Global review finds freshwater fish in peril

(CN) — Many freshwater fish worldwide are in crisis, according to a new global assessment, but varied solutions based on specific environmental needs may curb further population decline.

On Tuesday, researchers with the Convention on the Conservation of Migratory Species of Wild Animals (CMS) — an environmental treaty of the United Nations — formally introduced the first comprehensive report specifically designed to identify all transboundary migratory freshwater fish of conservation concern. The assessment evaluates the conditions and health of over 15,000 species.

The release of the assessment coincides with the CMS Conference of the Parties (COP15) taking place March 23 through March 29 in Campo Grande, Brazil.

Three hundred and twenty-five species of migratory freshwater fish are singled out in the assessment and need international protection, the researchers say.

These fish — including different species of catfish, salmon, sturgeon and eels — are well known to people around the world, because they often end up on their dinner plates.

“Migratory fish support fisheries that feed millions of people,” Zeb Hogan, lead author of the analysis and a research professor in the Department of Biology at the University of Nevada, Reno, said in an email.

“Before this assessment, knowledge about threatened freshwater fish came mostly from regional studies, species-by-species conservation assessments, or local fisheries monitoring," Hogan said. “This assessment fills a major gap and provides an up-to-date global overview that can guide international cooperation and conservation policy.”

Hogan pointed to the “iconic” Mekong giant catfish (Pangasianodon gigas), found in the Mekong River basin in Southeast Asia. It is one of the largest freshwater fish in the world, but it’s at risk, and fishing bans in Thailand, Laos and Cambodia have proven to be ineffective and sustaining its population.

“Its migratory spawning journeys are blocked by dams, and overfishing has reduced its population dramatically,” Hogan said.

In Central and Eastern Europe, the Danube salmon (Hucho hucho) need free-flowing rivers for spawning, and habitat fragmentation and hydropower make migration increasingly difficult for the species, Hogan said.

Tropical eels that migrate from rivers to the open ocean to reproduce, and Amazon catfish, such as the widely popular dorado and the spotted sorubim of the Paraná River in South America, all face the impediments to healthy life cycles — dams, overfishing, altered river flows and habitat degradation that threaten future generations of the species.

“These examples show the diversity of migratory fish, from iconic giants to ecologically critical species that underpin food security and highlight why international cooperation is essential for their survival,” said Hogan.

In their assessment, researchers found an estimated 81% drop in migratory freshwater fish populations worldwide since 1970 —one of the sharpest declines recorded for any major vertebrate group.

Hogan said freshwater fresh often live in ecosystems that are “heavily modified by human activity,” but the population decreases go unnoticed because they are gradual and spread out geographically.

“Freshwater fish are often out of sight and out of mind, compared with terrestrial or marine species,” he said. “Because their migrations cross national boundaries, no single country bears responsibility for protecting them, making coordinated protection surprisingly rare.”

Hogan noted the Convention is in a unique position to protect migratory fish, like other migratory animals, because its structure is transnational and relies on countries working together to come up with real-world solutions in viable timeframes.

Some potential solutions for the fish in peril include dam removals to allow migrations to continue, protecting floodplains, deep pools and other known spawning areas, maintaining natural river dynamics that keep ecosystems healthy, and community-led fisheries management that integrates local practices with conservation measures.

For the Mekong giant catfish and other Southeast Asia species, Hogan said key habitats in Cambodia may help while upstream pressures are addressed.

“There are still leverage points where conservation can make a real difference,” Hogan said.

“This is also a story about people, scientists, communities, and local managers, working at these critical points to sustain migrations and migratory fish," he continued. “Their actions, though small in scale compared to dams or basin-wide development, can have outsized impact on species survival.”

Ultimately, Hogan said, the assessment “reminds us that freshwater ecosystems are globally important, heavily threatened, but not yet beyond repair, and that cross-border cooperation is essential.”