Sea lice infestation level alters salmon swimming depth in sea-cages

Abstract

Host-parasite systems are often characterised by a co-evolutionary arms race, with avoidance behaviour being the first line of defence for hosts. In aquatic ecosystems, the rapid rise of fish farming has elevated host abundance, altering the context of host-parasite interactions. Behavioural defences in host fish may adapt to combat infection pressure in the captive environment. We tested whether farmed Atlantic salmon Salmo salar altered their swimming depth in response to the ectoparasitic sea louse Lepeophtheirus salmonis. Parasite loads were manipulated on individual fish, which were implanted with internal tags that recorded swimming depth. During daylight hours, salmon exhibited identical swimming depths irrespective of parasite load. However, fish with higher parasite loads (12-18 lice fish-1) swam deeper at nighttime, compared to fish with no or moderate parasite loads (0-6 lice fish-1). As infective sea louse copepodids are concentrated near the surface, our results suggest that the preference for deeper water in fish with higher parasite loads is an avoidance mechanism to prevent further infestation. Host behavioural responses to parasites are predicted to shift with changes to the system and artificial selection of host phenotypes; our study provides the first evidence of a parasite avoidance response in salmon held in sea-cages.