Fighting an invasive fish parasite in subarctic Norwegian rivers – The end of a long story?

The introduced monogenean Gyrodactylus salaris (Malmberg, 1957) is categorized as one of the most severe threats against Atlantic salmon ( Salmo salar Linnaeus, 1758) in Norway and has almost eradicated salmon populations in the Skibotn and Signaldalen Rivers in northern Norway. The parasite was unintentionally introduced to the Skibotn River in 1976 via release of infested Atlantic salmon smolt from Sweden. The parasite is restricted to freshwater, and survives at most a few days without its host. Therefore, eradication of all hosts in the infected river systems has been the preferred strategy to eliminate the parasite. After two failed eradication attempts in 1988 and 1995, the parasite spread further to neighbouring rivers. This, along with several other failed rotenone treatments in Norway in the 1990s, resulted in severe criticism of the national eradication strategy for G. salaris . Still, the eradication program continued, and the failed eradication attempts were analysed for possible improvements. Arctic char ( Salvelinus alpinus, Linnaeus, 1758) has proved to be a potential long-term host for the parasite and infested char were documented to have survived in small, groundwater-fed tributaries and ponds during the first two eradication attempts in the Skibotn River. Low limits on allowed rotenone concentrations set by the pollution control authorities might also have contributed to the failures. A third attempt at eradicating the parasite from River Skibotnelva was made in 2015 and 2016, using new knowledge about the parasite and its hosts, renewed strategies to map and deal with dilution from groundwater intrusion and an official acceptance of increased concentrations of rotenone. Treatments for two consecutive years was the main strategy improvement from previous eradication attempts. Water samples showed sufficient levels of rotenone concentrations at all sample points during the treatment periods. Significant efforts in collecting all possible surviving fish from the first-year treatment and screening them for G. salaris revealed no surviving parasites at the time of the second-year treatment. The national G. salaris eradication campaign includes a surveillance programme for eradication confirmation. The results so far are positive for the Skibotn Region, but the earliest an eradication confirmation can be issued earliest is 2021.