Research
Turbulence Effects on Reef Fish
Coral reef fish face turbulence in the ocean and we are studying how they cope with it in order to capture plankton. To do this, we are conducting flow measurements in the fish's natural habitat and behavioral studies in the laboratory.
Effects of turbulence
As we discussed here, water flow is an important factor that affect behavior and metabolism of fish including garden eels. Especially, effects of flow speed have been well studied. Depending on flow speed, fish changes their swimming mode, feeding behavior, energy consumption, and sheltering behavior.
Although previous studies have mainly focused on effects of mean flow speed, recent research has begun to explore effects of complex flows that fish experience in natural habitats. One of the studies showed that fish can optimize body beat frequency to synchronize with the vortical flow patterns behind a cylinder (this study won Ig Nobel Prize in 2024!). Here is the footage of a dead fish “swimming” behind a cylinder due to motion induced by a specific flow pattern.
However, in such studies, the flow patterns are relatively predictable so that fish may be able to learn or predict these patterns to optimize their movements. Our study investigated effects of turbulence that characterized by random and stochastic motion of flows. We focused specifically on feeding behavior (as introduced here), and compared swimming blue-green chromis and stationary, anchored garden eels.
To examine effects of turbulence in a lab, we first measured turbulence that these fish experience in each natural habitat. Then, we produced grid-generated turbulence in a flume (flowing tank) and assessed it with particle image velocimetry (PIV). We eventually generated three turbulence level (dissipation rate) at each of the two mean flow speeds (0.05, 0.15 m/s).
Finally, we examined fish behavior under these flow conditions, following our aforementioned study on garden eels. At strong turbulence, blue-green chromis reduced feeding rates at slow flow whereas garden eels showed the same response at fast flow. Based on the behavioral analysis, the reduction of feeding rates was caused by reduced swimming area in blue-green chromis and increased prey search time garden eels.
These results suggest that each fish species is adapted to flows at each habitat. Garden eels live in sandy bottom at depths more than 10 m, where flow is relatively slow and turbulence is weak. In contrast, blue-green chromis form a colony over corals, where flow is faster and turbulence is stronger. Garden eels maintained feeding behavior under any turbulence tested under slow flow but decreased their feeding rates at fast flow and strong turbulence. They may live in slow flow condition to avoid.
In garden eels, feeding behavior remained consistent across different levels of turbulence under slow flow. However, their feeding rates decreased under conditions of fast flow speed combined with strong turbulence. This suggests that garden eels inhabit slow flow environments to avoid conditions that reduce their feeding rate. In contrast, blue-green chromis showed stable feeding behavior across turbulence levels under fast flow, but showed reduced feeding rates under slow flow speeds combined with strong turbulence. Unlike garden eels, chromis can actively swim to locate microhabitats with weaker turbulence, thereby possibly mitigating the negative effects. These different responses to turbulence between species with different feeding strategies highlight their respective flow adaptations. Our findings emphasize the importance of incorporating turbulence into estimation of fish adaptive strategies and habitat use.