Hiding in the bush, in fear of a predator! Vegetation and predators influence shoaling among wild zebrafish

Social responses of aquatic group-living organisms are critically dependent on predation risk and vegetation cover in their habitat. To gain insight into how these factors shape their immediate responses, we exposed wild zebrafish shoals to either vegetation, predator cues or both these factors simultaneously. Control treatments were not exposed to the above factors. By analyzing 60 unique shoals across 270 experimental trials, we found that while shoals formed significantly larger subgroups and were significantly more polarized in the presence of predator cues, both these properties decreased when shoals experienced predator cues in presence of vegetation. Furthermore, foraging was significantly lower when predator cues and/or vegetation were present. Tracking of all individuals in treatments devoid of vegetation revealed that: (i) compared to control treatments, individuals within shoals receiving predator cues had a significantly higher probability to continue being in a group and (ii) individuals occupying the front positions deviated lesser from their median position within a shoal as compared to other individuals. Anti-predator responses and foraging are critical for survival and therefore, this study provides important insights into shoal dynamics in changing environments.