The development of pyrethrum-based treatments against the ectoparasitic salmon lice Lepeophtheirus salmonis in sea cage rearing of atlantic salmon Salmo salar L

Motile salmon lice Lepeophtheirus salmonis were successfully removed by an oil‐based pyrethrum treatment mixture. The experiments described here are all based on skin exposure of the salmon Salmo salar L. The in‐cage method, in which salmon would delouse themselves by jumping through a layer of treatment mixture, gave up to 86.9% delousing effect under low levels of solar radiation (October). However, at higher levels of radiation (May), the comparable effect decreased to 31.8%. Studies of exposure time vs. delousing effect showed no difference between 2‐s and 10‐s exposure and gave an overall delousing effect of 89.5%. Individual delousing procedures have thus been developed with a 4‐ to 6‐s dip in the pyrethrum treatment mixture. A commercial method is described based on commercial Py‐Sal 25 mixed with an anaesthetic bath before a vaccination or sorting operation. The overall delousing efficiency with this large‐scale method was 85%. This is the method now used in commercial delousing with pyrethrum as the active component.