Over a 40-year period, nearly 90 percent of the world’s largest freshwater species have declined – twice the rate of vertebrate populations on land or in the oceans.

Freshwater lakes and rivers are among the most diverse and dynamic systems on the planet, covering just 1 percent of Earth’s surface but providing a home to one-third of all vertebrate species. But these waterways are among the most threatened.

In the first comprehensive study tracking changes in populations of freshwater megafauna at a global scale, an international team of scientists compiled data for more than 120 freshwater megafauna species around the world and compared it with historical and modern geographic distribution of 44 species in Europe and the US. Their results, published in Global Change Biology, revealed between 1970 and 2012, 88 percent of the world’s megafauna species have declined with some parts of the world seeing as much as 99 percent of species dropping. Among the most threatened are large fishes like sturgeons, salmonids, and giant catfishes which have seen declines of 94 percent, followed by reptiels with a72 percent decline.

"The results are alarming and confirm the fears of scientists involved in studying and protecting freshwater biodiversity," said Sonja Jähnig in a statement.

The Indomalaya realm, a geographic region encompassing most of Southeast Asia and parts of East Asia, saw declines by 99 percent while populations of mega-fishes in the Mekong and Amazon have dropped close to zero due to deteriorating environment. Species in the Palearctic realm, which extends across Europe and most of Asia into parts of northern Africa, have declined by up to 97 percent. The report also notes major range contractions. The distribution range of 42 percent of all freshwater megafauna species in Europe contracted by more than 40 percent of their historical range.

The lifestyle, complex habitat needs, and slow life history of large freshwater species make them particularly vulnerable to environmental changes and more prone to extinction. Larger species are also targeted for their meat, skin, and eggs.

"Furthermore, the decline of large fish species is also attributed to the loss of free-flowing rivers as access to spawning and feeding grounds are often blocked by dams,” explained study co-author Fengzhi He.

“Although the world's large rivers have already been highly fragmented, another 3,700 large dams are planned or under construction—this will exacerbate the river fragmentation even further. More than 800 of these planned dams are located in diversity hotspots of freshwater megafauna, including Amazon, Congo, Mekong, and Ganges river basins.”

The study does present several challenges worth noting, including a lack of long-term monitoring data and evident biases in existing data.

Researchers note that some species have seen improvements in population levels due to conservation efforts; green sturgeon and American beaver are stable or increasing in the US and the Irrawaddy river dolphin in Mekong basin is increasing for the first time in two decades. However, current conservation efforts are largely inadequate and political boundaries tend to make large-scale efforts difficult to implement.

"According to the International Union for Conservation of Nature's Red List of Threatened Species, over half of all assessed freshwater megafauna species are considered as threatened with extinction. Nonetheless, they receive less research and conservation attention than megafauna in terrestrial or marine ecosystems," said Jähnig.

The authors note that their report highlights a need for conservation action, as well as improved monitoring and population trends and distribution analyses in the future.