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A SMALL orange fish may soon take on the job of the cleaner at salmon
farms around the coast of Britain. The fish will not only help to keep salmon
healthy but it might also help to clean up the industry’s image as a polluter
of coastal waters.

The gold sinny wrasse, Ctenolabrus rupestris, is one of several species
of wrasse that offer an alternative to pesticides as a means of removing
sea lice from salmon in fish farms. Sea lice, a type of crustacean parasite,
are a serious problem for salmon farmers. Fish kept at close quarters in
sea cages are prone to the parasite, which can cause extensive damage to
the skin. Sometimes the parasites penetrate the brain and kill the fish.

Fish farmers control the parasite by dosing the cages with an organophosphorous
pesticide, dichlorvos, which is sold under the name Nuvan. Regular treatments,
every three to four weeks, are necessary because the parasites reproduce
rapidly.

But Nuvan does not stay in sea cages for long, and it is a serious threat
to other marine organisms in the vicinity. Results of a recent study suggest
that it is the cause of the sudden upsurge in the incidence of cataracts
among wild fish (New Å®ÉúСÊÓƵ, Science, 26 August). The industry is now
looking for alternative ways to control sea lice.

This month, trials began with the gold sinny wrasse, a fish that is
about 12 centimetres long. The wrasse will pick off external parasites from
other fish. These fish, which generally live inshore and feed in kelp beds,
are not specifically ‘cleaner fish’. They would not normally come into contact
with salmon, which spend most of their lives in the open sea.

However, says Peter Smith of the Sea Fish Industry Authority’s Marine
Farming Unit at Ardtoe in Argyll, ‘they are very inquisitive and will eat
anything, even if it happens to be attached to another fish.’ A gold sinny
wrasse is as likely to remove parasites from a salmon as from other fish
it more normally associates with, if the two are together in a cage.

Last year, in Norway, Asmund Bjordal tried out several species of wrasse.
After some small-scale trials in tanks, he tried a larger experiment with
600 wrasse in a sea cage containing 26 000 salmon. The salmon did not need
treatment with Nuvan, whereas a control group had to be treated several
times to keep the lice under control.

The Norwegian results encouraged the Shetland unit at Ardtoe to carry
out some local trials. Two experiments are now under way, and will continue
through the winter. One trial, commissioned by the Shetland Salmon Farmers’
Association is near Lerwick in Shetland, where 100 wrasse are caged with
5000 of this year’s salmon smolts. The other, in Loch Sunart, on the west
coast of Scotland, involves 70 wrasse in a cage with 3000 older salmon weighing
about 2 kilograms each. If the results are as good as Bjordal’s, the fish
farmers want to extend the trials through the summer, when sea lice are
especially damaging.

Cleaner fish are an attractive alternative to Nuvan, and not just because
they are more acceptable ecologically. Nuvan is expensive. ‘It can cost
Pounds sterling 100 a year to treat one cage with pesticide,’ said Smith,
‘so reusable wrasse are sensible financially as well as ecologically.’ The
gold sinny wrasse may soon be proving its worth in the waters of Scotland.