Larval fishes must encounter food soon after hatching to survive. Newly hatched fish subsist on limited supply of yolk and thus must encounter and successfully capture food before depleting their energy resources. In general, larval fishes differ from adults in their interaction with the physical environment. Larval fishes hatch at a small enough size to experience the water as a mainly viscous medium. The flow regime around an organism depends on its size and its swimming speed and is characterized by the dimensionless Reynolds number, denoting the ratio of inertial to viscous forces. As larval fish mature, increases in size and speed facilitate a transition into an area of higher Reynolds numbers. This transition affects fundamental processes such as gas exchange and swimming style, and is associated with rapidly increasing capture success during early development. Likewise, morphological changes during ontogeny likely influence prey capture mechanics and hydrodynamics.I propose to characterize prey capture mechanics and hydrodynamics in larval fishes and determine how ontogenic changes and the transition between inertial to viscous medium affects suction hydrodynamics and capture success. I will combine laboratory experiments with hydrodynamic modeling and computational methods to characterize the flow field in front of the mouth of suction feeding fish larvae and investigate whether and how these patterns change with larvae size and Reynolds numbers. Hydrodynamic modeling and performance testing in the laboratory will be used to understand how changes in flow patterns affect feeding success. Lastly, larval size and age at the time of first feeding will be compared across species to determine whether suction feeding hydrodynamics places constraints on feeding success. The anticipated results will promote our understanding of how changes in the marine environment following global cooling and warming affect larval survival and sustainability of fish stocks.