Sea lice infestation has been an increasing problem in salmon aquaculture, with strict thresholds requiring extensive and repetitive treatment events during sea production phases. An alternative and more sustainable strategy to reduce sea lice infection is through selective breeding. The aim of this study was to investigate the expected reduction in lice infestation and associated treatments in sea pen raised Atlantic salmon selected for high genetic resistance.
Three genetic groups, i.e., 0, -1.5 and -2.5 genetic standard deviations (gSD) of genomic breeding values (gEBVs ) for lice resistance were created from Mowi nucleus population and subsequently, transferred to hatchery and nursing units until they smoltified and reached 150 g. In April 2023, 4, 500 f ish were PIT- tagged and distributed evenly across nine sea cages (5x5x5m3 ) located at LetSea AS in Solfjellsjøen , Norway. A latin square experimental design was used where each row and column contained all genetic groups with three replicates per group. Growth and lice count were monitored using artificial intelligence underwater camera units (Bioscope model, OptoScale AS). When lice count approached the treatment threshold, 50 random fish per cage were recorded for body weight and manual lice count and all fish were subsequently deloused using mechanical suction. The lice count data from six delousing events (Figure 1) was fitted with a linear model . There was no significant difference in lice count due to row and column in the latin square design. The mean lice count from -1.5gSD (-0.02 lice) and -2.5gSD (-0.32 lice) was lower than 0gSD but it was not statistically significant. In conclusion, we observed no significant difference in lice resistance in groups reared separately with higher and lower gEBVs for this trait. These results support previous findings of little or no difference in average lice numbers per fish between groups divergently selected for lice resistance that were challenge tested in separate tanks. These results could be explained by the ability of lice to find a host regardless of the host resistance level of the fish when there is no other choice and suggest s further study of the attraction and attachment dynamics are needed when gEBVs from mixed group challenges.