6 days ago
Thousands of river species at risk as dams block critical migration routes, study finds
Dams are severely disrupting life cycles of aquatic species that migrate between rivers and oceans, according to a major global review of scientific research.
The review, published in Biological Reviews, finds that dams are reducing the populations, species diversity, and genetic health of diadromous species – fish, eels, crabs and snails that depend on connected river and marine systems to feed or reproduce. These disruptions are fragmenting habitats and cutting off access to vital breeding and feeding grounds.
'Dams fragment rivers and block essential routes between coastal waters and upstream habitats,' Dr Jia Huan Liew, freshwater ecologist at the University of Tasmania and a co-author of the review, said.
'This is making it harder for migratory species to survive and complete their life cycles.'
The review, led by researchers from Hong Kong, analyses nearly 100 studies. It's the first to comprehensively assess the global impact of dam-induced fragmentation on these species.
The findings reveal that even widely used mitigation tools such as fish ladders – designed to help aquatic animals bypass dams – frequently fail to protect vulnerable species.
'Despite their widespread use, fish passes often underperform, particularly when designed without understanding the specific behaviours and traits of local species,' Dr Liew said.
He added the impact was especially severe for species that could not survive in landlocked habitats, or those with poor climbing abilities.
The review concludes that the most effective way to restore river connectivity is dam removal but acknowledges this can be expensive and complicated by human needs for hydropower and irrigation.
The review also highlighted a significant gap in global research: most existing data is based on temperate fish such as salmon while little is known about how dams affect tropical species or non-fish diadromous animals like migratory snails and crabs.
'There are many ways to measure the impacts of dams before they're built and to optimise their design and location,' said Dr Liew.
'What we really need to do is be as thorough as possible during planning stages to understand and mitigate harm to our wildlife before it happens.'
As dam-building surges globally, the review warns that freshwater biodiversity will continue to decline unless all future infrastructure projects account for the ecological needs of migratory species.
