Transformation of Dnepr (Zaporizhia) reservoir`s fish fauna: retrospective review and current status

Creation of reservoirs by regulation of the Dnieper River and small rivers caused significant changes in the conditions of existence and affected on fish biodiversity of pondsof Prydniprovya. To the anthropogenic factors influencing the species composition of fish fauna factors of technical and fishery character can be included. Technical impact on water bodies is associated with the creation of new artificial lakes, ecosystems of which are not stable and are influenced by invasive processes. Fisheries management measures include work on the introduction of new species of fish that primarily have economic importance for the development of industrial fishing. Work on the introduction of new species have both positive and negative effects, but nevertheless new species affect natively on fish fauna. This transforms biota of reservoir and creates conditions for further spread of new species, increasing their numbers, creating new ecological relationships in the ecosystem of the pond. In the formation of the Dnieper (Zaporizhia) reservoir`s fish fauna it is traditionally defined five stages: the first stage is before building a dam to the Dnieper (until 1931) when there was a natural Dnieper rapids area, inhabited by migratory, semi-migratory and local fish; the second stage is the beginning of formation of reservoir`s fish fauna with the gradual disappearance of reophilic and dominance of limnophilic species (1931–1941); the third stage is the restoration of fish populations of Dnieper rapids due to destruction of dam during the Second World War (1941–1947); fourth stage is the secondary formation of ichtiocenosis of reservoir after recovery of dam (1947–1960); fifth stage is reforming of the structure of fish fauna in terms of cascade (creation of Dnieper reservoirs) and anthropogenic pressures (from 1961 to the present period). The first stage is defined by period of filling the reservoir. After the construction of Zaporozhye hydroelectric station the process of rebuilding fish fauna took place. In the early years of the reservoir existence migratory and semi-migratory fish have naturally disappeared. The species composition of fish fauna has decreased by 11 taxons. The number of reophilic species has markedly reduced and remained predominantly in the upper river part of the reservoir. The lower part of reservoir with sustained hydrological regime has been being actively assimilated by fish of limnophilic complex. The second and third stages are associated with the destruction of the dam during the Second World War and the short restoration of hydrobiological regime of Dnieper rapids, until the re-filling of the reservoir and its final transformation into a regulated pond. The flow of these two phases has not influenced significantly the structure of industrial ichtyocomplex total number of species at that time was 38 taxons. The fourth stage involves fishery exploitation of reservoir. With the aim of the purposeful impact on formation of industrial fish fauna of the Dnieper (Zaporizhia) reservoir, considering sufficient development of natural fodder the works on artificial introduction of new species were carried out in the years 1950-1960. The fifth stage of development and current state of fish fauna of reservoir is characterized by the emergence and spread of invasive species. During the period of existence of the Dnieper (Zaporizhia) reservoir ichthyofauna of the reservoir has substantially transformed. In the modern fish fauna of reservoir there are 52 species of fish which are representatives of 14 families. Compared with the period of existence of Dnieper River before its regulation the number of species remained at the same level, but the species composition and structure of fish fauna changed drastically due to the loss of some species and the emergence of other species, mostly unwanted invaders. Changing the number of species is related to several factors. Firstly, process of spread of species upstream took place after the disappearance of Dnieper Rapids and rise of mineralization. Secondly, with the deliberate introduction of fish with purpose of fishery exploitation of the reservoir, and as a consequence randomly invasion of some species that came from fish farms. Thirdly, emergence of new species is also connected with the deliberate release of fish to the open water, similarly Pumpkinseed Sunfish Lepomis gibbosus (Linnaeus, 1758) has appeared in the Dnipro (Zaporizhia) reservoir, which is well acclimatized and has broadly extended its habitat in reservoirs of Dnipropetrovsk region. Today about 31 % of fish species of Dnieper (Zaporizhia) reservoir is adventitious. The process of genesis of fish fauna of the Dnieper (Zaporizhia) reservoir is still ongoing and it is connected with the subsequent emergence of new species and rise of their numbers. Such changes in the fish fauna of reservoir can harm fisheries because the vast majority of fish aliens are competitors by food for young commercially valuable fish species.