How River Plumes Benefit Recruitment of a Coastal Fish Population: The Relative Importance of Top-Down Versus Bottom-up Processes

Tributary inputs are thought to promote fish recruitment in large lake and coastal marine ecosystems through formation of offshore plumes that create nursery habitat for larvae and juveniles. While previous research has primarily hypothesized that river plumes benefit fish recruitment through bottom-up mechanisms (via nutrient subsidies that promote prey production), with little attention to other potential regulatory mechanisms, our understanding of how river plumes benefit recruitment remains scant.  Herein, we present results from a multidisciplinary investigation that used field collections (2006-2009), laboratory approaches (experimentation, otolith microchemistry, genetics), and spatially-explicit individual-based modeling to explain a strong, positive relationship (R² = 0.98) between open-lake river plume formation and yellow perch (Perca flavescens) recruitment to the fishery in Lake Erie. Our findings demonstrate that nursery habitat created by river plumes consistently contributes a disproportionally greater number of larvae to the juvenile stage (at which time recruitment to the fishery is set) relative to non-plume nursery areas.  Unexpectedly, bottom-up processes emanating from temperature or prey production effects on larval growth were relatively unimportant in explaining these differential contributions.  Instead, turbid nursery habitat created by river plumes promoted survival by reducing predation mortality, with significantly higher levels of selection against slow-growing larvae occurring outside of river-plume nursery habitat than inside of it.  Overall, our collective field, lab, and modeling results provide a first demonstration of the importance of top-down processes in mediating the impact of river plumes on recruitment of a fish population.