Fish predation cues induce drifting and emergence in an experimental stream mesocosm system

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Abstract Predator–prey interactions are important drivers of adaptation in aquatic communities, shaping the behavior of invertebrates with cascading effects on community dynamics. Behavioral responses, such as moving with the downstream current (drift) or altering the timing of emergence, are strategies that reduce the risk of predator encounters. These prey responses have been extensively studied over the last decades. However, much of the previous research was conducted in laboratory settings or focused on individual taxa, potentially overlooking the complexity of natural predator–prey dynamics. To address this, we here examined the effects of indirect and direct predation pressure on drift behavior and emergence patterns in a natural macroinvertebrate community using an outdoor mesocosm system ( ExStream ). This mesocosm system enables a more ecologically realistic assessment of these interactions. We analyzed how the number and size of drifting invertebrates and emerging insects changed with elevated levels of chemical fish cues (indirect predation, Gasterosteus aculeatus and Cottus rhenanus ) and subsequent additional direct predation by C. rhenanus over 9 d. Within the framework of the ExStream system, we demonstrated that most invertebrate groups displayed increased drifting behavior to indirect, direct, or both forms of predation. Furthermore, our study revealed a significant predator impact on emergence patterns, reducing the number and size of emergent insects. Drift and emergence are key processes in invertebrate dispersal and contribute to the transfer of resources to downstream habitats and adjacent terrestrial systems. Our findings show that predation affects both processes, altering community dynamics and potentially reshaping resource availability across ecosystem boundaries.

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