Impact of thermoclines on the vertical distribution of salmon lice larvae

Abstract

Salmon louse Lepeophtheirus salmonis, a key parasite of salmonids, is managed by multiple methods at both salmon farm- and ecosystem-scale that are informed by an understanding of the abundance and distribution of the infective, planktonic stage of the lice. Dispersal modelling using hydrodynamic models relies on accurately estimating larval depth and how environmental variables modify distributions. Larval responses to temperature could modify dispersal distances by altering their depth in the water column and thus exposure to depth-dependent oceanographic processes and the duration of their temperature-dependent development. Using column experiments, we tested how L. salmonis nauplii and copepodids responded to different thermoclines by establishing a bottom layer of 12°C with an overlaying layer varying from 6 to 18°C in 2°C steps. Nauplii moved upwards in high proportions and aggregated in the surface layer when the overlying layer was 10°C or cooler. In contrast, nauplii moved downwards and aggregated at the thermocline when the overlying layer exceeded 12°C. Temperature did not influence the vertical distribution of copepodids. When nauplii behaviour towards temperature was integrated into a dispersal model, dispersal distances increased. Temperature should be considered when calculating depth distributions. Further, nauplii and copepodids behave differently and should be configured separately in dispersal models.