Migratory freshwater fish populations have declined 81% since 1970. Of the 58 migratory fish species listed by the Convention on the Conservation of Migratory Species, 97% are threatened with extinction. A 2026 CMS report presented at COP15 in Brazil identifies 325 species requiring coordinated international conservation, with habitat fragmentation — primarily dams — accounting for half of all threats.
The number that matters isn't the decline percentage. It's the scale of dependency: migratory freshwater fish provide sufficient sustenance for roughly 200 million people worldwide. These aren't ocean species managed by fishing fleets with alternative catch. They're river species whose populations depend on uninterrupted corridors — spawning grounds upstream, feeding areas midstream, floodplain nurseries downstream — often crossing multiple national borders through shared basins like the Amazon, Mekong, Danube, Nile, and Ganges-Brahmaputra.
A dam doesn't remove the fish. It removes the corridor. The spawning habitat still exists above the dam. The feeding habitat still exists below it. The fish that need both habitats to complete their life cycle lose access to one or the other. The population decline is not from habitat destruction in the usual sense — the habitats are intact. It's from disconnection. The corridor between them was the resource, and the corridor is what was built through.
The report's recommendation — managing rivers as connected systems — faces a structural problem. The dam provides benefits (electricity, flood control, irrigation) to the country where it sits. The fish provide benefits to every community along the corridor, often in different countries. The entity that decides to build the dam captures the benefit locally. The entity that loses the fish absorbs the cost distributively. The decision-maker and the affected population overlap only partially, and often minimally.
This is not an information problem. The 81% decline is known. The mechanism is understood. The 325 species are named. The corridors are mapped. What's missing is a governance structure where the decision to fragment a corridor requires the consent of everyone the corridor feeds.