Variation of Atlantic salmon families (Salmo salar L.) in susceptibility to the sea lice Lepeophtheirus salmonis and Caligus elongatus

2270 Atlantic salmon smolts representing 30 mostly full-sibling families were transferred to a sea cage in central Norway in May 2003. In the period between transfer to sea and termination of the experiment in August 2003, 42.2% of these fish died, mostly as a result of an outbreak of IPN virus. 1198 of the survivors were investigated in their abundance of the sea lice Lepeophtheirus salmonis and Caligus elongatus. In total, 6000 L. salmonis and 238 C. elongatus were recovered from the fish, giving an average combined lice abundance of 5.2 and a range from 0 to 25 lice per fish. Significant differences in abundance of L. salmonis were observed among the families (P<0.0001), with mean family values ranging from 3.84 to 6.51. This equates to a 70% variation in infection between the lowest and highest infected families, and gave a “heritability” of 0.074 (±0.022). Inter-family differences in abundance of C. elongatus ranged from 0.09 to 0.37; however, these differences were not significant (P=0.67). A significant positive correlation between family sample size, reflecting to some degree family susceptibility to IPN virus, and mean family weight was observed (R2=0.61, P<0.0001); however, no correlation between mean family sample size and family abundance of L. salmonis (R2=0.04, P=0.29), or family abundance of C. elongatus (R2=0.11, P=0.08) were observed. No relationship between individual fish size measured as length or weight, and intensity of L. salmonis infection was observed in the pooled material, within each family or between families using mean family values.