Holding on, long gone, and back from the dead: Ecosystem implications of persistence, loss, and resurgence of fish migrations
Fish migrations are or once were major annual phenomena in most of the world’s river and lake systems. Today, enormous numbers of barriers limit where fish can migrate, spawning habitats are often degraded even when accessible, and populations of many species are further depressed by fisheries. In the face of these pressures, some species have proven remarkably resilient, while others persist but have experienced large reductions in range size and local abundance. Still other migrations have disappeared entirely despite historical records of enormous numbers of fish, but there is recent evidence of resumption of migrations in at least one species. The ecosystem-level implications of these changes remain largely unknown. Here, we offer examples of each pattern drawn from the diverse potamodromous species of the Great Lakes. Comparison of traits across these species enables us to evaluate whether there are any recurrent patterns in life history, habitat requirements, or other factors that predict which species have proven most resilient to environmental change. The trait-based approach allows general inferences about potential ecosystem-level implications of losing or gaining fish migrations in this diverse fauna.
Many fish migrations in the Great Lakes remain strong, but the range and abundance of some species has become limited. Dozens of native migrants are iteroparous, while many introduced species are semelparous. Suckers have remained prominent and widespread, and spawn successfully in all sizes of tributaries and substrates including below impassable barriers. Field studies reveal substantial inputs of nutrients from suckers despite low adult mortality, and a range of ecosystem process responses even when watershed nutrient loading is substantial. Other native species rarely rival the migratory biomass of suckers, even when supplemented by stocking. Historically, lake trout, whitefish, and sturgeon migrations may also have provided important subsidies to Great Lakes tributaries, but habitat loss and fishing pressure reduced or eliminated these migrations a century ago. Remarkably, lake whitefish have resumed fall breeding migrations in several tributaries of Lake Michigan, offering a hopeful example of resurgence but with perhaps less potential influence than spring migrants. We conclude that ecological flexibility is a hallmark of resilient migratory fishes, but basic differences in life history, spawning phenology, and population abundance mediate contributions to nutrient dynamics. Export of fish larvae back to the Great Lakes also deserves further attention from an ecosystem flux perspective.