Use of cleaner-wrasse (Centrolabrus exoletus (L) and Ctenolabrus rupestris (L)) to control infestations of Caligus elongatus Nordmann on farmed Atlantic salmon

Trials were conducted in experimental tanks and in commercial scale sea cages to evaluate the ability of wrasse to control sea lice infestations on farmed Atlantic salmon smelts. In a 3 h tank trial rockcook wrasse, Centrolabrus exoletus (L,), removed an average of 19 lice per fish h(-1) from salmon smelts infested mainly by Caligus elongatus Nordmann (93%) and with lower numbers of Lepeophtheirus salmonis (Kroyer) (7%). Goldsinny wrasse, Ctenolabrus rupestris (L.), did not remove lice from salmon over this 3 h period. Chalimus stages I and II were removed at a lower rate than were older and larger life history stages. Both species of wrasse failed to prevent rapid increases in infestations of C. elongatus in commercial cages in 1992 and 1993 at initial wrasse to salmon ratios ranging from 1:37 to 1:146. However, SCUBA diving observations confirmed that both wrasse species did remove C. elongatus from salmon in these cages. In the 10 m deep cages, wrasse occurred mainly at depths greater than 6 m while salmon occurred primarily between 2 and 5 m. Goldsinny had a significantly shallower distribution than rockcook. Both species associated closely with the cage net and with objects in the cage and interactions with salmon appeared to be relatively uncommon. Rigorously controlled trials on commercial farms need to be carried out in order to define protocol for the use of wrasse under these conditions.