Dispersal of sea louse larvae from salmon farms: modelling the influence of environmental conditions and larval behaviour

Abstract

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.

Reduced Lepeophtheirus salmonis larval abundance in a sea loch on the west coast of Scotland between 2002 and 2006

“…This correlation indicates that the gravid L. salmonis on the farmed salmon were an important source of sea louse larvae. Plankton surveys have suggested that larval Lepeophtheirus salmonis can be carried several kilometres landward from their source (McKibben & Hay 2004a, Penston et al 2004, and this suggestion is supported by numerical models (Asplin et al 2004, Murray & Gillibrand 2006, Gillibrand & Willis 2007. This implies that self-sustaining L. salmonis populations on farms (Tully 1989, Bron et al 1993a,b, Jackson et al 1997, Revie et al 2005) might be contributing to infestation pressure in the wider environment, not just in their immediate vicinity.…”

Densities of planktonic Lepeophtheirus salmonis before and after an Atlantic salmon farm relocation

“…It is unlikely that the copepodids recovered at Stn H in Cycle 2, or anywhere within the Loch Torridon management area during the present study, were transported there from farms in neighbouring management areas as the closest ones are > 30 km distant. A 10-yer study of sea lice infection on sea trout on the west coast of Ireland indicated that lice could disperse up to between 25 and 30 km from farms (Gargan et al 2003), and most sea lice dispersal models do not predict larvae to travel > 30 km (Krkošek et al 2005, Murray & Gillibrand 2006, Gillibrand & Willis 2007, Amundrud & Murray 2009). …”

How sea lice from salmon farms may cause wild salmonid declines in Europe and North America and be a threat to fishes elsewhere

“…Gillibrand & Willis (2007) mathematically demonstrated such a model for the dispersal of sea lice larvae under typical coastal environmental conditions (including tidal, riverine and wind-driven currents) and showed that inclusion of larval behaviour in the model best explained field observations of copepodite distribution. In addition, their hydrographic model showed how below the seaward freshwater current is an upstream mid-depth current that can transport larvae towards land and into estuaries (figure 1).…”