Volumetric velocity measurement of aquatic predator-prey interactions

Abstract

The aim of this study is to develop a novel multi-scale volumetric measurement system and flow facility, and apply it to improve understanding of aquatic predator-prey interactions. A combined infrared tomographic PIV + 3D PTV system was developed and demonstrated. Tomographic PIV was used to obtain the volumetric velocity field of the flow, while the 3D PTV was used to track the prey, and the eye of the predator, in the same volume. A visual hull technique was implemented to mask out the objects (such as fish) appearing within the reconstructed tomographic PIV volumes, ensuring that velocity vectors near the object/fish were not contaminated during PIV cross-correlation. Copepods, which make up the majority of the oceanic zooplanktons, are known to sense flow disturbed by approaching predators and can execute sudden high-speed swim (or jump) to escape predation. Although their response to local flow disturbances has been studied, their sensing and swimming response to live predators (fish) is not well understood. Three series of experiments were implemented - (1): Copepod interactions with a wall-mounted cylinder in cross-flow; (2): Predator-prey interactions in still water; (3): Predator-prey interactions in unsteady/turbulent flow. From the experiments, copepods appeared to respond (jump) to a large and sudden increase in local maximum principal strain rate (MPSR) of the fluid, instead of a fixed threshold quantity. For fish predation in still water, zebrafish were first observed to approach slowly, followed by sudden acceleration (ram feeding) to feed on copepods. Using a potential flow model, it was found that this strategy might not be sufficient to capture copepods successfully. Thus, zebrafish were found to execute suction feeding simultaneously to increase chances of predation success. In uniform cross flow, a coral reef fish (blenny) used a similar slow approach - ram feeding strategy to feed on copepods, but it rarely captured them. However, turbulent cross flow increased the chances of predation by allowing the fish to approach closer to a copepod, and preventing the copepod from detecting hydrodynamic signals from the approaching predator.