Understanding the threats to fish migration: Applying the global swimways concept to the lower Mekong

The Mekong River basin is a biodiversity hotspot and supports the largest inland capture fishery globally. The fish and fisheries, especially migratory species that underpin the capture fisheries, are, however, under threat from multiple pressures, not least hydropower development, expansion of irrigated agriculture and aggregate mining. In this paper the Global Swimways concept was used to understand migratory patterns in different Mekong fish species and provide insights for management and conservation of migratory species in the basin. Information was collated from existing databases, FishBase, and literature searches to determine the significance of migration routes of the river system. A total of 1393 fish species was recorded. About 21% of these are truly migratory species, mostly potamodromous and amphidromous species, and contribute ≈70% of catch. Distribution of fish in the Mekong River exhibits a well-defined zonation pattern, with species diversity highest in the lower floodplains and delta reaches. Three main migration zones occur in the Lower Mekong Basin (LMB) but with considerable migration of some species between zones. Some species adopt multiple migration strategies as opposed to simple longitudinal or lateral migrations, with sub-populations occurring in different reaches of the LMB exhibiting different migratory strategies. Approximately 11% of native fish species are threatened; among migratory species 35% are threatened. There are multiple challenges to maintaining swimways in the Mekong, including improved understanding of migratory pathways, managing intensification of environmental pressures and managing heavy fishing pressure. Measures to conserve and protect the migratory fish species in the Mekong are suggested.