UDC 597.2/5 (282.247.324.044) FISH FAUNA OF THE OSTER RIVER BASIN (DESNA RIVER BASIN, UKRAINE) – RECENT DATA

According to recent data, fish fauna of the Oster River basin numbers 21 species, including two introduced species: Perccottus glenii and Pseudorasbora parva. 17 species of fish were registered in the lower section of the river (4 of them for the first time), 21 species were found in the middle section (12 of them were not previously noted for this section) and only 11 species were found in the upper section (6 species were not previously noted for this section). Six species – Pseudorasbora parva, Misgurnus fos­ silis, Pungitius platygaster, Gymnocephalus cernua, Babka gymnotrachelus and Protero­ rhinus semilunaris – are recordered in the Oster River for the first time. Fish population of the lower part differs by qualitative composition, primarily due to its link with the Desna River. Only 11 species were registered in three tributaries of the Oster River and in their basins. Nosivochka River has 6 species, including introduced Cyprinus carpio and Hypophthalmichthys molitrix. Five species were recorded in the Divytsia River basin and six species are found in the V’iunnytsia River. The most frequently caught species in the Oster River basin were Leucaspius delineatus and Rhodeus amarus (F = 75.0), Scardinius erythrophthalmus and Tinca tinca (F = 66.7).


INTRODUCTION
The Oster River is a left tributary of the Desna River which flows within the Kozelets', Nosivka, Nizhyn and Borzna regions of Chernihiv district. It is 226 km long [2] with the basin area of 2,950 km 2 and the slope of 0.17 m/km. The river has four main tribataries: V'iunnytsia River (26 km long), Divytsia River (20 km long), Rudka River (19 km long), Nosivochka River (14 km long), and 9 tributaries with the length less than 10 km [2]. In addition, a big number of reclamation channels is integrated with it. The Oster River is connected with the Veresoch River (tributary of the Desna River) in the north by the reclamation channels of Smolianka swamp. In the south (at Kozelets' town, see Figure), the Oster River is united with the Trubizh River (from the Dnieper River basin) by the reclamation chanal.
At present, the river is shallow and partially dring up during summer. It is turning into a cascade of reservoirs, especially in the upper part. The Oster riverbed is marshy, regulated by dams which prevent its complete dehydration during summer-autumn period.
The Oster River fish fauna is poorly known. Some fragmentary data on the composition of the Oster River fish populations are presented in the publications of Kessler [3] who revealed Silurus glanis Linnaeus, 1758 in the Oster River, Belynh [1], who registered Rhynchocypris percnurus (Pallas, 1814) and Leucaspius delineatus (Heckel, 1843) and Güldenstäedt (Cited by [1]), who revealed Anguilla anguilla (Linnaeus, 1758) in the Oster River at Nizhyn town. However, Kessler [3] questioned this finding and considered as an erroneous data. In fact, the only work who presenting data on the distribution of fish in this river is an article of Poltavchuk and Shcherbukha [6]. The authors cited a list of 20 species of fish: 19 species of fish listed for lower section of the Oster River (at vicinities of Kozelets' town), 9 -for middle (Mryn village -Nizhyn town) and 7 -for upper one (Ombysh village -Ivangorod village) respectively (Table 1). Also we [8] registered the Perccottus glenii Dybowskii, 1877 in the Oster River basin for the first time, with remarks about some other species.
It should be noted that researchers were often confused about the Oster Riverthe tributary of the Desna River, with other Oster River which is the tributary of the Sozh River. The data on fish fauna of the latter are presented given in publications of the last century [13,14] and some modern works [4].
The aim of this work is to analyze the current state of fish fauna of the Oster River and its changes over the last 30 years, that occurred under the influence of significant changes in the hydrological regime of the river, as a result of land reclamation and anthropogenic loading.

MATERIAL AND METHODS
The original material analyzed in the present work was collected during 2012-2015 in spring-summer-autumn period. Fish were caught in the river channel and in the reclamation chanals adjacent to it, as well as in lakes and ponds located within the drainage basin of the river. Totally 638 specimens of 24 species from 42 points were collected and processed. Places of sampling are marked on the map (see Figure). All specimens were collected using nets and draggers. We have also studied the material from fish collections of Nizhyn Gogol State University [7] and those of the Zoological Department of National Museum of Natural History at the National Academy of Sciences of Ukraine [5]. We used frequency factor (F) which essentially reflects a percentage  of species to the total number of points, to describe their frequency in separate locations. The Sorensen similarity coefficient (S) calculated as S = 2M/(2M+N) where M is the number of matches and N is the total number of columns with presence in just one row, was used to compare species compositions of three sections of the Oster River. The Oster River was divided into three sections [6]: the lower, the middle, and the upper one. Such division is arbitrary because it is almost never associated with the hyd rologic conditions or any other factors. We accept it only for the comfortability of comparison our data with literature (Table 1). However, we consider as an appropriate to attribute river section from the mouth to the first dam to the lower reaches [6].

