Science: Salmon and spice, and no sign of lice: Garlic's potent properties are helping fish farmers in the war against parasites, says Danny Penman

Garlic may turn out to be the unlikely saviour of the Scottish salmon industry by acting as an eco-friendly pesticide, say researchers at Liverpool University.

Scientists at Liverpool and in the Shetlands have discovered that salmon lice, which cost the industry more than pounds 10m a year in lost production, hate the smell of garlic.

The lice find victims through their acute sense of smell. If salmon are given food containing garlic extract - equivalent to several cloves - the powerful odour confuses the lice, which fall off their hapless victims.

The fish apparently love the nouvelle cuisine and are happy to eat it three times a day as required. The salmon smell of garlic, but a few days in clean water gets rid of the taste from the fish.

At present, sea lice are controlled by a group of pesticides known as organophosphates. Farmers in the Faeroes were the first to experiment, using onions to replace the expensive and environmentally damaging man-made chemicals employed in salmon farming.

At first, the idea of using onions was met with derision, as Chris Young of the Shetland Salmon Farmers Association explains: 'It was viewed with some hilarity at the time - but on the other hand, anything was worth a try.'

Mr Young contacted researchers at Liverpool University to investigate the farmers' claims. Dr Hamish Collin, senior reader at Liverpool University, said: 'A preliminary study found that onions were not particularly effective, but then we decided to try garlic.' This proved effective, and held promise as an organic pesticide.

From there, a grant from the university funded a larger study to provide the necessary hard evidence for a grant application to the Government's Clean Technology Unit (CTU). Dr Collin said: 'The CTU wants to reduce the amount of chemicals being added to the environment - so we went ahead and did the trials in the Shetlands and those were successful.'

Farmers have tried other environmentally friendly ways of controlling salmon lice. The most successful alternative is the salmon lice predator, wrasse, which eats the lice, but can introduce other pests and diseases.

Alastair McIntyre, professor of fisheries and oceanography at Aberdeen University, said: 'There has been a movement to employ organophosphates where possible as something more environmentally friendly (than organochlorines), but nevertheless they are in fact biocides - designed to kill.'

The two main dangers, according to Sian Pullen, marine conservation officer for the World Wide Fund for Nature (UK), is that they kill animals other than sea lice, and pose a health hazard to salmon farmers.

Ms Pullen says: 'The effect is small-scale, but the problem is you have salmon farms in virtually all of the sea lochs on the west coast. Although the effect may be fairly localised, the cumulative result of this in fish farms all the way up the west coast is quite high.'

The organophosphates do not infiltrate the food chain and affect humans. However, they can wipe out a whole layer of the food chain, with devastating local effects, Ms Pullen says.

Another alternative is being developed by scientists at Aberdeen University who are trying to produce a vaccine to protect the fish. The idea is simple in theory but complex in practice. The scientists hope to isolate proteins from the lice that are capable of provoking an effective immune response in the fish. This would allow them to make the proteins artificially and then produce a vaccine.

If the final research at Liverpool proves successful, then the door is open for similar natural compounds to replace the plethora of man-made chemicals used by farmers and aquaculturists alike. Mr Young said: 'Nobody likes to be using any chemicals at all - so an organic, environmentally sound compound would be most welcome.'

(Photograph omitted)