Conservation physiology of freshwater fishes: an illustration of pressing questions and implications for management

Rivers, lakes, and wetlands are facing threats that continue to grow in intensity and frequency from climate change, habitat fragmentation, invasive species, changes in food availability, natural disasters, various forms of pollution (<i>e.g.,</i> trace metals, light, noise), and emerging infectious diseases. These disruptions to freshwater environments are driving population declines in freshwater fishes as well as threatening migratory species that need freshwater habitats to complete their life cycle. To improve freshwater fish conservation efforts, it is essential to understand the magnitude and nature of the threats fish are currently facing. Here, we present a series of case studies that illustrate the utility of employing physiological methods to assess both the threats facing freshwater fishes, and the conservation efforts being used to help preserve freshwater biodiversity. We present an array of physiological tools that can be used across multiple levels of biological organization, from molecular to population-level, to address a variety of questions. Finally, we share what we view to be pressing questions in freshwater fish conservation physiology and highlight strategies to help bridge gaps across different user groups.