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Fish farmers have a new approach to tackling a parasite. Sea lice affect farmed and wild salmon. So can "cleaner fish" be used to guard the salmon? 50,000 have been reared to do just that.

The growth of salmon farming on Scotland's west coast has been as dramatic and fast as the fish itself.

The rich flesh of salmon - "the king of fish" - used to be enjoyed by only a select few. But that has changed.

In just 45 years, the amount of farmed salmon reared in Scottish waters alone has risen from zero to around 150,000 tonnes.

Scotland is now a world-leader in the industry. The total value of Scottish farmed salmon is now around £500m, annually. And with demand growing in emerging markets such as China, the future of fish-farming looks bright.

But this extraordinary growth has given rise to environmental challenges around the Scottish coast, and in the waters off Chile, Ireland and Norway.

Salmon are reared in cages in sea lochs. Thousands of fish grow and fatten in these inshore waters. No surprise, perhaps that they can fall prey to parasites - among them, the sea louse, a marine cousin of the wood louse.

Sea lice can latch on to salmon, eat their skin and blood, and cause infections. Anglers and conservation groups highlight the deadly menace these sea lice can pose to wild salmon stocks as they move to and from their spawning grounds.

Sea lice are tough, and they've been resistant to many of the new treatments devised to defeat them. Scientists around the world have developed environmentally-friendly disinfectants, in-feed treatments, growth regulators, bath treatments, and research is well underway on possible vaccines. But none, so far, has been perfect.

On the shores of Loch Fyne, a natural approach is being pioneered. The long sheds of Otter Ferry Seafish contain dozens of large green fish tanks, each bathed in electric light.

The scene is rather like a pet shop aquarium, but on an enormous scale. In each tank, there's a shoal of fish - some so small, they're almost transparent. The two year olds are golden-red and about the size of a goldfish.

They're called ballan wrasse, and they're being reared to protect the salmon. Ballan wrasse love eating sea lice. That useful behaviour is being harnessed to create a natural defence. Mix salmon and wrasse, the thinking goes, and the sea lice will lose.

The director of Otter Ferry Seafish, Alastair Barge, has been rearing ballan wrasse on a commercial scale for four years now. It is a complex process - the water temperature, artificial weed, feed and light levels all have to be right. But he has worked it out, and he now has 50,000 of the fish maturing.

He said: "It is quite an old idea. At the beginning of salmon farming, people seemed to realise that wrasse were a potential cleaner fish. But they are quite tricky to rear, so that's why the idea went on the back-burner at that stage. Studies have found they will pick sea lice off farmed salmon, without harming the salmon.

"We raise them on a special fish food. Initially we were concerned that they might not adapt to a sea lice diet, but we found that when they have a choice, they prefer the sea lice every time. When wrasse are mixed with salmon, the stocking level is 2% - so one wrasse can protect fifty salmon from sea lice.

"We've been very fortunate, since we're a small company and couldn't launch out on this research ourselves, so we've been supported by the salmon industry, as well as the Crown Estate and Highlands and Islands Enterprise, so all the stakeholders have helped."

Ballan wrasse are now being tested in fish farms by the industry's biggest salmon producers. The Scottish Salmon Company (SSC) has introduced wrasse at their site at Stockinish off Harris in the Outer Hebrides.

Next year, they expect to deploy 5,000 wrasse - that's enough to effectively service one fish farm. After that, their plan is to service tens of sites over the next few years.

The introduction of wrasse is just one of many changes in salmon farming.

At SSC's site at Ardyne, just south of Dunoon on the Clyde coast, the Chief Executive Stewart McLelland points out the sophisticated feeding systems, controlled from shore, so that even in the worst winter storms, the fish don't miss a meal.

"Forty years ago you would be standing at the side of a steel or wooden cage, hand-feeding the salmon. Here, you've got recyclable plastic materials, automated feed systems - computer controlled so there's no feed wastage.

"That helps with the environment. And wrasse are part of the science that will hopefully provide a good biological solution to a natural parasite. And it will benefit farmed salmon and potentially the wild salmon as well."

So amid all the technology, it seems there's a place for a natural treatment for sea lice. Within the next few years, farmed salmon will live alongside wrasse, and both should benefit from the arrangement.