Migrations of fishes throughout stream networks support natural and human communities globally. Despite their importance, freshwater migratory fishes are increasingly under threat from stream habitat degradation due to land use and climate change and stream habitat fragmentation from barriers, with the latter being especially threatening to migratory species. This is because barriers can block migratory fishes from accessing the distinct stream habitats located throughout networks needed to complete their life cycles. Strategies to conserve freshwater migratory fishes are needed, including identifying specific threats and, in some cases, opportunities for barrier removals. Because barriers can also benefit human communities and protect native species from species invasions, fishery managers must attempt to balance the benefits and costs for migratory fishes, natural systems, and people when making barrier removal decisions. To make effective decisions, fishery managers need to know the diversity of migratory life histories in stream fish assemblages. Additionally, fishery managers need to know the relative influence of barriers and additional threats to migratory fish assemblages across the large spatial extents in which barriers and migratory fish habitat are distributed. The goal of my dissertation is to address these needs. I aim to increase our understanding of distributions and abundances of migratory fishes along with the ways in which steam fragmentation by barriers affects those fishes over large regions. In my first chapter, I characterize the migratory life histories of 1,250 fish species across the continent of North America. This information supports analyses of my second chapter, in which I characterize regional migratory fish assemblages and the relative influences of natural factors, human land uses, and stream fragmentation by barriers on those assemblages in the conterminous United States. I then focus my research within the Great Lakes region, an area where fishery managers must balance restoring connectivity for migratory fishes while controlling for invasive Sea Lamprey Petromyzon marinus. In my third chapter, I evaluate habitat suitability in streams above barriers for six migratory fish species, including Sea Lamprey, across the state of Michigan and a portion of Wisconsin. I use these habitat suitability estimates in my fourth chapter along with costs of barrier removal and an alternative method to control Sea Lamprey, lampricide, in an optimization analysis to prioritize barriers for removal in Great Lakes tributaries. Collectively, my dissertation emphasizes the prominence and variation in migratory fish life histories in stream fish assemblages, identifies the relative importance of barriers and other landscape-scale threats on migratory fish assemblages, and demonstrates the value and challenges associated with barrier removal decision-making across large spatial extents. In addition, the approaches developed and implemented in my dissertation can be applied in other regions that require similar information for barrier removal decision-making to improve the conservation and management of freshwater migratory fish species in fragmented stream networks.℗
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