The coastal fish community in the Baltic Sea has changed drastically during the last decades, including a pronounced decline of the top-predator northern pike (Esox lucius) and exponential increase of the mesopredator three-spined stickleback (Gasterosteus aculeatus). One consequence of this regime shift seems to be that the three-spined stickleback impairs the capacity of pike populations to recover, attributed to a predator-prey role reversal. It has been shown that the mesopredator can reduce the recruitment of pike, suggesting the potential to shape the abundances and size distributions of their own predator. Despite the potentially major ecological and evolutionary consequences of this predator-prey role, underlying mechanisms are still rarely understood.

Here, I conduct a laboratory preference experiment to investigate the role of chemical communication in the predator-prey role reversal, especially focused on the observed aggregations of three-spined stickleback outside wetlands used for spawning by anadromous pike in the Baltic Sea. 

The data suggest that three-spined stickleback do not use chemical information released by juvenile pikes to locate the spawning grounds. Since previous studies show that stickleback aggregate outside the wetlands before the pike larvae started emigrating (i.e., before visual detection of the larvae is possible), the results question whether pike recruitment is the driver of the observed three-spined stickleback aggregations.

Further, the data suggest that three-spined sticklebacks need alarm pheromones from conspecifics to identify adult pike as predator using olfaction. Since pike cannot predate on three-spined sticklebacks within the wetlands (three-spined stickleback are not able to enter the wetland), no alarm pheromones are released from the wetland. This may explain why stickleback do not avoid aggregating outside the wetlands that inhabit adult pike. 

These findings suggest that chemical communication is an important source of reliable information for stickleback when assessing predation risk emanated from adult pike, however, not for detecting juvenile pike. This knowledge helps to understand the underlying mechanisms of the predator-prey role reversal and thereby contributes to the understanding of the dynamics of the shifting coastal fish community in the Baltic Sea.