Background. Aparasitological survey was carried out in a shallow, eutrophic Oświn Lake (north-eastern Poland), within the Seven Islands ornithological reserve. We intended to compare the occurrence of eyeflukes in fish from two, distinctly demarcated pools of the lake, differing in environmental conditions. The effect of fish size and sampling season on the infection parameters was analysed and the present state of the parasite faunas was compared to the literature data. Materials and Methods. Within 1998 and 1999, a total of 1091 fishes representing 8 dominant species was examined. Samples were collected four times a year (in May, July, August, and October) simultaneously from the eastern- and western parts of the lake. Results. Diplostomum spp. occurred in all the fish examined, numerously in roach, rudd, white bream, and carp bream. The parasite was less abundant in northern pike, crucian carp, and European perch. Tylodelphys clavata was found in roach and it sporadically occurred in northern pike, rudd, white bream, carp bream, and European perch. The infection rates of rudd and white bream in the eastern part of the lake were significantly higher than the respective values from the western part. The infection with both eyeflukes did not correlate with the fish length. The infection of roach, rudd, white bream, and carp bream with Diplostomum spp. and roach with T. clavata varied significantly in different experimental periods. Conclusion. The eyeflukes as well as P. cuticola were sensitive indicators, reflecting the environmental pressure-both, spatial differences in ecological conditions within the same water body and long-term alterations. Diplostomum spp. did not accumulate with the age of fish. No seasonality was found for Diplostomum spp. or Tylodelphys clavata. Locationdependent occurrence of Diplostomum spp. in rudd and white bream indicated a limited fish interchange between the two pools of the lake.
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