Diagnostic monitoring of mass fish mortality in north-western russian aquaculture farms and strategy of molecular genetic detection

Salmon farming and fish farming in the Northwestern Federal District (NWFD) account for over 70 % of Russia’s commercial trout production and a significant share of whitefish and sturgeon products. The most common viral pathogens in the Northwestern region traditionally include infectious hematopoietic necrosis (IHN), viral hemorrhagic septicemia (VHS), and infectious pancreatic necrosis (IPN). The survey covered fish farms in the Leningrad Region (cage and tank farms), the Republic of Karelia (cage farms on lakes and rivers), and the Novgorod Region (flow-through farms). The study was conducted from 2021 to 2025; sample collection was timed to coincide with periods of recorded fish mortality with at least a 10% loss in 7 days. Rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar), whitefish (Coregonus lavaretus), and other species were studied. Samples included groups of various ages, from juveniles weighing 1–5 g to commercial fish. Renibacterium salmoninarum, motile aeromonads of the Aeromonas hydrophila complex, as well as viral agents IHN, VHS, and IPN, were not associated with the epizootic situation we recorded during our survey of fish farms in the Leningrad Region, the Republic of Karelia, and the Novgorod Region in 2021—2025. The main etiologically significant agents of mass fish mortality in the surveyed farms were: Lactococcus garvieae, Aeromonas salmonicida subsp. salmonicida, Yersinia ruckeri, Flavobacterium psychrophilum, and Pseudomonas spp. Collectively, these pathogens accounted for at least 80 % of the recorded livestock mortality. This paper describes the clinical presentation for each disease entity, as well as strategies for using PCR protocols and molecular targets for molecular genetic verification of the pathogen.

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