Temperature controls the rate of all biochemical processes, and thus can significantly influence the physiology and ecology of all living organisms on earth. Understanding how temperature influences fitness can therefore provide insight into mechanisms affecting population size, dynamics, and geographic distributions, especially in the context of climate change. Early life stages of fishes experience significant selective pressure to grow large and accumulate energy reserves in order to avoid predation and starvation. Increasing temperatures will increase routine metabolic rates, which may result in less energy being available for growth and energy storage. In this thesis, I examine the effects of temperature, food availability, and other factors on the ecophysiology of anadromous juvenile alewife (Alosa pseudoharengus) and blueback herring (A. aestivalis), collectively known as river herring. River herring have been targeted in restoration efforts along the eastern United States as a critical forage fish species to support both aquatic and marine ecosystems. Significant data gaps in the basic thermal ecophysiology of anadromous river herring populations make it challenging to strategize climate-ready management actions, as well as identify other abiotic and biotic factors that may interact with temperature to influence river herring population dynamics. In five research chapters, I report how temperature affects juvenile blueback herring survival, growth, and scaled energy reserves (Chapter 2) and then describe the effects of both temperature and ration on juvenile alewife and blueback herring (Chapter 3 and 4). I report findings from a direct comparison of thermal limits and performance in juvenile alewife and blueback herring originating from southern Connecticut nursery habitats (Chapter 5) to determine if the two species are similarly climate vulnerable. Finally, I describe which abiotic and biotic factors were related to variation in juvenile river herring size, growth, and scaled energy reserves in individuals collected from five sites in the Connecticut River (Chapter 6). Collectively, my findings indicate that both temperature and food availability are important determinants of juvenile growth and energetic condition. In the context of climate change, restoration of river herring populations may be more effective by increasing available nursery habitat to reduce density dependence and intraspecific competition.