Largemouth bass (Micropterus salmoides) is an important predator and a popular sportfish. However, adult survival is often poor because of size-structured interactions in the first year of life. To test the role of size-structured interactions among young largemouth bass, fish prey, invertebrate prey, fish competitors, and fish predators, we compared habitat-specific size through time, diet, stable isotope values, and distribution among three groups of age-0 largemouth bass in Hillsdale Reservoir. This research matters because largemouth bass is an important sportfish and this whole-system manipulation was a high-profile interest for our fisheries management colleagues. Our partners in this research were the Kansas Department of Wildlife, Parks, and Tourism. This research informs decisions. For example, all sizes of first year largemouth bass used beach and vegetated habitat and ate a similar diet of invertebrates and fish. Thus, our extensive and comprehensive data set had impact for sportfish management in Kansas by showing that fish size is not a growth or survival bottleneck during the first summer. In addition to aiding management in their decision-making related to stocking, this project advanced basic ecological knowledge about controls on first-year survival of a common and important predator.