Migratory fish populations have crashed by more than 80% since 1970, new findings show.
Populations are declining in all regions of the world, but it is happening fastest in South America and the Caribbean, where abundance of these species has dropped by 91% over the past 50 years.
This region has the world’s largest freshwater migrations, but dams, mining and humans diverting water are destroying river ecosystems. In Europe, populations of migratory freshwater fishes have fallen by 75%, according to the latest update to the Living Planet Index.
Migratory freshwater fish partially or exclusively rely on freshwater systems – some are born at sea and migrate back into fresh water, or vice versa. They can in some cases swim the width of entire continents and then return to the stream in which they were born.
They form the basis for the diets and livelihoods of millions of people globally. Many rivers, however, are no longer flowing freely due to the construction of dams and other barriers, which block species’ migrations. There are an estimated 1.2m barriers across European rivers.
Other causes of decline include pollution from urban and industrial wastewater, and runoff from roads and farming. Climate breakdown is also changing habitats and the availability of freshwater. Unsustainable fishing is another threat.
Herman Wanningen, founder of the World Fish Migration Foundation, one of the organisations involved in the study, said: “The catastrophic decline in migratory fish populations is a deafening wake-up call for the world. We must act now to save these keystone species and their rivers.
“Migratory fish are central to the cultures of many Indigenous peoples, nourish millions of people across the globe, and sustain a vast web of species and ecosystems. We cannot continue to let them slip silently away.”
A quarter of freshwater fish species are threatened with extinction, according to the International Union for Conservation of Nature (IUCN), with migratory fish disproportionately threatened.
The report looks at population trends of 284 freshwater fish species. Researchers also noted there could have been substantial declines prior to 1970 but there was no data for this.
There was also insufficient data to calculate population changes in Africa, but researchers wrote that many species in that region faced multiple stressors.
Previous research has found similar “catastrophic” declines. Authors of the latest report call for better long-term monitoring, rivers to be restored and protected, and the removal of barriers to migration.
Researchers want to find renewable energy alternatives to the thousands of new hydropower dams being planned across the world. Last year, a record 487 barriers were removed in 15 European countries.
Michele Thieme, deputy director of freshwater for WWF-US, said: “We have the tools, ambition and commitment to reverse the collapse of freshwater fish populations … Prioritising river protection, restoration and connectivity is key to safeguarding these species.”
Dr David Jacoby, a zoology lecturer at Lancaster University, said that while the report confirmed widespread concerns about freshwater bodies, “the extent of decline, both regionally and globally, is still shocking”.
“The threats posed by barriers to migration, pollution, water abstraction and climate change become cumulative,” he said, adding that the “huge” impact on migratory species and the impact on the fisheries they sustain required increased monitoring to help reconnect freshwater and marine ecosystems.
Dr Anthony Acou, of the National Research Institute for Agriculture, Food and the Environment (INRAE) in France, pointed out that, as many species migrating between salt and fresh water spent most of their lives in the sea, it was also important to consider “pressures such as oceanic current modification, decrease of productivity, offshore windfarms, climate change [and] bycatch”.
“To preserve/conserve the species, it is critical to better understand the impact of the pressures on both marine and freshwater habitats to enhance our understanding and target efficient management measures,” he said.
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