A fisherman on the Japurá River in Brazil hauls a freshwater megafish into a boat.Pulsar Imagens/Alamy

Freshwater megafish — giants weighing more than 30 kilograms that can live for decades — declined by more than 94% between 1970 and 2012, according to a recent study1.

The findings, published on 8 August in the journal Global Change Biology, are part of an analysis that looked at the populations of enormous freshwater animals in the world’s rivers and lakes. The drop-off reflects a broader downward trend in the populations of freshwater megafauna — such as caimans and giant salamanders — around the world (see ‘Plunging populations’). The study authors estimate that the populations of big freshwater animals have fallen by 88%.

“It is sadly a very shocking result,” says Fengzhi He, a fish ecologist at Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin and the lead study author. His team estimates that the megafauna populations that started dropping in the 1980s across swathes of Asia — including Cambodia, southern China, India and Afghanistan — have plummeted by 99%.

The team collected data on the populations of 126 large freshwater species from 72 countries. They analysed the data using the Living Planet Index, a statistical measure that adjusts for imbalances in the amount of information from various areas.

They expected megafish to be hit the hardest by human activities such as overfishing and loss of habitat, because many giant fish species mature late, have relatively few offspring and require large, intact habitats for migration. Their movements are increasingly hampered by hydroelectric dams in the world’s greatest river basins, such as the Mekong, Congo, Amazon and Ganges.

Of the more than 200 freshwater animal species in the world larger than 30 kilograms, 34 are categorized as critically endangered on the International Union for Conservation of Nature’s Red List of Threatened Species. But He cautions that researchers just don’t have enough information on many of the large freshwater species to determine their conservation status. It might be possible to help some species in danger of extinction to recover if scientists have accurate data on the animals’ populations, He says.

He hopes that his team's findings prompt more-detailed assessments in places such as southeast Asia before it’s too late. “I actually think it might be worse than we think in the Mekong Basin, where megafish like the Mekong giant catfish and the giant salmon carp have almost been eliminated,” He says.