Swim or hide: predator cues cause species specific reactions in young fish larvae

The anti‐predator behaviour of first‐feeding (9 mm total length) hatchery‐reared pike Esox lucius larvae and wild‐caught three‐spined stickleback Gasterosteus aculeatus larvae was studied in the presence of chemical and visual signals from a fish predator. The results clearly showed that both fish species detected the predator by chemical signals alone but the combined chemical and visual signals caused stronger and more diverse reactions. Subsequent to predator detection, their swimming activity decreased, they attacked zooplankton less frequently and spent more time in the vegetation. Fishes differed in their anti‐predator responses. Pike reacted more clearly to chemical signals alone while three‐spined stickleback needed both chemical and visual cues to assess the predation risk. The strongest reaction was the reduction in swimming activity in the three‐spined stickleback (38% decrease) and a decrease in attack rate of the pike (39% decrease), but only when a refuge was available. Pike were more dependent on the vegetation cover showing almost no anti‐predator responses in the absence of a refuge. In addition, there was a difference in the refuge use of three‐spined stickleback between different macrophytes, indicating a complex or dense structure, which was difficult to penetrate or chemical excretion in one of them.