Scottish Marine and Freshwater Science Volume 2 No 3 BACTERIAL KIDNEY DISEASE: ASSESSMENT OF RISK TO ATLANTIC SALMON FARMS FROM INFECTION IN TROUT FARMS AND OTHER SOURCES

Marine Scotland is the directorate of the Scottish Government responsible for the integrated management of Scotland's seas. Marine Scotland Science (formerly Fisheries Research Services) provides expert scientific and technical advice on marine and fisheries issues. Scottish Marine and Freshwater Science is a series of reports that publish results of research and monitoring carried out by Marine Scotland Science. These reports are not subject to formal external peer-review. 1 Objectives Bacterial kidney disease (BKD) is a disease of salmonid fish caused by the bacterium Renibacterium salmoninarum (OIE 2006). In Great Britain (GB) BKD occurs in both Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) farms and has been historically recorded in wild Atlantic salmon (Smith 1964; Bruno 1986). In 2004, the European Union granted GB additional guarantees for the control of BKD, subject to a control program for existing cases of infection (Munro 2007). The impacts of BKD on trout are considered less serious than impacts on salmon where higher levels of mortality occur mainly in valuable sea grown fish. It is therefore desirable to minimise the exposure of farmed salmon to R. salmoninarum, but control of infection in trout may be less cost effective. It may be possible to manage infection on salmon and trout independently, but the practicality of this depends on the infection risk that farmed trout pose to farmed salmon. The objective of this exercise is to assess the factors affecting the exposure of farmed salmon to R. salmoninarum. It is only worth controlling infection on trout farms with the objective of protecting salmon IF transmission from farmed trout is a significant risk in absolute terms AND it is significant relative to risk from other sources. That is, assuming control of R. salmoninarum in trout is not considered worthwhile to protect trout as an end in itself. This risk analysis identifies routes whereby farmed salmon could be exposed to R. salmoninarum infection. These risk routes may apply to freshwater or marine sites. We use case histories and analysis of potential contact patterns (networks, shared drainage basins, survival in water or transmission by wild/escaped fish) to derive the relative importance of different routes of pathogen introduction into freshwater salmon sites such as: 2  Wild or escaped fish populations  Farmed rainbow trout  Introduction of ova  Import of fry or parr to freshwater site(s) The importance of routes of introduction of pathogens into marine salmon …