Eric von Elert

The fish‐derived bile salt 5α‐cyprinol sulfate (CPS) has been identified as a kairomone inducing the predator avoidance behavior “diel vertical migration” (DVM) in Daphnia magna in response to fish. However, conclusions about the ecological significance of CPS have been derived from laboratory experiments only. Using a mesocosm approach, we investigate whether the role of CPS as a kairomone can be confirmed in the field. We demonstrate that CPS induces downward migration during daytime in a field‐derived Daphnia community mainly consisting of Daphnia longispina and Daphnia cucullata within the experiment. In the study lake, the actual population of D. longispina shows a similar pattern of DVM, while concomitant quantification of CPS by HPLC‐MS confirms that CPS in situ concentrations are sufficiently high for induction of daytime downward migration of D. longispina in this oligo‐mesotrophic lake. Together, these observations infer that CPS is a significant kairomone‐inducing DVM‐like behavior in Daphnia .