Stochastic simulation of live salmonid movement in England and Wales to predict potential spread of exotic pathogens.

The anthropogenic movement of live fish has been identified as the most important route for the transmission of disease between river catchments. To assist in contingency planning for exotic salmonid disease outbreaks, a stochastic model was developed to assess the potential geographic distribution of an introduced pathogen with time to first detection. The Live Fish Movement Database (a resource funded by the UK Department for Environment, Food and Rural Affairs [Defra] and managed by the Centre for Environment, Fisheries and Aquaculture Science [CEFAS] and the Environment Agency [EA]) was used to establish details of live fish movement in England and Wales. A contact network was created for farm to farm and farm to non-farm (fishery) movements of rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta and Atlantic salmon Salmo salar, and probability functions were used to model the timing and destination of movements from farm sites, based on these trading activities. Monte Carlo simulations were run to track the progression of potential disease transmission from single index farm inputs through river catchments with time. Two hundred farms exported fish to 1653 destinations in 147 of the total 198 river catchments. The median number of catchments contacted after 3 and 12 mo were 3 and 6, respectively. In 5% of simulations 63 or more catchments were contacted, and in 1% of simulations 75 or more catchments were contacted after 12 mo. These results may be used to underpin the development of contingency plans for exotic disease outbreaks.