The role of chemical communication in the predator-prey role reversal of northern pike (Esox lucius) and three-spined stickleback (Gasterosteus aculeatus)

Predator-prey interactions can modify population dynamics and influence ecosystem functioning. An interesting, but less understood, aspect of such species interactions is predator-prey role reversal where a mesopredator can reduce the recruitment success, and ultimately the abundance, of their own predator. Such role reversal is ongoing in the Baltic Sea where increasing abundances of mesopredatory sticklebacks (Gasterosteus aculeatus) have resulted in intense predation on the recruits of their predator the northern pike (Esox lucius). Still, the mechanistic underpinning of this reversal remains poorly understood. Here we investigate the role of chemical communication in this predator-prey role reversal with the aim to understand whether and how sticklebacks use olfactory cues to assess whether pike constitutes a predator or prey. By performing a laboratory preference experiment, we show that chemical communication is indeed used by sticklebacks for assessing the predation risk emanated by adult pike but that it requires additional alarm cues (Schreckstoff) informing that sticklebacks are predated upon. Adult pike kairomones or alarm cues alone did not result in any adaptive response by sticklebacks nor did olfactory cues of pike as prey (pike larvae kairomones and dietary cues of zooplankton). This knowledge contributes to the understanding of predator-prey interactions as well as the dynamics of the shifting coastal fish community in the Baltic Sea. The results also suggest that addition of Schreckstoff to confined pike (and perch) spawning and recruitment habitats may deter sticklebacks from entering which should be further explored by management.