The use of cleaner-fish to control sea lice on two Irish salmon (Salmo salar) farms with particular reference to wrasse behaviour in salmon cages

Corkwing (Crenilabrus melops (L.)) and goldsinny (Ctenolabrus rupestris (L.)) wrasse successfully controlled sea lice infestations on farmed Atlantic salmon smolts (Salmo salar L.) on two commercial fish farms off the west Irish coast. Lice levels were generally maintained below five mobile stages per fish using ratios as low as one wrasse to 250 salmon, and cleaner-fish were shown to be a more effective lice control method than conventional chemical treatments in the case of diseased or stressed salmon. High wrasse losses were recorded from cages, presumably a result of escaping through the nets, or by smaller individuals being consumed by large smolts. Aggressive behaviour of smolts towards the wrasse was observed when the cleaner-fish were first introduced to the cages. Cleaning behaviour was observed throughout the day though a peak in cleaning activity was recorded in the early morning for corkwing and at mid-day in the case of goldsinny. Specialist cleaners were recorded by SCUBA observations and by gut content analysis, with up to 58 lice consumed by individual wrasse. Cleaning behaviour was encouraged by maintaining clean nets, so limiting alternative food sources for the wrasse. Artificial shelters were utilized by both species, particularly goldsinny, and some evidence of competition for resting spaces was observed at dusk. A decline in cleaning activity was observed in October, probably a result of lower temperatures and reduced daylength. The results are discussed with regard to wrasse husbandry in the salmon cages.