The Siberian Dace Leuciscus baicalensis in Watercourses of Different Order of the Middle Ob Basin and its Role in Opisthorchiasis Circulation

The results of the comparative analysis of the size, age and sex composition of the Siberian dace Leuciscus baicalensis from the rivers of the Middle Ob basin (Basandaika River, a second-order tributary, Tom River, a first-order tributary, Ob River, the main watercourse) in 2016–2019 are presented. Differences in the growth rates of dace depending on the type of watercourse were found: the larger the river, the higher the growth rate of specimens. The sex composition is similar in all watercourses; the female-to-male ratio is 1 : 2. Dace from the Tom River is distinguished by the highest prevalence and intensity of invasion by larvae of Opisthorchis felineus (a parasite of the hepatobiliary system of humans and carnivores), 88% and 13.1 ind./fish, respectively, compared to 52% and 4.8 ind./fish in fish from the Ob River. Infection does not depend on the sex of dace. Infection rates increase with the advancing age of fish: the infection prevalence of fish at the age of 5 years reaches 100%. Currently, the epizootological situation remains tense due to high rates of fish infection.