Baseline activity and shoal type determine antipredator behaviors in bluegill from a southern Ontario lake

Abstract

Shoaling is an evolved behavior in fishes that has several adaptive advantages, including allowing individuals to avoid predation through risk dilution. However, factors such as size disparity and the presence of heterospecifics may influence the behavior of individual fish within shoals following exposure to elevated predation risk. Using bluegill Lepomis macrochirus as a model species, we measured changes in area use, shoaling index, and movement of a focal individual in isolation, in single-species shoals with two conspecifics, or in mixed-species shoals with two congeneric pumpkinseed L. gibbosus. The experimental shoals were exposed to one of three chemical cues selected to present graded levels of risk: lakewater controls (lowest risk), Northern pike Esox lucius predator odor (kairomones; intermediate risk), or conspecific chemical alarm cues (highest risk). Within the individual bluegill, we found that the multivariate response of area use and post-stimulus activity (line crosses) of the focal fish was significantly influenced by pre-stimulus activity, but not by cue type or fish size. As univariate responses, post-stimulus activity varied positively with pre-stimulus activity. Post-stimulus activity was greater in single-species shoals compared to mixed-species shoals, and again varied positively with pre-stimulus activity. Contrary to predictions, bluegill did not demonstrate graded antipredator responses to the chemical cues. Our findings suggest that prey fish may alter their risk-aversive behaviors in response to chemical stimuli based on shoal composition and provide further insight into the role of intra-prey guild interactions in response to predators in co-occurring prey species.Significance statementWhen faced with predation, individuals in groups may experience lower levels of risk than solitary individuals. Using bluegill as a model organism, we examined how antipredator behaviors, in response to chemical cues indicating different levels of risk, varied between focal individuals as singletons and in single- and mixed-species shoals. Contrary to our prediction of graded stepwise responses indicative of differing levels of risk posed by the cues, we found that individuals with greater baseline activity levels demonstrated weaker antipredator responses independent of cue type, while fish in single-species shoals had higher post-stimulus activity levels than fish in mixed-species shoals. Our results suggest that studies examining changes in antipredator and shoaling behaviors will benefit from including ecologically relevant scenarios involving sympatric prey guild members.