PARASITES OF PIKE PERCH STIZOSTEDION-LUCIOPERCA (LINNAEUS 1758) FRY REARED IN 2 DIFFERENT TYPES OF NATURAL FOOD PONDS IN SOUTHERN FINLAND

Pike perch fry ( < 4 months old) reared in two different types of natu ral food ponds were examined for parasites in Rauma and at Porla, sout hern Finland, during June-August in 1990 and 1991. The ponds differed in size and drainage. The highest number of observed parasite species per sample was 11 at Porla and 4 in Rauma. Altogether 4 species of pro tozoan parasites were detected on the skin and/or gills: Ichthyobodo n ecator, Ambiphrya ameiuri, Trichophrya piscium and Trichodina sp. The two major species were I. necator and A. ameiuri. The intensity of I. necator infestation was mostly rather low. Infestation by A. ameiuri w as very heavy at Porla in 1990. In July, the skin and particularly the fins were filled with this ciliate and the fins were frayed. Ten spec ies of helminth parasites occurred at Porla: Diplostomum sp. (lens), D iplostomum sp. (vitreous humour), Tylodelphys clavata, Bunodera luciop erae, Proteocephalus sp., Eubothrium sp., Raphidascaris acus, Argulus foliaceus, Piscicola geometra and Nematoda sp. Only two helminth speci es, Diplostomum sp. in lens and Diplostomum sp. in vitreous humour, we re found in the pike perch in Rauma. The composition of the parasite s pecies and their influence on cultured pike perch in the natural food pond seemed to be connected to various environmental factors. Some ind ication was obtained for Ambiphrya ameiuri-induced mortality. The resu lts of this study indicate that drainage of the pond between the growi ng seasons, minimizing the number of fish species in the pond and havi ng sufficient nutrition are good methods for keeping the parasite load small.