The invasive salmon parasite Gyrodactylus salaris is among the worst threats to Atlantic salmon (Salmo salar) today. G. salaris is a small (0.5 mm) ectoparasitic monogenean which is found on fins and skin of Atlantic salmon in its freshwater phase. In Norway, G. salaris has caused epidemics that have devastated stocks of Atlantic salmon in 48 rivers. The density of salmon parr in infected rivers has been reduced on an average of 86 % and the catch of salmon are reduced on an average of 87 %.
The occurrence of G. salaris in Norwegian salmon rivers causes a yearly economic loss in the range of 34-40 million €. This loss is primarily due to the lost salmon fishing in infected rivers and loss of sea fishing in the adjacent sea areas. The most visible factor is the loss of local economic ripple effects of salmon fishing in the rivers. It is worth noting that the economic loss mainly affects communities around the infected watercourses. In addition, the Norwegian Government spent yearly about 9 million € for measures to limit the damage caused by the parasite. The most important measures are surveillance, preventing the spread of the parasite, eradication of the parasite from infected rivers, and conservation and restoration of fish populations that are directly affected by the parasite or indirectly as a result of the actions implemented for the parasite eradication. During the 35 years history of G. salaris in Norway, the expenditure has reached 90 million €. If we include the loss of income on salmon fishing and loss of local economic ripple effects in the same period, a rough estimate suggests that Norway has a total economic loss of 430-538 million € as a consequence of the introduction of G. salaris in 1975.