RESULTS AND DISCUSSION
According to our data, fish fauna of the lower, middle, and upper sections of the Oster River lists 29 species (  , 1758), and B. barbatula in the upper section of the Oster River, though they were listed by Poltavchuk and Shcherbukha [6].
Additionally, our study did not reveale previously listed Rh. percnurus and N. fluvia tilis in the river and its reclamation chanals. Nevertheless, both species are presented in fish collections of Nizhyn State University -they were caught in a lake near Lypiv Rih village in 2005 by Ruhmedeva O.P. (see Fig.).
Three species (Abramis brama (Linnaeus, 1758), Chondrostoma nasus (Linnaeus, 1758) and Leuciscus leuciscus (Linnaeus, 1758) were not registered in the middle and upper sections of the Oster River. They were listed by Poltavchuk and Shcherbukha [6] only for the lower section (Table 1). Also, we have not revealed S. glanis which was noted for the Oster River by Kessler [3].
Fish fauna of three parts of the Oster River was compared by Sorensen coefficient ( Table 2). The similarity of the lower and middle sections was 0.56, lower and upper -0.41, and middle and upper -0.71. Poltavchuk and Shcherbukha [6] have got similar values: 0.64, 0.46 and 0.75, correspondingly. Based on these data, we concluded that fish popu lation of the lower section differs by a qualitative composition, primarily due to its link with the Desna River. Our data confirm this, since the value of the coefficient of Sorensen for similar comparisons is 0.56, 0.41 and 0.71, respectively. Some discrepancies presented in Table 2 can be explained by the fact that we considered the lower part as a free section of the river from the mouth to the first dam (other authors also included a part with tree dams in the lower section). When comparing the quality of fish population in the same areas over the years, including 1988 and 2015 (Table 1), one can see that the similarity between the sections in the direction from lower to upper ones is reduced, while Sorensen coefficient is equal to the lower part of 0.72; only for the top 0.53. It can be explained by much higher species richness in the relevant parts of the Oster River. Table 3 presents data on the composition of fish fauna of the Oster River. The highest number of species was recorded at river section from the Oster River to the mouth (it is actually the entire lower current). 16 species were found in the river at the territory of Nizhyn town ( Table 3). Here, high diversity of habitats is seen and the sampling process was very intensive. The most frequently caught species at this area were L. deli neatus and Rh. amarus (F = 75,0), S. erythrophthalmus and T. tinca (F = 66,7). Thus, they can be attributed to the most numerous industrial species in the Oster River. At some other areas, they can be considered as background species. There was also a number of fish which had only one sample point, mainly in the lower part of the river: L. leuciscus, P. platygaster, G. cernua, B. gymnotrachelus, N. fluviatilis, P. semilunaris, A. brama, C. carpio (noted only by personal data of local fishermen) and introduced Hypophthalmichthys molitrix (Valenciennes, 1844). Obviously, these species could pe ne trate into the studied area from the Desna River. On the other hand, L. delineatus, M. fossilis and some other fish were not registered at lower section, though they are common at middle and upper parts of the Oster River. We also examined three of four tributaries of the Oster River ( Table 3, X-XII) -Nosivochka, V'iunnytsia and Divitsia Rivers (excluding Rudka River). Only 11 species were registered in these rivers and their basins. Nosivochka River has 6 species including C. carpio and H. molitrix which were introduced into the Nosivochka River pounds. Five species were recorded in the Divytsia River basin and six species were found in the V'iunnytsia River. However, the composition of fish fauna of this rivers cannot be considered as constant, because they are subjected to a periodic drying.
Thus, at present fish fauna of the Oster River numbers at least 29 species including 2 introduced species (P. glenii and P. parva) and 4 species (C. carassius, Ch. nasus, L. leuciscus and Rh. percnurus) listed as endangered [11]. We suppose that the ichthyofauna of the lower section of the river can be even more diverse because this part of the river is not separated from the Desna River, as opposed to the middle and upper sections which are cut from the Desna River by numerous dams.