Behaviour in Atlantic salmon confers protection against an ectoparasite

Altered behaviour is believed to be the first form of defence against parasite infection, through reducing opportunity for infection or deflecting parasites to sub-optimal sites on hosts. To determine whether the suite of behaviours fish exhibit deter or deflect infection, we tested for differences in salmon louse (Lepeophtheirus salmonis) infection level and attachment location on three groups of Atlantic salmon (Salmo salar) that were either normal, behaviourally modified via injection of ketamine hydrochloride or sham-injected procedural controls. The frequency of behaviours differed between behaviourally modified and normal hosts, which resulted in 26-31% fewer lice in normal fish. Louse attachment locations were unaffected by behaviour. Jumping and motionless behaviours were more common and the frequency of burst swimming behaviour was reduced in non-manipulated fish, suggesting that these behaviours restrict parasite attachment success. Our results show that the suite of fine-scale behaviours that normal salmon display at the time of parasite encounter are associated with reduced infestation.