The conservation of migratory birds has long relied on the understanding and protection of flyways, the aerial routes connecting their breeding, feeding, and resting grounds. These carefully mapped pathways have been instrumental in habitat preservation and species monitoring for nearly a century. However, a parallel world of migration exists beneath the surface of our rivers, a hidden network of swimways crucial to the survival of migratory freshwater fish. These underwater journeys, obscured from human view, have received significantly less attention, even as migratory freshwater fish populations face a dire crisis, having declined by over 80 percent since 1970. This alarming drop is attributed to habitat fragmentation caused by dams, overfishing, and pollution, all of which threaten species vital to both aquatic ecosystems and human livelihoods. The emerging concept of global swimways aims to address this oversight and provide a crucial framework for protecting these essential migration routes.
Scientists are now working to map these underwater highways, creating detailed, species-specific migration routes in rivers across the globe. Unlike bird migration, which can be observed directly, mapping fish migration requires painstaking data collection and synthesis. Researchers are combining existing data from various sources, including FishBase, the IUCN Red List, and existing studies, to piece together a comprehensive picture of these complex movements. These maps will serve as critical tools for sustainable infrastructure development, conservation planning, and habitat restoration, highlighting crucial migratory corridors and identifying potential bottlenecks where fish may be particularly vulnerable. While acknowledging the progress made so far, researchers emphasize the significant gaps that remain in our understanding of fish migrations, particularly in less-studied regions like the tropics and the Southern Hemisphere. Technological advancements for tracking freshwater fish migrations are also lagging compared to the tools available for studying bird movements, further highlighting the challenges faced by researchers in this field.
One of the most significant threats to swimways, and consequently to migratory fish populations, is the construction of dams. These barriers fragment river systems, blocking access to critical breeding and feeding grounds and disrupting established migration routes. While dam construction has slowed in developed countries like the United States and Europe, it continues at an alarming pace in developing regions, particularly in Southeast Asia. The Mekong River, home to the world’s largest inland fishery and sustaining millions of people, has seen hundreds of dams built in the past two decades. This surge in dam construction has had devastating impacts on highly migratory species like the Mekong giant catfish, which is now considered critically endangered. The construction of dams impedes the catfish’s access to its breeding grounds, leading to a drastic decline in its population. However, recent conservation efforts, such as the tagging and release of a record number of giant catfish in Cambodia, offer a glimmer of hope, demonstrating the resilience of these species and the potential for their recovery if their migratory pathways are protected.
The complexity of river ecosystems poses further challenges to mapping and protecting swimways. Rivers are not simply linear channels; they are dynamic, multidimensional systems, connecting laterally to floodplains, varying in depth, and changing seasonally. Fish migrations are intricately linked to these environmental fluctuations, with fish moving between habitats based on specific conditions. The swimway concept aims to capture this complexity, recognizing that not all fish migrations are the same. The vast Amazon Basin, with its extensive flooded forests and wetlands, exemplifies this intricate interplay between fish migration and habitat. The seasonal flooding of these areas creates crucial feeding and breeding grounds for numerous fish species, highlighting the importance of considering the entire river system, not just the main channel, when mapping swimways. Furthermore, many rivers, including the Amazon and the Mekong, span multiple countries, underscoring the need for international cooperation in conserving these shared resources and ensuring the unimpeded movement of migratory fish across political boundaries.
The success of dam removal projects in restoring historical swimways provides further evidence of the resilience of both fish and their habitats. The recent removal of four dams on the Klamath River in the northwestern United States allowed salmon to return to spawning grounds inaccessible for over a century, demonstrating the potential for river ecosystems to recover if given the opportunity. This success story underscores the importance of prioritizing habitat restoration and connectivity in conservation efforts. The Global Swimways Initiative, launched in 2023, aims to synthesize data on all 2,400 known migratory freshwater fish species, representing about 13% of all described freshwater fish species. This ambitious undertaking seeks to create a global framework for understanding and protecting these vital migration routes.
The researchers involved in the Global Swimways Initiative argue that, much like migratory flyways revolutionized bird conservation, the concept of global swimways has the potential to reshape how we manage and protect our rivers. By shifting the focus from individual rivers to the interconnected network of migration routes, this approach emphasizes the need for a more holistic and integrated approach to river management. Mapping these underwater highways is paramount for the survival of freshwater fish and the health of the ecosystems and human communities that depend on them. Just as bird conservation has benefited from the understanding and protection of flyways, a similar focus on global swimways promises to be crucial for the conservation of migratory freshwater fish, ensuring their long-term survival and the continued provision of essential ecosystem services they provide. The future of these species depends on our ability to recognize and protect the hidden highways beneath the surface of our waters.
