Susceptibility of Atlantic salmon and rainbow trout to the salmon lice Lepeophtheirus salmonis

The susceptibility of Atlantic salmon and rainbow trout to Lepeophtheirus salmonis was investigated in a controlled infestation test in two replicated tanks (each 3 m diameter, water depth 1.8 m) of in total 1094 Atlantic salmon and 58 rainbow trout (tank 1) and 1112 Atlantic salmon and 61 rainbow trout (tank 2). The Atlantic salmon were 1+ post smolt that had been reared on seawater for 1.5 months while the rainbow trout were 0+ post smolt that had been reared in net cages in the sea for six months. Egg sacs from sexual mature salmon lice were harvested from Atlantic salmon. On average 74 copepodids per fish were added to tank 1 and 36 copepodids per fish to tank 2. When the lice reached the attached chalimus II–III stage (12 days after infestation) the lice count per fish was recorded on anaesthetised fish and the individual body weight of the fish recorded. The relationship between the lice count per fish and body weight was positive but low in both species. For Atlantic salmon the average lice count and body weight per fish was 27.1 lice (tank 1) and 13.9 lice (tank 2), and for rainbow trout 78.9 lice (tank 1) and 92.8 lice (tank 2). For Atlantic salmon average body weight was 260.6 g (tank 1) and 260.2 g (tank 2), and for rainbow trout 430.5 g (tank 1) and 480.2 g (tank 2). The predicted lice count per Atlantic salmon of the same average body weight as the rainbow trout (38.4 lice in tank1 and 23.8 lice in tank 2) was much lower than the observed average lice count per rainbow trout. Longer lasting infestation studies should be carried out with the two species, preferably with fish of the same average body weight, to see if observed differences between the two salmonid species in susceptibility to L. salmonis are similar or varies for the different development stages (sessile, motile, sexual mature) of the lice.