Seasonal shifts in collective tendencies of floodplain-associated schooling fishes

Seasonal flood pulses are a key driver of environmental variability in large river-floodplain ecosystems, yet their influence on the behavior of many floodplain associated fish remains poorly understood. We used high-resolution imaging sonar to test the hypothesis that, despite the irregular and episodic nature of inundation in the Lower Mississippi River Basin, floodplain-associated schooling fishes would exhibit shifts in collective tendencies in response to changes in floodplain connectivity. As schooling is known to be an adaptive strategy, greater behavioral plasticity would allow schooling individuals to appropriately react to changes in environmental conditions, predation risk, and foraging opportunities. We recorded 56 hours 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 (polarization), and inter-individual distance (nearest-neighbor distance) were quantified from 5,085 schools comprising 120,114 individuals using a semi-automated approach. School areas did not differ across conditions, except for smaller schools observed at near sites during disconnected-fall. Fish swam in a more aligned fashion during connected-summer, while inter-individual distances were lowest during connected-spring and disconnected-summer. Altogether, our results indicate schooling fishes inhabiting the floodplain exhibit behavioral changes at the level of their collective in response to changes in the local environmental conditions. Given the inter-annual variation in flood pulse dynamics, unravelling the functional explanations (e.g., antipredatory, foraging) of the observed modifications of schooling tendencies mediated by water level and river-floodplain connectivity will require more work.