The UN is sounding the alarm about migratory fish, the forgotten inhabitants of rivers.

On Tuesday, March 24, a UN report shone a spotlight on a discreet but consequential issue: migratory freshwater fish are among the most threatened species in the world. Atlantic salmon, mountain trout, large fish of tropical basins… the same situation, the same fragility. Their lives hang by a thread, or rather by a continuous ribbon of water, made up of rivers, wetlands, and sometimes estuaries—that in-between space where fresh water meets the sea.

But this ribbon is tearing apart. Dams, embankments, the draining of wetlands, pollution, overfishing in some regions—the pressures are piling up and fragmenting habitats. The reader can guess: a migratory fish doesn't just need clean water; it needs passage, continuity, a clear route. When the waterway becomes a maze of obstacles, migration is cut short, and reproduction follows the same path.

Waterways cut off abruptly

The report illustrates this dependence with a spectacular case: the golden catfish. This fish can reach two meters in length and its round-trip migration exceeds 11,000 kilometers between the Andes and the Atlantic, crossing several Latin American countries. The authors present it as the longest known freshwater migration. On this scale, the slightest habitat disruption resembles a customs barrier placed in the middle of a national highway, except that here, no one turns back for convenience.

This warning also comes at a time when environmental diplomacy is taking center stage: the COP15 of the Convention on the Conservation of Migratory Species (CMS) opened on Monday, March 23, in Brazil. The stakes are real: 325 species of freshwater migratory fish require coordinated efforts between countries to prevent their extinction. Coordination is often a contentious word, because a river basin ignores borders while public policies scrupulously respect them.

The document is presented as the first global assessment offering such a detailed overview of the conservation status of these fish. Since the previous assessment in 2011, the number of species evaluated by the International Union for Conservation of Nature for its "Red List" has increased from approximately 3,000 to nearly 15,000, including almost 900 migratory species. A biodiversity crisis can remain invisible for a long time in river basins, until the day the river still seems to be flowing, but carries little more than silence, and then it is already too late.