Review of sterlet (Acipenser ruthenus L. 1758) (Actinopterygii: Acipenseridae) feeding habits in the River Danube, 1694–852 river km

The aim of this work is to present the variation in the sterlet’s (Acipenser ruthenus L. 1758) diet along the course of the River Danube, from 1694 to 852 river km. The diet of sterlet mostly comprised 12 bottom fauna taxa. Sterlet in the Danube feed mainly on larvae of Trichoptera, Chironomidae (Diptera) and Gammaridae (Amphipoda), with a reduction in food composition variability close to the Djerdap I dam and in the reservoir between the two dams. The other components of the diet were Corophium sp., Asselus sp., Mollusca, Oligochaeta, Annelida, Insecta, Nematoda and Hirudinea. The organisms typical of lithorheophilic and psammorheophilic biocoenoses play a considerable role in the diet of the sterlet. Composition of bottom fauna as food items varies due to changes in environmental conditions induced mainly by the construction of two dams: Djerdap I (943 river km, 1970) and Djerdap II (863 river km, 1984).


Introduction
The sterlet (Acipenser ruthenus) is a potamodromous and bottom-feeding sturgeon species for which a commercial fishery still exists in the River Danube (Ristić 1970). However, as have other sturgeon species in the River Danube, the sterlet has been affected by a combination of hydropower development, over-harvesting, habitat degradation and pollution (Lenhardt et al. 2006). In the past, the Danube sterlet population extended from Ulm to the Danube delta (Hensel and Holcík 1997). Currently, this species is the most widely distributed sturgeon in the Danube even if its occurrence in the mid and lower sections is limited. Significant changes in sterlet populations were observed in the Serbian part of the River Danube after construction of the Djerdap I [943 river km (rkm), 1970] and Djerdap II (863 rkm, 1984) dams. Mass migration was observed toward upstream regions with faster river flow rates (Janković et al. 1994). Dams are also blamed for a decrease in sterlet catches, preventing their migrations from the lower parts of the River Danube downstream from the Iron Gate gorge (Guti 1995).
The first food that larval sterlet consume are small oligochaetes and larval Chironomidae (Fieszl et al. 2011). The main food of the sterlet in all rivers is benthic organisms. The most important of these are insect larvae, chiefly species of Chironomidae, Trichoptera, Ephemeroptera and Simuliidae. Also included in the diet are larval Plecoptera and Heleidae; small molluscs of the genera Sphaerium, Pisidium and Viviparus; Oligochaeta, Polychaeta, Hirudinea, and other invertebrates (Holčik 1989). During the spawning periods of other fish, sterlet feed readily on the eggs, including those of other acipenserids (Holčik 1989).
Construction of two dams (Djerdap I, Djerdap II) significantly changed the flow regimen in the middle and lower part of the River Danube and the present study was conducted to analyse how this impacted feeding of sterlet in different sectors of the River Danube.

Material and methods
For this work, data were used from four publications (Janković 1958;Janković et al.1994;Fieszl et al. 2011;Streĺnikova 2012) as well as from our material collected from professional fishermen, using nets of different mesh sizes (32-50 mm), at river sectors 1173-1132 rkm during the period 2002-2003. Maps showing nine locations along the River Danube (rkm 1694-852) where diet of sterlet was analysed are presented in Figure 1.
Data concerning total length, total weight, number of examined stomachs and period of catch of the analysed sterlet in the River Danube, 1694-852 rkm are presented in Table 1.
To analyse benthic fauna, frequency of taxa groups (F-%) was presented and calculated as the percentage contribution of the groups to the total abundance at each location (rkm).

