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The aim of this study was to explore whether the widespread zooplankter Daphnia magna is able to differentiate between the kairomones of planktivorous common bleak (Alburnus alburnus) and omnivorous black bullhead (Ameiurus melas). We expected different responses in some plastic defensive traits because the presence of a planktivorous and an omnivorous fish pose a different degree of threat to daphnids. Our mesocosm study showed that the size distribution of D. magna in the bleak treatment tended to differ from both black bullhead and control treatments, while there was no apparent difference between the latter two treatments. The eye diameter/body length ratio and tail spine/body length ratio of D. magna were highest in the bleak treatment, differing from both black bullhead and control treatments, and without any significant difference between the latter two treatments. We found that only bleak triggered detectable responses in D. magna, suggesting either that kairomones of the studied fish species could differ in their chemical composition, or that black bullhead may exude less kairomone than bleak. [ABSTRACT FROM AUTHOR]