Garonne River Ecological Corridor Backfires on Salmon Due to a Predator

The Golfech fish ladder was intended to safeguard the biodiversity of the Garonne. Yet, it has turned into a formidable trap. Indeed, catfish have developed a relentless hunting strategy there. They decimate a crucial share of migratory fish.

Why the Garonne’s ecological infrastructures make life easier for this formidable predator

EDF commissioned the Golfech fish pass in 1987. It was meant to offset the ecological impact of the hydroelectric dam. However, it forms a bottleneck two meters wide. According to a study in PLoS ONE, this extreme confinement deprives the salmon of space to dodge.

Research from 2016 is clear. Attacks carried out outside the channel frequently fail. Indeed, prey know how to flee in open water. By contrast, the predation rate reaches 35% inside the device. A fierce hunt thus becomes a guaranteed feast.

An unprecedented behavioral shift that proves the astonishing biological plasticity of the river giant

These animals were spotted in the structure as early as 1995. Since then, they have gradually colonized the site through learning. These long-lived fish have refined their tactics. They even manage to ascend the current of the pass. They intercept migratory fish directly.

Moreover, this adaptation unsettles the species’ internal clock. These predators are typically nocturnal. Yet, many individuals now hunt in broad daylight, from 8 a.m. to 5 p.m. They align themselves with the migration schedules of their targets. This temporal flexibility demonstrates a formidable adaptive intelligence.

The sizes of these specimens physically block the passage. They can reach up to 2.7 meters and 130 kilograms. Aloses also suffer from this scourge. Their attack rate surpassed 37% in 2019. The stomach analysis of the large catfish confirms that they make up the bulk of their spring diet.

A fatal environmental pressure for a salmon population already on the brink of extinction

This pressure weighs on a species that is already collapsing. In fact, the Nouvelle-Aquitaine populations dropped by half in 2025. Reproductive renewal is now threatened, with fewer than 600 breeding adults recorded. This historic decline has also seen the salmon’s habitat shrink by 70% since 1900.

The Loire population was once thriving. It counted 100,000 annual migratory passages in the 19th century. Today it totals fewer than 1,000 individuals. The losses at Golfech thus amplify the vulnerability of these fish. The disruption of ocean cycles also weighs on them in the open sea.

The regulatory challenge in the face of an ecosystem deeply altered by humans and climate

Indeed, crisis management pits river users against one another. In 2024, a national council suggested banning the practice of releasing caught fish back into the water. This measure aimed in particular to curb the species. However, targeted removal campaigns show a disappointing record.

Local eradication proves ultimately ineffective. New predators immediately replace the fish removed. They are drawn by this easy access to prey. Yet the core knot of the problem remains the existence of the dam. Global climate warming is heavily exacerbating this structural anomaly.

The rising temperatures of the Garonne favor this giant. Cold-water species, by contrast, suffer. Moreover, increased mortality in the North Atlantic helps explain the declines observed since 2024. The future of the migratory fish therefore depends on factors that go far beyond our rivers.