Seasonal Shifts in Collective Behaviour of Floodplain‐Associated Schooling Fish

ABSTRACT Seasonal flood pulses are associated with environmental variability in large river‐floodplain ecosystems, yet the influence on schooling fish remains poorly understood. We used high‐resolution imaging sonar to conduct the first in situ assessment of collective behaviour in a Lower Mississippi River Basin floodplain habitat in order to test the hypothesis that floodplain‐associated schooling fish alter their collective behaviour in response to changes in floodplain connectivity. As schooling is known to be an adaptive strategy, greater behavioural plasticity would allow schooling individuals to appropriately react to changes in environmental conditions, predation risk and foraging opportunities. We recorded 56 h of video across sites located near and far from the initial point of floodplain inundation during connected (high‐water) and disconnected (low‐water) periods spanning all four seasons. School area, alignment (polarisation) and inter‐individual distance (nearest‐neighbour distance) were quantified from 5085 schools comprising 120,114 individuals using a semi‐automated approach. Smallest school areas were observed at near sites during disconnected‐fall. Schools at the far sites during connected‐summer were typically of a larger area than disconnected‐summer at near or far sites. Fish swam in a more aligned fashion during connected‐summer, while inter‐individual distances were lowest during connected‐spring and disconnected‐summer. Results suggest schooling fish inhabiting the floodplain exhibit behavioural changes at the level of their collective in response to changes in the local environmental conditions. Our findings extend the flood pulse concept beyond traditional assemblage‐level response by demonstrating that floodplain connectivity is associated with shifts in the collective behaviour of schooling fishes inhabiting this dynamic environment.