A simple model of the role of area management in the control of sea lice

Sea lice are parasites whose treatment is a major cost on farming salmon, and sea lice from salmon farms can significantly increase lice numbers on wild salmonids in their vicinity. Effective sea lice control is thus an important consideration for sustainable salmon aquaculture. Sea lice have free-living planktonic larvae that are transported by currents and for this reason co-ordination of farm activities within management area (MAs) is increasingly used to control lice. Here we develop a simple model of co-ordinated management and resultant frequency of treatment required to maintain control of lice numbers in order to assess the benefits of using MAs. The model consists of a circle of salmon farms nodes that exchange larva lice with their neighbours on both sides. The farms are grouped into MAs, whose sizes are based on those of Disease Management Areas currently operating in Scotland. Transmission across MA boundaries is reduced, but is not generally stopped completely. Using this model, co-ordinated management within MAs is shown to reduce the number of treatments required to keep lice burdens under control. Co-ordinated fallowing is always effective at reducing treatment requirements, however the benefits of co-ordinated treatment depend on hydrodynamic mixing regimes and the efficacy of medicines used. Benefits of co-ordinated management apply even when MA boundaries are epidemiologically ineffective, but strong boundaries can greatly increase MA benefits. There are thus robust clear benefits of the use of MAs and so simple modelling supports their use as a general policy. However, specific benefits in a particular area depend on the specific local environment which requires the use of more sophisticated hydrodynamic and population modelling to evaluate.