Turbulence effects on free and anchored zooplanktivorous fish in coral reefs

Water flow is a key driver of fish behavior and metabolism, which can consequently alter fish distribution and community structure in coral reefs. Previous studies of fish behavior have predominantly addressed uniform flows, overlooking turbulence despite its potentially complex effects. Here, we investigated effects of small-scale turbulence with unidirectional flows on feeding by two types of zooplanktivorous reef fish: free-swimmers (damselfish) and anchored fish (garden eels). Flow speed and turbulence intensity were measured in situ and partly reproduced in a custom-made flume. Using a grid, we generated small-scale turbulence, kinetic energy of which is similar to that found in natural habitats. Feeding rates were measured under three levels of turbulence at two flow speeds, during which feeding movements were recorded and reconstructed in 3D. While the strongest turbulence in the experiment caused a significant reduction in the feeding rate of free-swimming damselfish under slow flows, anchored eels were affected only at fast flows. Although turbulence had no significant effect on strike distance, strike time, strike speed, or reactive distance in either fish, we found that reductions in feeding rates were due to reduced foraging area for damselfish and reduced search time for garden eels. The different responses between the two fish species corresponded well with hydrodynamic differences in their respective habitats. Our study demonstrates that small-scale turbulence differentially affects feeding of the two types of fish, highlighting the importance of considering its effects to understand fish ecology in coral reefs, such as adaptation strategies and habitat distributions.