In a four year experiment (1988)(1989)(1990)(1991) carried out in a shallow, eutrophic lake, an increase in youngof-the-year pike (Esox lucius) density was used to control planktivorous fish assemblages consisting mainly of small-sized cyprinids: roach (Rutilus rutilus), Leucaspius delineatus, and white bream (Blicca bjoerkna). Stocking with pike fry of ca 30 mm in three successive springs resulted in large-scale mortality among prey of vulnerable sizes. Rotenone treatment at the end of the fourth year showed that roach and white bream populations contained few 0 + to 2 + fish, but very high numbers of 3 + to 6 + fish. By this time, short-lived, slow-growing L. delineatus, with no size refuge from predators, had been driven almost to extinction.

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A whole-lake experiment was carried out for seven years (1988)(1989)(1990)(1991)(1992)(1993)(1994) in a shallow, eutrophic lake. In the first phase (1989)(1990)(1991), repeated introductions of young-of-the-year pike (Esox lucius) were used to control the density of juvenile stages of the dominant planktivores (roach Rutilus rutilus, white bream Blicca bjoerkna and Leucaspius delineatus). The successive introductions of juvenile pike were accompanied by selective removal of large pike, roach, bream Abramis brama and white bream. No visible improvement in water quality was registered in the first three years. Assessments made after the lake was treated with rotenone, revealed the high efficiency of juvenile pike in controlling prey of vulnerable size. Though the first three age-classes were nearly exterminated, older fish (3+ to 6+) remained abundant enough to keep their planktonic prey in check. Indeed, the non-altered density of dominant cladocerans indicated that they were still heavily preyed upon. Algal biomass remained high.In October 1991, rotenone was applied to remove all fish. Immigrating fish were gradually recolonizing the lake from spring 1992 onwards. However, the process was kept under control by continued stocking with juvenile pike and intensive fishing. The years following the rotenone treatment witnessed a considerable improvement in water quality, with water transparency 30% higher, a significant 2.2-fold decrease in seston dry weight and 2.8-fold decrease in algal biomass, averaged for the summer months. These events can largely be attributed to substantial changes in the herbivorous zooplankton, above all increased density of Lake Wirbel's largest cladoceran, Daphnia cucullata.

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The presence of A. crassus larvae in herring, stickleback, perch, ruffe, pike-perch, Mots-clés : Anguillicola, cormoran, poisson proie, Baie de Vistule.

Chair of Zoology, Academy of Agriculture and Technology in Olsztyn Studies on food composition of two unexploited species, ruffe and gudgeon, were carried out in 1982 and 1984 in Wl:ocfawek Darn Reservoir. Diet of both species was essen tially composed of Chironomidae larvae, Cladocera and Copepoda. Diet composition of ruffe and gudgeon suggests that the two species grazed mostly in shallow water, with ruffe penetrating the bottom, and gudgeon feeding in areas overgrown with submerged plants.

The paper presents the results of studies on spined loach-Cobitis taenta (L.) diet in Zegrzynski Dam Reservoir. Food composition of the fish collected at four stations was analysed and compared. The stations differed as to the environmental conditions. Changes of diet components in particular months were described for two sampling stations. Habitat conditions prefered by spined loach were described on the basis of the re sults and the literature data. A description was also given of its feeding grounds in the reservoir, and of the food of this fish in different places of its range of occurrence.