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In the present study, an evaluation was made of the dependence between infections by various types of parasites and the sex and age of the host. The parasitic community of Abramis brama includes 24 species of parasites. We assessed the degree of infestation by parasites in groups of male, female and juvenile specimens. It was established that only male bream were infested with Zschokkella nova (Klokacewa, 1914), Hysteromorpha triloba (Rudolphi, 1819), Allocreadium isoporum (Looss, 1894), Ichthyocotylurus platycephalus (Сrерlin, 1852). The ciliates of Trichodina nigra (Lom, 1960) were noted on the gills and fins of all the examined fish, the extensiveness of infection in the group of juveniles exceeded that of the groups of females and males. Specimens of Myxobolus rotundus (Nemeczek, 1911) were found on the gills and in the kidneys, and M. parviformis only on the gills of the fish. The level of infection in males was three times the EI in females and juvenile fish. The infestation on the fins by the metacercaria of Rhipidocotyle campanula (Dujardin, 1845) in male bream was more than 6 times higher than infection in females and juveniles. An increase in the extensiveness of infection in males by the nematode Philometra ovata (Zeder, 1803) was also noted in comparison with groups of females and juvenile fish. Species-specific monogeneans: Dactylogyrus falcatus (Wedl, 1857), D. wunderi (Bychowsky, 1931), D. zandti (Bychowsky, 1933), Gyrodactylus elegans (Nordmann, 1832); the trematodes Sphaerostomа bramae (Muller, 1776) and Diplostomum chromatophorum (Brown, 1931) infested all individuals of bream in approximately the same proportion, whereas an increase in the EI of the Caryophyllaeus laticeps (Pallas, 1781) cestode in males was observed. Infection with metacercariae Metorchis sp. in males exceeded that in females and juvenile fish. At the same time, only females and juvenile bream were infected by Chilodonella sp., metacercariae Opisthorchis felineus (Rivolta, 1884) and glochidia Unionidae gen. sp. During the studies in females, the following species of parasite were observed singly : Proteocephalus sp., Azygia lucii (Miiller, 1776), Ergasilus sieboldi (Nordmann, 1832). Parasitization by Raphidascaris acus (Bloch, 1779) was recorded only in males and juveniles, whereas in females this parasite was not observed. The dominant parasite in males was G. elegans, in females – the representatives of the genus Dactylogyrus, in the juvenile individuals the trematode S. bramae dominated. Analysis of the parasite fauna in various age groups allowed us to establish that at the age of 5+–6+ bream were infected by 20 species of parasites, in age group 3+–4+ there were 15 species, at the age of 7+–8+ the fish were infected only with 11 parasitic species. The fish in all three groups were infected by T. nigra, Dactylogyrus spp., G. elegans, D. chromatophorum, S. bramae, R. acus almost at the same level and had no sharp differences in extensiveness and abundance of infection. Unionidae gen. sp. were observed only in group 3+–4+. Parasitization by Proteocephalus sp., metacercariae H. triloba and I. platycephalus, A. isoporum, and A. lucii was found in bream of age group 5+–6+, the species did not occur in bream at other ages. Nematode P. ovata and crustacean E. sieboldi infested the fish in age groups 3+–4+ and 5+–6+ with a slight increase in the extensiveness of infection at low abundance rates. The same age groups were observed to have decrease in the extensiveness of infection with Chilodonella sp. and metacercariae O. felineus. Myxozoan Z. nova parasitized in age group 5+–6+ and had an increase in extensiveness in age group 7+–8+. In all groups, infection was noted by representatives of the genus Myxobolus, R. campanula and Metorchis sp. with EI increasing with age. The obligate parasite of bream – C. laticeps infested fish the most in age group 3+–4+ in comparison with age groups 5+–6+ and 7+–8+. In the younger age group, the dactylogyruses were dominant parasites, in the age group 5+–6+ Gyrodactylus dominated, in the older age group – Metorchis sp.

Sexual and age characteristics of the parasitofauna of Abramis brama (Cypriniformes, Cyprinidae) of the Lower Irtysh (Russia)