An assessment of the risk of spreading the fish parasite Gyrodactylus salaris to uninfected territories in the European Union with the movement of live Atlantic salmon (Salmo salar) from coastal waters

The freshwater, monogenean fish ecto-parasite, Gyrodactylus salaris, was introduced into Norway through the importation of juvenile salmon from Sweden in the 1970s and resulted in dramatic declines in the number of Atlantic salmon (Salmo salar) in 45 Norwegian rivers. From May 1, 2004, a change in European Union (EU) legislation (EC decision 2004/453/EC) allowed movements of salmonids to territories free of G. salaris from coastal sites where (i) salinity does not fall below 25 parts per thousand (‰), or (ii) rivers draining into the estuary have been declared free of G. salaris. This risk assessment was undertaken to establish whether exports of live Atlantic salmon from coastal sites increased the risk of G. salaris introduction and establishment in uninfected EU territories. A scenario tree of events necessary for the introduction and establishment of the parasite was constructed, relevant information was identified, and the probability of each step was assessed. Salinity was shown to be the key environmental determinant of parasite survival; at 25‰ the parasite survives for approximately 22 h (at 1.4 °C, temperature at which parasite survival is longest). Transmission from seawater sites, where salinity is greater than 25‰, is only possible if infected fish were introduced hours before export. Furthermore, transport via wellboat in full-strength seawater (33‰) reduced the risk of introduction to a negligible level. Similarly, establishment at the site of destination will also be largely determined by salinity. At high salinities the parasite has only hours to reach freshwater (e.g. via migration on wild fish), and therefore the risk of establishment is extremely low. The risk of G. salaris introduction with the importation of Atlantic salmon from coastal zones where salinity is less than 25‰, and where rivers draining into the region have been declared free of G. salaris, is largely determined by the risk of G. salaris introduction in the freshwater zone. Since reproduction and the survival of G. salaris are negatively associated with increasing salinity, the risk of introduction of G. salaris is lower for movements of live Atlantic salmon from coastal zones, compared with the rivers declared G. salaris free, which drain into those coastal zones. Therefore, the change in legislation did not create routes with a higher level of risk of G. salaris introduction, compared with existing routes (i.e. from approved G. salaris free freshwater zones). However, the change in legislation may increase the volume of trade and thus increase the absolute risk of G. salaris spread. This work demonstrates how qualitative risk assessment can be used to support policy development and decision-making in fish health management.