesman Mark Ruskell MSP said: "This is welcome research that further demonstrates the profound impact industrial fish farming is having on Scotland's marine environment.

"However, the proposals to tighten regulations for fish farms simply don't go far enough and may result in even larger farms.

"Further expansion of this industry whilst such serious concerns remain is unjustified and a moratorium on new farms is urgently needed."