Sea lice are a major parasite of salmonids, and are associated with large losses to the salmon farming industry. Even though levels on farmed salmon are often low, because their population of is large they contribute a large fraction of lice egg production. The extent of interaction of lice populations on farmed and wild fish, and hence the risk to wild fish, depends on the transmission of larval lice between these populations. This transmission in turn depends on the larval lice's movement through the water.
Marine Scotland Science are developing a model to simulate the movement of lice through the waters of Loch Torridon. Extensive data are available on adult lice population and larval lice distribution in this area. The model consists of a and a particle tracking model; results are processed by post-processing routines (Fig. 1).
Fig. 1. Flow of data from hydrodynamic model to transport to post-processing analysis programs.
How the Model Works
The hydrodynamic model generates water current velocities in response to tides and under different assumptions of wind forcing. The results are saved as a table of current velocities defined on a grid at 100 metre intervals, defined at half hour intervals. The results of the hydrodynamic model are fed into a particle tracking model which uses the generated currents to calculate movement of particles. This movement includes direct advection by currents, interpolated to the exact location of the particle, and turbulent diffusion, which disperses particles, calculated from gradients in current velocities.
Advection and diffusion are combined with the particle's own properties to calculate a new position for the particle in space and time. This is repeated, building up a trajectory of the particle. The particle's age is also recorded so that the larval lice can mature from a non-infectious naupulus to an infectious copepodid.
Areas in which fish are at risk of infection from lice produced at a given source, for example, a fish farm, can be found from combining runs from many lice particles (Fig. 2). A variety of programs are used to do this. The model's results can be used to identify risky sites and the behaviours that are necessary to explain the observed distribution of larval lice.
Fig. 2. Particle dispersal from a farm site in Loch Sheildaig, a basin of Loch Torridon.
What Early Results Show
Although the model is still under development, early results show that tidal currents are capable of dispersing lice over large areas, but are insufficient to explain high concentrations found in shallow waters. The recently produced wind-generated currents may play an important role in this.