Results and discussion
During the investigation period, in different seasons, the stomachs of 850 sterlet were analysed.
Comparisons of sterlet diet in the 1950s and in 1986 in the River Danube are presented in Table 2. The main prey taxa as representatives of aquatic macrozoobenthos in the studied river sectors, after and before dam construction, were Trichoptera, Chironomidae, Oligochaeta, Hirudinea, Gammaridae and Corophium sp. (Janković 1958;Janković et al. 1994).
Bottom fauna groups such as Trichoptera, Chironomidae and Gammaridae were present during both sampling periods, in varying proportions. Corophium sp. was abundant in the 1950s along the investigated sector of the river (1215-991 rkm) (Janković 1958) whereas investigations from 1986 to 2003 revealed Corophium sp. only in the Hungarian part of the Danube (1694-1676 rkm, Fieszl et al. 2011).
The present study on the sterlet's diet in the River Danube revealed that food composition depended on river sector. The larvae of trichopterans and chironomids, along with gammarids and leeches, were the main prey of the studied fish. Molluscs, oligochaetes, amphipods, isopods and nematodes slightly enriched the feeding spectrum of danubian sterlet.
Comparing seasonal aspects of sterlet feeding in the River Danube shows that the same bottom fauna groups participate in its diet throughout the year with some differences in prey proportions according to season. Stomach contents depend on the bottom fauna composition of each river sector as well as of sampling season. In May, Trichoptera, Amphipoda and Hirudinea were most abundant, whereas in September and October, representatives of Chironomidae, Oligochaeta, Gammaridae and Corophium sp. were mainly present in the diet (Janković 1958;Janković et al. 1994; Strelnikova 2012).   important in the downstream sector (Janković 1958;Janković et al. 1994;Strelnikova 2012). The large amount of Gammaridae found in the stomach contents in the downstream river sector is in line with a statement by Holčik (1989), who noticed that in the section of the Volga near its mouth, gammarids play a large role in the diet of the young fish, accounting for over 90% of all contents. Under natural conditions, Acipenser ruthenus feeds mostly on larval Ephemeroptera and Diptera, other aquatic insects that fall onto the water surface, and small benthic organisms including small snails and leeches (Ermolin 1977). Prey selection in sturgeon often follows a seasonal sequence due to prey availability. Hence, the samples from the river sectors above dams, or samples from below the dams, came from different times of the year and different years, as well. Analysis of data on the structure of the macrozoobenthos community before construction of the Iron Gate Dam on the River Danube and after its construction indicates that certain global changes have occurred in the structure of this community. Among these changes are decreases in the abundance of populations of amphipods, some species of gastropods and bivalves, and increases in the abundance of pelophilous and phytophilous forms of the group Oligochaeta and pulmonate gastropods (Simić and Simić 2004). The abundance of amphipod crustaceans, especially species of Ponto-Caspian origin, has declined in the Iron Gate region. Representatives of Gammaridae and Corophium sp. were encountered in later years (Simić and Simić 2004). It can be seen from the results that Ponto-Caspian species such as Corophium sp. is more sensitive than Gammaridae (Simić and Simić 2004). During research conducted under the international project "Joint Danube Survey 1 and 2", Mollusca dominated both in terms of the number of species and their relative abundance, followed by representatives of Chironomidae and Oligochaeta (Paunovic et al. 2010). Janković (1958) showed that Trichoptera, Chironomidae and Amphipoda were present in the sterlet diet throughout the year and represented the main part of its diet and are partly in accordance with works of Rusev (1963), Usynin (1978) and Nagy (1987), which found that the diet comprises mostly larval Chironomidae, Trichoptera and Ephemeroptera, as well as with the most recent studies of sterlet nutrition (Lapkina et al. 2005) which showed that Chironomidae and Hirudinea were dominant sterlet food. A remarkable reduction of variability in sterlet diet composition occurred after the dams' construction and the formation of reservoirs, probably resulting from changes in the invertebrate benthic fauna due to alterations in the environmental conditions (Jankovic et al. 1994). Dam building and habitat destruction highly impacted the sterlet's spawning places through the accumulation of silt, as stated by Strelnikova (2012), as well as its feeding grounds. This work presents a review of sterlet diet in the lower and middle parts of the River Danube, which are impacted by dam building.