Gyrodactylus salaris Malmberg, 1957 (Monogenea, Gyrodactylidae) spreads further – a consequence of rainbow trout farming in Northern Russia

The monogenean freshwater parasite Gyrodactylus salaris Malmberg, 1957 is endemic to Atlantic salmon ( Salmo salar ) east of the Baltic Sea, but has spread outside this area via transport and stocking of fish. In Norway and Russia, infections with G. salaris have had catastrophic consequences for many salmon populations. The parasite is also common on farmed rainbow trout ( Oncorhynchus mykiss ) where it can persist in low numbers and without clinical signs. The transport and movement of infected rainbow trout is an important factor in the spreading of G. salaris in Europe. Due to increasing interest in establishing rainbow trout farms in White Sea drainages in Murmansky Oblast, Russia, and the potential subsequent unintentional spreading of G. salaris , parasitological examinations of salmonids were carried out. Farmed rainbow trout (n = 48) and Atlantic salmon (n = 375) from River Tuloma and farmed rainbow trout from Lake Imandra (n = 10), were examined in the period from 2015 to 2019. Additionally, environmental DNA monitoring was conducted for the detection of G. salaris in 2018. Gyrodactylus specimens were first detected in 2015 on Atlantic salmon from the tributary River Pak. Specimens obtained from Atlantic salmon in River Tuloma and from rainbow trout in River Tuloma and Lake Imandra the following years were confirmed to be G. salaris by sequencing of the nuclear ribosomal internal transcribed spacer (ITS rDNA) and mitochondrial cytochrome oxidase 1 (COI). All specimens carried the same COI sequence, which was identical to a strain (GenBank Accession number AF479750) frequently found on farmed rainbow trout. The prevalence varied, but reached 100% in some samples. Maximum intensity observed was 899, but intensities were generally lower than intensities expected to lead to mortalities. There was good correspondence between eDNA monitoring and conventional methods. The results indicate that G. salaris has spread to River Tuloma via transport of live rainbow trout.