The use of cleaner fish by Norwegian salmon farming has increased in recent years. Lumpsuckers, ballan wrasse (Labrus bergylta) and other members of the wrasse family are used as a biological control for salmon lice.

These cleaner fish are used as a preventive measure to keep lice numbers down. The fish snack on lice, which helps limit the numbers of lice that are able to attack farmed salmon in their cages. "Wrasses are one of the most important measures we have to remove lice," said Espen Lie Dahl of the aquaculture company Salmar, which reuses some of their cleaner fish.

A mortality rate of 40 per cent in cleaner fish is a minimum number, says researcher Lars Helge Stien from the Institute of Marine Research. But these fish die in droves.