Migratory fish populations have fallen by 81% between 1970 and 2020, mainly due to the degradation, change, and loss of habitat, a study has found, calling for strengthening of monitoring efforts and enhancing river protection and restoration efforts.

Latin America and the Caribbean region observed the steepest decrease, followed by Europe, according to the latest update on the Living Planet Index (LPI) for migratory freshwater fish.

The index includes information on 1,864 populations of 284 native migratory freshwater fish species, and builds on the IUCN Red List, among the most used indicators in conservation policies.

“Migratory fishes that spend all or parts of their life cycle in freshwater are highly threatened,” said the study, funded by conservation organisations The Nature Conservancy, WWF, World Fish Migration Foundation, and Wetlands International.

“Almost one in three of all freshwater species and 25% of freshwater fish species are threatened with extinction, and migratory fishes are disproportionately threatened compared to non-migratory fishes.”

Latin America and the Caribbean witnessed a 91% decline in migratory fish populations since 1970, while in Europe, the figure stood at 75%.

Although Asia and Oceania witnessed a lower decrease (28%), the study said the lack of reliable data could suggest a potential underestimation. As well, insufficient data prevented researchers from determining trends for Africa.

North America observed a 35% decrease, though the study argued that much of the overall abundance of migratory freshwater fishes in the region may have been lost prior to 1970.

Excessive harvesting during the period of post-European colonialisation, together with the impacts of the industrial revolution and dam construction, contributed to a decline in fish populations in North America before 1970, while after that, the trend remained fairly stable, said the researchers.

Abundance of monitored migratory freshwater fishes between 1970 and 2020 per region. Source: Living Planet Index for migratory freshwater fishes

MAIN THREATS

Overall, the downward trend in migratory fish populations has been consistent over the last three decades, though it has not affected all species indiscriminately.

Notably, 65% of the species included in the LPI are, on average, less abundant in 2020 than in 1970, while 31% have increased over that period.

“While there are plenty of species decreasing at a slower rate than the most extreme cases, instances of species increasing – ranging from around 5% to 100% – are observed much less,” said the study.

“Stable species – i.e. those changing by less than 5% over the monitoring period – were infrequently identified in the data. Overall, this suggests that there are not just more declining species, but that declining species are showing a greater magnitude of change than increasing species are.”

The study identified habitat loss and degradation, including fragmentation of rivers by dams and other barriers, as the primary threat to migratory fishes in all regions, followed by overexploitation, increasing pollution, and the impacts of the climate crisis.

Researchers stressed the need to ramp up efforts to bridge the data gap, as well as enhance protection and restoration efforts in free-flowing rivers to meet the goal of restoring 300,000 km of rivers set out by the global initiative Freshwater Challenge last year.

The study also called for expanding international cooperation for the conservation of freshwater migratory fishes, including adding more species to the Convention on Migratory Species (CMS).

In February, the 14th Conference of the Parties to the CMS announced a global initiative to identify, protect, and connect areas critical to migratory species.

The Convention covers a wide range of migratory species, focusing on the most threatened ones. Ahead of the meeting, 16 countries and the EU applied for a list of 18 species to be protected under the CMS, including nine fishes.

A report released during the summit highlighted 399 threatened species – mostly fish and birds – not currently covered by the Convention but that deserve greater attention.

By Sergio Colombo – sergio@carbon-pulse.com

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