Understanding key factors associated with the infection of farmed Atlantic salmon by the salmon louse Lepeophtheirus salmonis

The objective of the work described in the current thesis was to provide a better understanding of some of the key factors associated with sea louse, Lepeophtheirus salmonis, infection of farmed Atlantic salmon. In Chapter 2, initial work focused on establishing the vertical and horizontal distribution of sea lice copepodids and spatial patterns of on-farm infections. The louse distribution was investigated along the main current gradient across adjacent salmon production pens at three commercial sites. A depth profile for the distribution of larval lice was also established for the top 12 m of the water column at three different locations in close proximity to commercial salmon farms. Within all multi-pen sites there were clear patterns of distribution and infection along the main water current gradient with the abundance of lice in end pens at each site appearing to be different from the central pens. The vertical distribution pattern of free swimming L. salmonis larvae (nauplii and copepodids) showed that the surface 6 m harboured 85.5 ± 1.6 % of the lice present in the water body analysed (0 – 12 m depth), irrespective of sampling date and location. In Chapter 3, further environmental effects / influences on attachment success of the copepodids were analysed using controlled infection challenges. A flume with adjustable flow rates, and controlled light conditions was designed for this study. Flume current velocity was observed to be a significant factor in infection success, with higher infection levels observed at lower current velocities, while higher current velocities were demonstrated to reduce settlement success. At fixed velocity, higher copepodid exposure levels gave rise to higher infection levels, this having a linear relationship suggestive of a lack of competitive effects for space on the fish. Light was also shown to play an important role in host settlement. A positive correlation between increasing light intensity and higher louse attachment success was found for all tested light spectra / wavelengths (white - Halogen, blue 455 nm, green 530 nm and red 640 nm). Observation of an infecting cohort of copepodids showed maximal infection at four days post-moulting with a tail-off of infection by six days post-moulting. However, even under the optimal conditions represented by a flume challenge, including linear water flow, the constraint of copepodids to pass close to the salmon host and the very high exposure levels of copepodids per fish, louse attachment success was still relatively low. Chapter 4 examined implementation of a possible management approach based upon some of the environmental influences observed. This chapter described a study in which environmental manipulation of salmon swimming depth was employed on-site in an attempt to reduce farm infection of Atlantic salmon. The effects of submerged artificial lighting in combination with submerged feeding were tested with respect to salmon swimming depth and sea lice infection, following the hypothesis that L. salmonis infection in a commercial salmon population could be reduced through exposure to deep lighting and feeding. The results of the study suggest that swimming depth manipulation can indeed be used at a commercial scale to reduce salmon lice burdens on Atlantic salmon by physically minimising spatial interactions between the two animals. In the final research chapter (Chapter 5), this thesis examines the question of whether ploidy of the host impacts on sea louse infection levels and whether susceptibility of individual fish is consistent between replicate infections. Results showed that triploid salmon are not subject to higher sea louse infection levels under experimental challenge and farm infection conditions compared to diploid hosts. In addition, triploid fish subject to initial infection, did not become more or less resistant to infection compared to diploids when comparing repeated sea louse infections. In summary, this thesis describes work conducted to analyse key infection pathways and factors influencing infection of Atlantic salmon by sea lice and suggestions made as to how findings may be exploited to reduce louse burdens in Atlantic salmon farming. The practical solutions presented to exploit the results found in this work are currently under consideration by the Scottish salmon industry.