The parasitofauna of the Siberian sterlet Acipenser ruthenus marsiglii of the Lower Irtysh

In the Ob-Irtysh basin, studies on the parasitofauna of the Siberian sterlet are very limited and confined to the middle of the twentieth century. The decrease in the numbers of sterlet over the past half century may have led to a change in the qualitative and quantitative composition of the parasitofauna. The aim of this work is to study the parasitofauna of the Siberian sterlet in the rivers Irtysh and Tobol. 85 specimens of Siberian sterlet (L = 27.8-51.5 cm, 2+ - 6+) from the Tobol and Irtysh rivers were examined by the method of complete parasitological dissection during the periods from 5 to 21 June and from 14 to 22 July 2017. A comparison of the biodiversity of parasites was performed using the indices: Berger-Parker. 11 species of parasites were found, including four species specific to sturgeons: Cryptobia acipenseris (Joff, Lewashow, Boschenko, 1926), Haemogregarina acipenseris (Nawrotzky, 1914), Crepidostomum auriculatum (Wedl, 1858), Capillospirura ovotrichura (Skrjabin, 1924). Seven broadly specific species: Trichocilna sp., Proteocephalus sp. (plerocercoids), Diplostomum chromatophorum mtc. (Vrown, 1931), Echinorhynchus cinctulus (Porta, 1905), Piscicola geometra (Linnaeus., 1761), Ergasilus sieboldi (Nordmann, 1832), Unionidae gen. sp. Cryptobia acipenseris and H acipenseris were recorded for the first time in the Ob-Irtysh basin. C. acipenseris was found only in three fish from the total number of examined sterlet (3.5%). In the River Tobol the extent of infestation H. acipenseris was higher (22.7%) than in the River Irtysh 11.1%. The carriers of C. acipenseris and H. acipenseris in the Lower Irtysh are allegedly leeches of Piscicola geometra (10.6%). The most common parasites found were C. auriculatum (32.9%) and C. ovotrichura (15.3%). In the River Irtysh, metacercariae of Diplostomum chromatophorum (Vrown, 1931) (metacercariae) were discovered in the lens of the eye in the studied fish. The maximum EI of this parasite was noted in the fish sample near the city of Tobolsk - 12.5% with AI - 1 and IO - 0.12. In the Gomoslinkino area in June and July, the infection level was low (EI 4.3% and 8.3%, respectively). In the June sample of fish from the River Tobol no metacercariae of this species were detected. Single cases of infection of Proteocephalus sp. (plerocercoid), E. cinctulus, E. sieboldi, Unionidae gen. sp. were found. In the River Irtysh the parasitofauna was dominated by C. ovotrichura, in the River Tobol - C. auriculatum. The number of parasite species noted in the Siberian sterlet in the Ob-Irtysh basin according to our own and literary data is greater than that found in sterlet from the River Yenisei: 18 and 11 respectively. Common to the sterlet of the two basins are 8 species of parasites: Cryptobia acipenseris, Dichthothrium annatum, Crepidostomum auriculatum, Capillospirura ovotrichuria, Truttaedacnitis clitellarius, Echinorhynchus cinctulus, Piscicola geometra, Unionidae gen. sp. The fauna of the Ob-Irtysh basin was enriched by the "southern" narrowly specific parasite of sturgeon - H acipenseris. For many years the infection of C. auriculatum has practically not changed.