Some of the longest, most important migrations of species on Earth are happening beneath the surface of the world's rivers and many are rapidly collapsing, according to a major new assessment by the Convention on the Conservation of Migratory Species of Wild Animals (CMS), an environmental treaty of the United Nations.

The Global Assessment of Migratory Freshwater Fishes, being launched at the CMS 15th Meeting of the Conference of the Parties (COP15) in Brazil, finds that migratory freshwater fish—a group of species that maintain river health, underpin some of the world's largest inland fisheries, and sustain hundreds of millions of people—are among the most imperiled wildlife on the planet.

The Assessment identifies hundreds of migratory fish needing cross-border action, presenting authoritative evidence that species whose life cycles depend on connected rivers across national borders face accelerating declines driven by dam construction, habitat fragmentation, pollution, overfishing, and climate-driven ecosystem changes.

The analysis identifies 325 migratory freshwater fish species as candidates for coordinated international conservation efforts, highlighting a largely overlooked biodiversity crisis unfolding across the world's shared river basins.