The number of large fish species that live in rivers and lakes declined by 88% from 1970 to 2012, Global Change Biology magazine reported.
According to the scientific article, the rate of decline of vertebrate populations is much higher in freshwaters than in terrestrial or marine realms.
The freshwater megafauna is made up of animals that can reach a body mass of over 30 kg, like river dolphins, beavers, crocodiles, giant turtles and sturgeons.
The main author of the study, Fengzhi He, reiterated that public awareness on the freshwater biodiversity crisis is limited and many people ignore that these animals exist.
'They are not like tigers, pandas, lions, or whales â€' species that receive a lot of attention in the media and school education,' noted Fengshi, who is a freshwater ecologist at the Leibniz-Institute for Freshwater Ecology and Inland Fisheries (IGB) in Berlin.
During the research, the experts compiled data on time series available for 126 species throughout the world, in addition to reports on historic and contemporary geographic distribution of 44 species in Europe and the United States.
'This is a crisis of huge proportions that is not widely appreciated', said Zeb Hogan, a fish biologist at the University of Nevada, Reno, who has studied the difficult situation of giant freshwater fish for two decades.
'Once the largest animals go, it's a warning that we need to do something quickly to improve the ecosystem health of our rivers and lakes,' he added.
Among the biggest threats facing large freshwater species are overexploitation and habitat degradation, as many of these animals are targeted for meat, skin, and eggs, the study showed.
Megafish in particular tend to be more vulnerable than other fish to dams that block their migratory routes and limit access to spawning grounds, the research added.
'Large animals also tend to be slow to mature and have low reproduction rates, making them particularly vulnerable,' Fengshi pointed out.
However he added, 'We don't want this to be a situation of only doom and gloom,' stressing, 'We want to inform people about this biodiversity crisis but also show them that there is still hope to protect these giant freshwater species â€' that it can be done.'