The expansion of Atlantic salmon (Salmo salar) aquaculture systems has led to the success and proliferation of the ectoparasitic sea louse, Lepeophtheirus salmonis. Mediating infestation should have a biological basis, and this PhD investigates the fundamental behavioural interaction between host and parasite. It focuses on salmon behavioural defence and susceptibility, and effect of infestation on performance. Behaviour should be in the toolbox against pathogens in aquaculture, and this project has provided baseline knowledge of host-parasite interactions that will also help drive the development of biological tools. Sam is now a researcher in the Animal Welfare Group at the Institute of Marine Research, Norway.