Dr. Murai Éva - Gubányi András szerk.: Parasitologia Hungarica 28. (Budapest, 1995)

of the River Danube can be found in works of Roman (1955) and Vojtek (1959), while those for the River Tisza have been reported by Edelényi (1967) and Ergens et al. (1975). The results of this study indicate that the parasite fauna of fishes of the Palaearctic region is rather uniform. The fact that in Lake Balaton we could not detect numerous parasites recorded from other places, reflects differences between the intermediate host organisms living in the different habitats, rather than actual differences between regions. This work involved a larger sample of fish of the greatest economic and biological importance. Therefore, the data obtained on such species enable a more reliable evaluation of the parasite fauna than do the dissection data of fish of which a smaller sample was examined. However, the interpretation of even the former data requires some caution. The presence of endoparasitic helminths is always a reliable indicator of the degree of infection. At the other side, data concerning protozoans, crustaceans and monogeneans freely moving on the skin and gills must be evaluated critically. Namely, in laboratory the number of these organisms usually decreased if the fish were kept in flow-through water, while in aerated aquaria it increased. The parasites found generally correspond to those demonstrated by Molnár (1962, 1964, 1966a, 1966b, 1968, 1969, 1970a, 1970b), Matskási (1967, 1968), Matskási et al. (1971) and Mészáros (1967, 1968). However, the data obtained on the protozoan fauna so far indicate that the occurrence of several species hitherto not described or not detected in Lake Balaton should be reckoned with. The results of this study suggest that in some respects the parasite fauna of Lake Balaton fishes is characterized by stability, which is reflected in the continuing presence of species that were demonstrated decades ago. On the other hand, the changes occurring in the life of the lake, such as the immigration or introduction of new fish species, have led to the appearance and, in some cases, mass multiplication of new parasites. Of parasites which are outstanding because of their permanent presence, the commonest species is E. sieboldi which establishes itself on the gills and even on the operculum of the pike-perch. That parasite had already been described by Geyer (1939a). It was also Geyer (1939b) who reported the periodic occurrence of, and the skin lesions caused by Tracheliastes maculatus on bream. Already Jaczó (1940) pointed out the high prevalence of Myxobolus spp. living on the gills of the bream, although the spores found by us could not yet be identified with the species M. hungaricus and M. variabilis described by him. From Perca fluviatilis, Jaczó (1940) also detected Henneguya cysts which probably belonged to the species recorded by us from pike-perch as H. creplini. Molnár (1966a) mentions it as a characteristic feature of the Lake Balaton bream that the female specimens of Ph. ovata can reach their full maturity only in fish infected by the plerocercoid of Ligula intestinalis, while in other fish they remain in a stunted, "retarded" state. In the majority of cases, this rule applied also in the present studies; however, the two cases when larva-containing females were found also in breams not infected by Ligula suggest that completion of the cycle is possible also in Ligula-free fish. In the pike-perch and Volga pike-perch the most typical finding was the mass of Ichthyocotylurus metacercariae which established themselves in the abdominal cavity, primarily on the surface of the heart. This is obviously related to the increase of the bird population in the Lake Balaton region. As regards the new parasites demonstrated from Lake Balaton, the most important findings were the mass occurrence and pathogenicity of A. crassus in eel (Molnár et al. 1991, 1993, 1994, Székely et. al. 1991) and the high prevalence of its paratenic larvae in other fish species (Székely 1994, 1995). At the same time, it was surprising to see that the