Fishing ecology: the big ones are getting away.

Nature shares the sad fact that “freshwater megafish” – in other words, the really big ones, the 60-pounders out there in the rivers and lakes – are in the middle of a population crash:

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%.

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.