First report of Gyrodactylus teuchis (Monopisthocotyla: Gyrodactylidae) on farmed rainbow trout Oncorhynchus mykiss (Walbaum, 1792) in Russia.

We recorded Gyrodactylus teuchis Lautraite, Blanc, Thiery, Daniel & Vigneulle, 1999 on one-year-old farmed rainbow trout Oncorhynchus mykiss (Salmoniformes, Salmonidae) from Lake Kalmozero (White Sea drainage system, Republic of Karelia, Russia) in May and October 2023. Morphological identification of these specimens was confirmed by molecular data on the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene and the ribosomal internal transcribed spacer 2 (ITS2). Analyses of the newly obtained cox1 gene sequences show they are identical to fragments of sequences of this gene for G. teuchis individuals collected from wild S. trutta in Slovenia and from farmed S. trutta in Russia. Sequencing of the ITS2 region revealed ribotype D found previously in wild S. trutta and S. fontinalis in Austria, and in farmed S. trutta and O. mykiss in Poland. The infection prevalences in the caged rainbow trout were 100% and 75%, and the mean abundances were 20.8 and 18.1 ind., in May and October, respectively. There were no significant differences between the parasite abundance distributions in these two months, likely indicating similar levels of resistance to the infection. This is the first report on G. teuchis from the White Sea drainage basin as well as in farmed rainbow trout in Russia. This new finding is a signal for veterinary authorities to strengthen parasitological monitoring of transported stocking material.