Dr. Murai Éva szerk.: Parasitologia Hungarica 25. (Budapest, 1992)

4 protozoan and 26 metazoan parasites of the sterlet (Acipenser ruthenus L.) described until then. Skrjabina (1974) wrote a review article on helminths described from acipenserids. Numerous data are available in the literature on the parasite fauna and diseases of acipenserid mother fish caught for artificial propagation, as well as of the acipenserid fry obtained from that propagation (Shestakovskaya 1981). While in the mother fish mainly the helminth infections acquired in natural waters present an animal health problem, the diseases of fry often supervene on indicator diseases, e.g. the excessive multiplication of ectoparasites on the body surface and gills. The present paper reports on cases in which a snail commensalist oligochaete caused total mortality of artificially propagated sterlet larvae. MATERIALS AND METHODS The studies were performed in the fish hatchery of TEHAG (Százhalombatta) on sterlet larvae kept in appr. 100-litre tanks with throughflow water, in the years 1991 and 1992. Water throughflow was ensured by a water-pipe system (Fig. 1) according to the technology applied on the farm. Water temperature was between 16 and 18 °C. Feeding of the larvae with tubifex crushing and starter feed was commenced after their first breath-taking. Larvae were regularly transported to the laboratory and examined for the presence of ectoparasites under stereomicroscope. Impression smears made from the body surface and from the surface of organs after opening the body cavity were fixed in methanol, stained with a tenfold dilution of Giemsa solution, then examined in a light microscope. The oligochaetes collected from the body surface and gill cavity of the larvae were immobilized with 5% MgCh and identified by microscopy after adding a drop of Amman-lactophenol (Ferencz 1979). To obtain permanent preparations, the helminths were fixed in 70% ethanoL Physa acuta snails occurring in the water-conduits and tanks in large numbers were also sampled and examined for the presence of Chaetogaster limnaei. From the active ingredients (formaldehyde, mebendazole and trichlorphon) used for the medication trials, stock solutions were prepared and added to the water. The treatments were carried out in the form of 30- and 60-minute exposures. RESULTS In 1991, appr. ten thousand larvae died before or immediately after taking the first breath, when starting to feed. If the infestation was less severe, the larvae were seen restlessly swimming close to the surface, their swimbladder did not contain visible air-bubbles, and gas bubbles were often present in their primitive oral cavity instead. Severely affected, exhausted larvae were lying on the bottom of the tank, swam less