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

The earliest studies were carried out by Rátz (1897) who reported the occurrence of some protozoan, metazoan and crustacean parasites in the lake. Subsequently, data on the occurrence of parasites were contained in the works of Jaczó (1940, 1941, 1949), who also described some new myxosporean species from fishes. Systematic studies on the parasite fauna of Lake Balaton were performed by Molnár (1962, 1964, 1966a, b, 1968, 1969, 1970a, b) who provided data primarily on the occurrence of helminth parasites. Of the other authors, Geyer ( 1939a b) and Ponyi and Molnár (1969) reported on the crustacean parasites occurring on Lake Balaton fishes. Faunistic studies have been conducted by Matskási (1967, 1968), Matskásietal. (1971), and Mészáros (1967, 1968). Mödlinger (1934) reported the frequent incidence of Apophallus donicus while Molnár et al. (1968) that of Ligula intestinalis. Relatively little is known about the protozoan infections of Lake Balaton fishes. Although the occurrence of some species has been described by Molnár (1970a, b), instead of conducting a systematic faunistic survey only the development of some species has been studied and some new species have been demonstrated (Molnár 1988, 1989). A new survey of the parasite fauna was called for by the massive eel mortality that occurred in Lake Balaton in 1991-1992, which Molnár et al. (1991, 1993) attributed to the swimbladder injury caused by the introduced eel nematode Anguillicola crassus. Of the fish species living in Lake Balaton, the pike-perch, which is the "heraldic fish" of the lake, and the introduced eel have outstanding economic importance. Other fish species whose health status can affect the biological balance of the lake include the bream (which accounts for 80% of the fishermen's catch), the ruffe and bleak serving as pike-perch food, and the river goby and gibel carp which are new arrivals (Bíró 1995). The aim of this work was to detect the parasites of these fish species and to collect data on their seasonal occurrence. MATERIALS AND METHODS The studies were conducted in 1994, in the fishing season which lasted from 16 April to 9 November. On a total of 12 occasions, 299 fish belonging to 15 different species were collected at several sampling sites of the lake (Alsóörs, Sóstó, Csopak, Tihany, Szemes, Keszthely) and subjected to complete parasitological examination. Of the species included in our test protocol higher numbers of fish were dissected: thus, the results contain parasitological data of 55 pike-perch (Stizostedion luciopercd), 29 Volga pike-perch (Stizostedion volgense), 82 eels (Anguilla anguilla), 60 breams (Abramis brama), 18 gibel carp (Carassius auratus gibelio) and 27 river gobies (Neogobius fluviatilis). The distribution and numbers of the fish species of which only a few specimens were examined were as follow: sheatfish (Silurus glanis) 4, ruffe (Gymnocephalus cernuus) 4, knife (Pelecus cultralus) 3, white bream (Blicca bjoerkna) 1, common carp (Cyprinus carpio) 4, grasscarp (Ctenopharyngodon idella) 1, silver carp (Hypophlhalmichthys molitrix) 2, tench (Tinea tinea) 2, and asp (Aspius aspius) 1. The body length of the fish species studied was not uniform (it ranged between 10 and 75 cm). The fish derived from the fishermen's catch included bigger specimens while those caught with a small net were of smaller size, due to the use of nets of different mesh sizes. The species distribution of the fish studied at the individual sampling times was unfortunately not uniform, as it depended on the catch. Most of the fish were caught from the basin of the lake with the help of the researc'i boat of the Limnological Research Institute of the Hungarian Academy of Sciences and of