Particle-based (Lagrangian) disease transmission
Particles can be released into the model and tracked continuously. We assign properties to the particles according to what is being simulated. Some are passive and simply follow the currents, while others may be active vertically or horizontally, or have a sinking velocity. Examples of such particles include sea lice, different viruses, emissions from aquaculture, plastic, drill cuttings from the seabed, dredged material discharged from the surface, wild salmon smolts, and more.
Concentration-based (Eulerian) disease transmission
In some cases, disease transmission is simulated as a concentration field. This approach is used when more particles would be required from a release source than is practical to represent individually in an ocean model. The advantage is that it provides a more detailed representation of the release and dilution processes in the water masses, while the drawback is increased computational time for the model. Examples include the release of sea lice larvae from aquaculture facilities, viral discharges, or the spread of dissolved medicines and pharmaceuticals from fish farming operations.
Each modelling approach has its advantages and limitations. Results from simulations of sea lice larval dispersal and impacts on wild salmon smolts (part of the SINMOD system) are used annually as input data for the Norwegian aquaculture Traffic Light System.
Analysis of disease transmission networks in aquaculture
Results from sea lice and virus dispersal simulations can be used to describe interactions between fish farming sites as a network. Each site is represented as a node, while modelled disease transmission between sites forms directed and weighted connections. Graph-theoretical analyses identify key sites, interconnected transmission areas, and connections where interventions may reduce further spread.
The methodology is applied at national, regional and local scales to assess production areas, coordinated fallowing, disease barriers, site structures, and combinations of open, closed and submerged farming systems.
SINTEF uses these analyses in major national collaborative projects, including OptiLok, which focuses on area-based management related to ILA and PD, as well as in local assignments for industry stakeholders. Local analyses can, for example, provide decision support when establishing new sites and assessing their connectivity with existing facilities.
Our disease transmission network calculations and analyses have been used by the Chilean authorities to determine the sequence for delousing aquaculture facilities.