Genetic variation in susceptibility of Atlantic salmon (Salmo salar L.) to salmon lice, Lepeophtherius salmonis, from field and challenge tests

Sea lice, Lepeophtheirus salmonis (L. salmonis) is the major threats for Norwegian salmon farming industry and the cost of the parasite was estimated to be around five billion NOK per year. Chemical treatments have been used to control sea lice infection in the last two decades, however, negative impacts on the economic, environment and animal welfare has also been reported. Results from several Atlantic salmon breeding programs suggest that there is substantial additive genetic variation in resistance to the sea lice. The aim of this study was to estimate the genetic variation in resistance to sea lice (L. salmonis) with dataset from field and tank challenges in marine harvest breeding populations. The specific objectives of the study were to estimate genetic (co) variation between repeated lice challenges of i) the same year class of families, ii) between different year classes, and iii) the importance on the use of lice count per fish (LC) as compared to lice density (LD; i.e. LC adjusted for body size of the fish). To achieve this, a total of 15457 individuals with complete phenotype information and 14269 individuals with genotype information from 1339 families of 644 sire and 1305 dams were measured for LC and LD across several year-classes of Atlantic salmon. Challenge tests were conducted in sea net-cages and tanks. The average LC in net cages ranged from 4.6 (SD = 2.9) to 15.5 (SD = 8.0) and in tank it was 17.2 (SD = 12.5) and 23.9 (SD = 15.5) for Matre and Veso, respectively. In net cages heritability estimates of LC was 0.05 to 0.18 and 0.06 to 0.15 using pedigree and genomic relationship matrix, respectively. In tanks, the heritability estimates for LC was 0.19 and 0.30 and 0.16 and 0.24 in Matre and Veso, using pedigree and genomic relationship matrix, respectively. LD heritability estimates ranged from 0.04 to 0.16 in net cages while in tanks it was 0.22 in Matre and 0.25 in Veso. Genetic correlation between LC and LD was 0.67 to 0.99 across all the year classes.\nThe genetic correlation between the two independent counts for year class 2015 and 2016 was found to be positive and significantly different from zero. In addition to this, we have found a moderate genetic correlation between the three locations of 2017- year class both for LC and LD. Phenotypic correlation of body weight with LC or LD were -0.05 to 0.2 and -0.05 to -0.41 for LC and LD respectively; suggesting that selection for increased body weight in Atlantic salmon would not cause unfavourable correlated response in lice resistance.