Invasive diseases of commercial fish in the Volga Delta

The transformation of the delta ecosystem, which has been taking place over the years, can lead to changes in all the components of the biotope. To assess the situation in the Volga Delta, studies were conducted using the method of incomplete parasitological analysis. Over a five–year period (2019-2023), the composition of parasites capable of causing the development of invasive diseases in commercial fish species of the Volga Delta included representatives of the class Cestoda, Nematoda, Acanthocephala, Crustacea. The highest incidence of diseases was recorded in the spring period, which is determined by the biological characteristics of helminths - causative agents of diseases, at the same time, the ecological and trophic factor was the leading factor in the formation of an epizootically significant fauna of parasites (cestodes, nematodes, scrapers), the development of which is associated with intermediate hosts and the duration of parasitization. Hydrological and hydrochemical parameters determined the occurrence of Pomphorhynchus laevis, a carp–specific carp, and Achteres percarum, a pathogenic crustacean. Invasive diseases occurred in a subclinical form, manifested by general pathological reactions in the body of the affected fish. The detected morbidity levels were heterogeneous, however, the composition of taxa (pathogens) did not undergo significant changes during the study period. In most cases, the incidence of the examined fish was within the background values with a slight increase in some years, with the exception of estrongylidosis of perch. The systematic identification of a constant range of parasites that provoke the development of pathological changes in the organs and tissues of their hosts, as well as a low incidence rate that does not exceed the annual average, indicate stable invasive processes and the preservation of pathogen transmission mechanisms in the hydrobiocenosis of the Volga Delta.