Alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis) populations, collectively “river herring”, have declined drastically over the last century due to overfishing, habitat degradation and impediments to migration. River herring provide critical ecosystem services in coastal watersheds, serving as forage for a wide variety of predators and as a link between marine and freshwater environments where adults import marine nutrients into freshwater lakes and their offspring export nutrients from freshwater back into the ocean. The factors driving young-of-year river herring productivity are not well understood, but are important for restoring populations and their critical ecosystem services. Although recent work in coastal New England has focused on estimating metrics of juvenile productivity in freshwater lakes, there are significant data gaps associated with the habitat use, growth rates, and survival of juvenile river herring as they migrate from freshwater to estuarine habitats. The first empirical study in this thesis examined juvenile growth rates across freshwater lake, river, and estuarine habitats within three coastal Massachusetts watersheds. Results suggested that access to downstream estuarine habitats provides opportunities for enhanced juvenile growth rates compared to lake-resident fish, particularly in systems with high in-lake juvenile densities and unimpeded connectivity to the estuary (e.g., Weymouth Back River watershed). In high-density systems with multiple barriers to downstream fish passage (e.g. Mystic River watershed), bottlenecks to juvenile growth may occur when high densities of emigrating fish occupy shallow, eutrophic river reaches. In contrast, in low-density systems, juvenile growth rates were similar between lake and estuarine habitats (e.g., Essex River watershed). Juvenile river herring that remain in freshwater lake habitat in high-density nursery environments are also faced with density dependent growth limitations due to low pelagic prey biomass. The second empirical study in this thesis explored dietary composition, prey selectivity, morphological traits, and growth in littoral and pelagic lake habitats across four sites. Stomach content and environmental zooplankton analyses revealed flexible foraging strategies within and among populations, where juvenile river herring engage in pelagic filtering, benthic or epiphytic foraging, and surface feeding depending on habitat and prey availability. Alewife generally selected larger prey, although they also exhibited signs of intraspecific resource partitioning in Great Herring Pond, a high-density system. In Upper Mystic Lake, where blueback herring and alewife co-occur, alewife primarily exploited littoral prey while blueback herring targeted more pelagic zooplankton, highlighting species-specific niche differentiation. These findings underscore the ecological importance of littoral habitats in lakes and the value of preserving habitat complexity and connectivity across freshwater and estuarine nursery environments. Moreover, this research provides novel insights into the mechanisms driving juvenile river herring production and offers practical guidance for protecting access to quality nursery habitats that will maximize juvenile river herring growth and survival.