The biggest problem facing the country's salmon farming industry, which is estimated to be worth more than pounds 100m, is a parasite smaller than a full stop. Normally free-swimming salmon are little troubled by the lice, which drop off anyway when the fish returns to fresh water.
But in enclosed sea cages, salmon are an easy victim. The lice clamp on and grow fat by eating away at their hosts. They cost the industry millions of pounds because a lice-chewed salmon is as saleable as a pit-bull terrier with Aids, and about as attractive. Heavy concentrations can even kill fish.
All sorts of remedies are being tried, from bags of onions to illegal pesticides. Traces of one of the chemicals, a nerve poison, have been found in supermarket salmon, and only this week, government scientists confirmed that Ivermectin, which can cause acute irritation to the eyes and which kills lice by paralysing their nerves, had been seized from a Scottish farm. But the most effective solution so far has been a fish that eats lice like sweeties.
There are several species of wrasse found on our coastline. They are perhaps the most spectacular fish in our waters, a dazzling mixture of blue, red, orange yellow, purple and gold. Until now they have been viewed as the peacocks of the sea, beautiful but useless because they are generally viewed as inedible.
The most astonishing thing about them is that they are generally born as males but, as they get older, they will often become female. Wrasse are one of the few creatures in the world with this capability. Dr Alwyne Wheeler, the country's foremost fish authority, says: 'When there are a lot of males around, or when the females die off, many species of wrasse can change sex to ensure a guaranteed supply of females. We don't know how they know, or how they do it.'
But it is their eating habits, not their strange sexual behaviour, that makes them attractive to salmon farmers. Wrasse are the Florence Nightingales of the seabed. Using formidable teeth that are strong enough to crush crab shells or barnacles, wrasse relish the parasites and bacteria that infest some fishes.
They even have their own 'hospitals' - a feature such as a large rock where other fish will come for a check-up. Even large bass or conger eel that would normally gulp down anything smaller will not harm their doctor, and allow wrasse inside their mouths to clean their teeth.
Some bright spark realised that if wrasse could do this in the wild, they might be able to do the same in captivity. Norwegian trials were remarkably successful, and similar experiments in Shetland in 1989 confirmed that wrasse would work as cleaner fish on infected salmon.
Although wrasse seem the ideal solution because they cause no harm to the environment and their effects are confined to the lice, there are still problems. For a start, wrasse have to grow up with the salmon. Put them in a cage with adult salmon and they usually get eaten.
With their technicolour dreamcoat colours, wrasse are easy prey for cormorants and seals. Captive wrasse need 'kennels' where they can shelter at night. Normally they burrow in the sand or hide in rock crevices. And if they do their job too well, they may start nibbling at the salmon if there are insufficient lice.
But these are annoyances rather than problems. Generally the salmon farming industry is convinced wrasse will save the day. Because of this, it has been paying up to pounds 2 for live fish only five inches long. In recession-hit Scotland, this has been a valuable extra source of income for many fishermen.
However, there is a serious worry that this demand for wrasse will have much wider problems. In an experiment where all specimens of a tropical species, the cleaner wrasse (labroides dimidiatus), were removed from an Indian Ocean reef, all the other fish disappeared too.
Wheeler says: 'Wrasse are very important to the ecology of a reef. In general terms I don't think this exploitation of wild fish is a good thing. Because wrasse are very long-living, we may not notice the effects of their removal for many years.'
To be fair to the fish farming industry, it is funding a wrasse- breeding programme at the Seafish Industry Authority's marine farming unit at Ardtoe, Argyll, while a commercial company has set up a similar venture at Hunterston, north Ayrshire.
But meanwhile, wrasse fishers are still plundering the Scottish coastline, selling the little fish into slavery and a diet of lice for breakfast, lunch and supper. It is not known if they still change sex while in captivity, but it is unlikely. Imagine the poor little tyke telling his macho salmon mates that things have changed, and you now want to be called Tracey.Reuse content