The objective of this EAGER proposal is to demonstrate the applicability of in-situ particle image velocimetry (PIV) technology to address fluid mechanics questions that are difficult to answer in the laboratory. We will demonstrate this applicability by linking eddy composition to fish habitat selection. Despite the ubiquity and importance of turbulence in the fluvial environment, accurate measures of turbulent eddy composition (eddy diameter, vorticity, and orientation) in rivers are lacking. In-situ PIV extends laboratory particle image velocimetry to the field by using synchronized portable laser and imaging systems. The result is a temporal series of two-dimensional vector fields that can then be used to describe the size, orientation, distribution, and rotational strength of eddies in a river. These turbulent eddy metrics will be collected at locations where brown trout are observed station holding in a river. The results will, for the first time, describe the eddy characteristics of habitat selected by brown trout. The eddy characteristics of trout habitat will be compared to regions not occupied by the fish in order to also describe the habitat the fish are actively choosing against. Laboratory experiments have demonstrated that the stability and energetic costs of swimming fish are affected by the eddy angular momentum, orientation, and distribution. We therefore seek to test three hypotheses: (1) brown trout are choosing habitat that has smaller average eddy diameters than surrounding habitat, (2) brown trout habitat is dominated by vertical eddies rather than the more posturally disruptive horizontal eddies, and (3) eddy angular momentum is at a minimum in locations chosen by brown trout. Data to evaluate these hypotheses will be collected from the West Maple River, MI. This study will build on a previous three year study on the same reach of river that correlated brown trout habitat selection to velocity fluctuations. In order to tie the proposed data to the existing data a snorkel survey will be combined with both acoustic Doppler velocimetry (ADV) data collection and in-situ PIV data collection. The benefits of this project will be: (1) Demonstration of the applicability of a new technology to fluvial research (2) Increased understanding of the interaction between turbulence and fish habitat selection (3) Improved stream restoration designs that increase biodiversity.