The greatest enemy to farmed salmon is a small copepod called the salmon louse of which there are two species which specifically attach themselves to the fish. Which is recognized by its horse-shoe shaped carapace and brown colour. Which is much smaller and causes very little damage unless present in very high numbers. Infected salmon show grey patches on the back and head where the copepods have been feeding.
Damage is caused by the lice eating the mucus,epithelium, and scales of the fish.
In severe cases the underlying dermis is also damaged and as a result osmotic control becomes impaired and tissue ' water-logging' occurs. Other invasive secondary infections may also occur. Usually in the first year at sea lice numbers are kept down by placing small wrasse in the sea cages with the salmon. This biological control is very effective with the wrasse cleaning the salmon and feeding on the lice. However, as the salmon get bigger the wrasse begin to shy away from feeding on the fish and their effectiveness is much reduced.