“She’s got the ticket to ride, but she don’t care” (The Beatles, 1965) The epidemiology of Gyrodactylus salaris in the best wild salmon river in the Baltic Sea basin

1 % 0.1 % 0.05 % 0.01 % Fig. 1 River Tornionjoki water system has an abundant and stable G. salaris infection • River Tornionjoki water system is the best wild salmon river of the Baltic Sea catchment area. The salmon stock is on the rise with ca. 100 000 broodfish counted on their spawning migration in 2014 and 2016, ca. 60 000 in 2015, smolt run of 2016 ca. 2-3 million smolt. • G. salaris infection most common in the upper tributaries (Fig. 1, Ref. 1). The occurrence pattern has remained like that for over a decade 2000-2012 (Table 1). There are no obstacles for the downstream spread of the parasites and smolt are heavily infected (Ref. 4). In spite of this, she (all G. salaris are females) is most prevalent in the north. • Water quality parameters maybe explanatory (aqueous aluminium ion concentrations during floods?)