FISH FARM SIZE AND SURROUNDING WATER CURRENT SPEEDS DICTATE THE SEPARATION DISTANCE REQUIRED TO AVOID TRANSMISSION OF DISEASE AGENTS BETWEEN PRODUCTION SITES.

摘要

Scotland is the largest Atlantic salmon (Salmo salar) producer in the EU with an output of over 150,000 t, contributing ￡500 million annually towards the economy. Production continues to increase across Scotland, predominantly due to the increase inoutput per farm and reduction in losses to infectious diseases. Farms are grouped within disease management areas (DMAs) whose boundaries are defined as being where the closest pair of farms is separated by more than twice the tidal excursion distance (TE) (TE is defined as 7.2 km, or 3.6 km in Shetland). The majority of salmon farms are located within relatively sheltered inshore areas where non-tidal advective current speed is minimal. However there is an aspiration for offshore production where current speeds will be greater and so TE models may break down, furthermore it may be possible to increase stocking levels. It has previously been demonstrated that the number of stocked fish alters the disease dynamics within a farm by altering transmission.However, an assessment has not been undertaken to assess how farm size impacts on the transmission of pathogenic agents between farms. A discrete-time SEIR model was developed representing fish farms. The model incorporates transmission, expression andrecovery parameters as well as pathogen shedding and decay. An expression is derived representing the time required for shed particles to decay below the minimum infective dose. The critical time expression is used to assess the distance travelled by a cohort of pathogen particles transmitted by a simplified hydrodynamic that incorporates residual advection, tidal advection and turbulent diffusion elements. Applying characteristics for a robust pathogen, infectious pancreatic necrosis virus type (IPNVt), and less robust pathogens such as infectious salmon anaemia virus type (ISAVt), and Aeromonas salmonicida type (ASt) pathogens it is possible to obtain separation distances whereby farms avoid infection. Simulation outputs indicate that separation distances increase to avoid disease as farm size and current speed increase. The more conserved EPNVt pathogen requires separation distances of hundreds of km, ASt tens of km, whilst the distances for ISAVt are within the scale of the current DMAs, which were developed for ISAV control. However, should production be moved in to areas of faster moving currents and increased farm production the current disease management area principles may need readdressing.