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By Graham Lawton

Robert Wilson/Wildestanimal/Getty Images

“THEY are the most threatened group of organisms on the planet,” says biologist Ivan Jarić. “More than 70 per cent of species are critically endangered, some are almost gone.”

He isn’t talking about the usual suspects: great whales, great apes or the corals of the Great Barrier Reef. He is talking about great fish. Specifically, sturgeons and paddlefish. Together they span 27 species, but 17 are in the most precarious category on the red list of endangered species.

Actually, make that 26 species. Earlier this year, a team including Jarić broke the news that one of the greatest of them all, the giant Chinese paddlefish, is almost certainly no more. It hasn’t been seen in the Yangtze river basin since 2003 and a recent exhaustive search failed to find any. “The chance it still exists is very, very low,” says Jarić, who is at the Czech Academy of Sciences Institute of Hydrobiology.

Sturgeons are the hardest hit of a group of animals that rarely make the headlines, even in conservation biology circles, but this group is declining faster than any other. They are collectively known as “freshwater megafauna” – monster fish such as sturgeons, giant catfish, river sharks and rays, along with river dolphins, porpoises, seals, manatees, crocodiles, alligators, snakes, turtles and salamanders.

“The river megafauna are hidden below the surface of human perception”

All told, there are more than 200 species of freshwater megafauna; most are in deep water and some are probably already doomed to extinction. Yet they are largely overlooked by efforts to save the world’s biodiversity. “It really is a neglected area,” …