Novel cypermethrin formulation for the control of sea lice on salmon (Salmo salar)

A novel formulation of cypermethrin was applied as a bath treatment to Atlantic salmon infested with sea lice on a commercial fish farm on the Isle of Skye, Scotland. Twenty 15 m × 15 m cages were treated with cypermethrin at a concentration of 5 μg/litre sea water. The numbers of sea lice of all stages were recorded on five fish per cage before the treatment and one, seven and 16 days after treatment. Statistically significant reductions in the numbers of chalimus III and IV pre‐adults and adults were recorded over the whole period; the average percentage reductions at one and 16 days after treatment were 59 per cent and 90 per cent (chalimus III and IV), 98 per cent and 95 per cent (pre‐adults), and 99 per cent (adults), respectively.