Abstract. The aim of this study is to examine the size selectivity of the pikeperch fed with one of the most widespread weed fish in fish farms, such as topmouth gudgeon (Pseudorasbora parva Temminck & Schlegel). Topmouth gudgeon (Pseudorasbora parva Temminck & Schlegel), separated in two size groups: large -group 1 (body weight of the specimens 8. 63 ± 2.68 g, total length 9.58 ± 0.92 cm, and During the experimental period it was observed that pikeperch preferred to feed on smaller individuals, the difference in the number of consumed small and large fish is approximately 2 times (70 to 34, respectively).Keywords: pikeperch (Sander lucioperca L.), prey, topmouth gudgeon (Pseudorasbora parva Temminck & Schlegel), size, selectivity, food AGRICULTURAL SCIENCE AND TECHNOLOGY, VOL. 9, No 3, pp -, 2017 209 212 DOI: 10.15547/ast.2017 IntroductionPredation as a form of relationship between different species is essential for the formation of fish populations. Predators can affect not only species composition, but also the size structure (Bronmark et al., 1995). The presence of predatory fish in water bodies affects significantly the entire ecosystem and can change the feeding behavior or habitat preference of other species (Magnehagen, 2006;Persson et al., 1996).The direct effect of their presence is expressed in reduction in the number of preys, important in this respect being the selectivity of eating, which is defined as total quality for predators (Beyerle and Williams, 1968;Ivlev, 1977; Hart and Hamrin, 1988; Einfalt and Wahl, 1997).The interaction between predator and prey in aquatic ecosystems is highly dependent on their size (Paine, 1976;Olson, 1996). According to Nilsson and Bromark (1999), pike (Esox lucius) prefers to eat smaller prey when there is such choice presented. It is believed that this is determined by the lower risk of failure to catch them. Those who are larger swim vigorously and cause real danger of kleptoparasitism (Nilsson and Bronmark, 1999;2000;Tuersson et al., 2006). The opposite conclusion is reached by Hubenova et al. (2013), who found that pike prefers to feed on larger specimens.The maximum size of prey that predator may eat depends on the morphological constraints of the oral apparatus (Werner, 1974;Nilsson and Bronmark, 2000). In this connection, from entirely anatomical viewpoint the pikeperch has significantly less capacity compared to European pike and Wels catfish.Essential for the choice of prey, except for size, is the shape and height of the body, swimming speed, accessibility, level of satiation of predators, the existence of competition, etc. According to Petrova et al. (1993;1993a) the food spectrum of pikeperch is a reflection of the species composition, abundance, biological and physiological state of the ichthyofauna in the environment. In practice, this choice is determined by a complex set of factors that occur in specific conditions.The issue of selectivity of pikeperch (Sander lucioperca L.) with respect to consumed fish species, including in terms of their ...