The control of sea lice Lepeophtheirus salmonis (Kroyer, 1837) (Copepoda: Caligidae) on Atlantic salmon Salmon salar L., production sites
Sea lice Lepeophtheirus salmonis are the major pathogen affecting the salmon farming industry. Current methods of control, especially in the UK, are largely ineffective as they do not affect the chalimus stages of L. salmonis and are toxic to fish. Greater control of L. salmonis through improved management, more effective compounds, disease control and stock selection were examined. The epidemiology of L. salmonis on Atlantic salmon Salmo salar production sites was examined at a number of farms within Loch Sunart. Intensity of infection was found to be influenced by both management practices and seasonal variations. By instigating a series of co-ordinated, synchronous, strategic treatments throughout the loch system during the winter, initial chalimus levels during the spring were significantly reduced by 90% (p<0.001). Lice numbers for the rest of the production cycle were significantly lower (p<0.01). In addition there was a reduction in the number of treatments needed, an increase in the interval between treatments, reduced fish mortalities and improvements in fish harvest quality. There was no significant reduction in mean chalimus numbers observed at the control site where treatments were not co-ordinated. The strategic treatments were repeated in the Loch Linnhe system during 1997 resulting in an 87% (p<0.01) reduction in initial chalimus numbers. The efficacy of the synthetic pyrethroid, cypermethrin (Excis RTM) was assessed against L. salmonis chalimus. Exposure of the chalimus to 5mug I-1permethrin for 1 h resulted in a significant reduction (83%, p<0.001) in the mean number of chalimus 12 d post-treatment. The developmental stages observed in the untreated group were predominantly pre-adult II males. The majority of the remaining lice observed in the treated group were chalimus III indicating a profound effect of cypermethrin on the rate of development of L. salmonis. At 1 pg I-1permethrin for 1 h there was no significant reduction in the mean number of lice compared to controls (p=0.161) although there was a significant delay in the rate of development. A number of immunosuppressants were administered to Atlantic salmon and their effect examined upon the settlement and survival of Lepeophtheirus salmonis. Corticosteroid was found to increase the mean numbers of L. salmonis observed post-challenge by 59% (p<0.05) compared to controls. Administration of immunosuppressive factors isolated from Aeromonas salmonicida induced a 70% (p<0.01) increase in mean numbers of L. salmonis observed. There was no effect on the rate of development, nor tissue distribution of L. salmonis observed on the test fish compared to controls. The control of L. salmonis on production sites is examined in relation to disease status of the fish as well as the potential for effective immune modulation. Different stocks and families of Atlantic salmon Salmo salar were examined for variable susceptibility to settlement and survival of L. salmonis. Differences in susceptibility of up to 55% (p<0.01) in mean lice numbers were observed between stocks. Variability in susceptibility to settlement and survival, as well as damage caused by L. salmonis was also observed between families of Atlantic salmon.