Dispersal of Sea Louse Larvae from Salmon Farms: Modelling the Influence of Environmental Conditions and Larval Behaviour.

Sea lice are ectoparasitic copepods of farmed and wild salmonids that cost the global salmon aquaculture industry more than US $100 million annually and have been implicated as a contributing factor in the decline of wild salmonid populations in the North Atlantic region. A coupled hydrodynamics-louse-transport model including varying physical forcing and sea louse larval behaviour was developed, and used to investigate: (1) the dispersal of sea louse larvae from a point source in an idealized coastal inlet; (2) the effect of physical processes on louse distributions; and (3) the effect of behavioural processes on louse distributions. The dispersal and distribution of sea louse larvae was strongly influenced by environmental conditions and larval behaviour. In particular, larval diel vertical migration increased the predicted infection risk for wild salmonids, as more sea louse larvae were retained within the inlet and were dispersed more widely with higher average abundance at the surface. When wind forcing was weak or channelled along the inlet, higher louse abundance was predicted along the boundaries, where migrating salmonids tend to congregate. Copepodids were intermittently transported to the river mouth at the head of the inlet during different combinations of river flow and wind forcing, either by surface flows or deeper landward-flowing currents. Mean predicted nauplius abundance peaked near the source of larvae and decreased rapidly with increasing distance from the source, whereas copepodids peaked 7 to 12 km seaward of the source. Realistic environmental forcing and larval behaviour must be incorporated into dispersal models if the spatial and temporal variability of larval sea louse abundance is to be reliably predicted.