THE FAUNA OF ENDOPARASITES IN ACIPENSER RUTHENUS LINNAEUS , 1758 FROM THE SERBIAN PART OF THE DANUBE RIVER

Helminths of sterlet (Acipenser ruthenus L.) from the Danube River were studied as a part of ichthyoparasitological research in Serbia. The fish were collected during the period of 2002 – 2003 along the Danube's course through the Belgrade region. A total of 517 specimens of sterlet of different ages (0+ to 1+) were examined. Helminths were found in 342 or 66.15% of the collected specimens. The examined fish yielded a total of 13 helminth species: four species of Trematoda, one species of Cestoda, four species of Nematoda, and four species of Acanthocephala.


INTRODUCTION
Acipenser ruthenus L. is a potamodromous freshwater species which prefers deep depressions in rivers with stony, gravely, and sandy beds.It originally inhabited Eurasian rivers, being widely distributed in ones flowing into the Caspian, Baltic, Black, and Arctic Seas, and the Sea of Azov.In the past, sterlets regularly occurred in the Danube River up to Vienna and even as far as Ulm.Recently, it has had a very limited distribution in the middle and upper Danube (H e n s e l and H o l c i k , 1997; R e i n a r t z , 2002; S o k o l o v and Va s i l e v, 1989).
In Serbia, A. ruthenus occurs in the Danube and lower parts of its main tributaries, the Sava and Tisza Rivers (J a n k o v i ć , 1993).This fish is the smallest of all Danubian sturgeon species (A n o n y m o u s , 1997).Significant changes in sterlet populations were observed in the Serbian part of the Danube River after construction of the Iron Gate dams: I (943 km) and II (863 km) (J a n k o v i ć et al., 1994; L e n h a r d t et al., 2004).The gives species is the only sturgeon which is a still numerous in the Serbian part of the Danube.
In this study, we establish the species of endoparasites in the sterlet (A.ruthenus L.) during 2002 and 2003, and possible differences between the examined sites on the Danube River in the Belgrade region.

Study Area
The catchment area of the Danube River in the Belgrade region is part of the middle sector of the Danube basin -the largest sector of the river's watercourse from Bratislava to the Iron Gate dams (Serbia/Romania).With its main tributaries, the Danube represents the most significant Serbian water resource.The Danube this part flows through a densely populated area, and a permanent risk of pollution is present.
The sampling area covered five sites along 49 km of the Danube River (Fig. 1

MATERIAL AND METHODS
We examined a total of 517 sterlet specimens, which were captured using nets with different mesh size (32 -50 mm).The material was analyzed using standard parasitological procedure.The stomach and intestines were examined for determination of the helminth fauna.During the parasitologi-During the parasitological examination, the intestines were cut open and examined under a stereomicroscope.The parasites The parasites found were bleached, stained, prepared, and fixed for determination and collection.
For isolation, sorting, and identification of the parasite fauna of sterlet, we used "Olympus BO61" binoculars and an "Olympus CO-01" microscope.We also analyzed the extensity and intensity of parasitic infestation of the fish specimens.

RESULTS AND DISCUSSION
The length of all captured sterlets ranged from 220 to 300 mm, their weight ranged from 13 to 30 g for juveniles; and from 300 to 380 mm and 351 to 370 g, respectively, for one-year-old specimens.Endohelminth parasites were found in 342 or 66.15% of all captured sturgeon specimens.Thirteen species of parasites belonging to four classes were registered in all.The highest percentages share of infected sturgeons were recorded at the Zemun and Dorćol sites: 87.13 and 64.21 %, respectively.
The species of endohelminths and intensity of parasitic infection per investigated sites are presented in Table 1.
According to M o r a v e c et al. (1989), three of the mentioned parasite species are specific for representatives of Acipenseridae -Skrjabinopsolus semiarmatus, contracaecum bidentatum, and Amphilina foliacea.K a k a c h e v a -Av r a m o v a (1983) cites the named species as well as the species cystoopsis acipenseris.The other parasite species found in the examined sterlets have many other fish hosts.
There has been much relevant research on the fauna of fish parasites in Danubian waters.In this paper, we present the data of: M o l n a r ( 2006 2004)] contain little information about the endohelminths of sterlets, and a few data are cited here (C a k i ć , 1986a, 1986b).The cited author analyzed the parasites of sterlets in the Serbian part of the Danube River and reported the presence of three parasitic helminth species -Skrjabinopsolus semiarmatus, contracaecum bidentatum and Acanthocephalus anguillae.
The catch of A. ruthenus in the Belgrade region of the Danube River nowdays is mostly composed of one-and two-year-old specimens, while three-yearolds are rare, a fact conformed by our results.
The intestines of the sterlets from the Danube River in the Belgrade region contained numerous and diverse representatives of helminths, which can be a consequence of pollution and poor water quality.The saprobic status of the Danube in the Belgrade area can be judged from the fact that the bottom fauna corresponds to β-meso-to α-mesosaprobic conditions, according to WQ M P.
The main purpose of our research was to study the helminth fauna of Acipenser ruthenus L. along the Serbian stretch of the Danube River in the Belgrade region.In our opinion, it is necessary to continue these examinations in order to determine the role of helminths in regulating the population density of sterlet.
Our investigation showed that the degree of sterlet parasite infestation is very high and that further examinations are needed to determine the role of helminth parasites in regulating the population density of sterlet.

Fig. 1 .
Fig. 1.Sampling sites along the Danube River in the Belgrade region.
Appropriate identification keys were used for determination of representatives of the parasite fauna to the lowest taxonomic level (B a u e r, 1984, 1985, 1987; B y k h o v s k a y a -P a v l o v s k a y a et al., 1962; L o m and D i k o v a , 1989; M o r a v e c , 1994).
); M o l n a r et al. (2006); M o r a v e c et al. (1999); M o r a v e c (2001); N e d e v a (2004); O n d r a c k o v a et al. (2005); and T h i e l e n et al. (2004).