By Jason Bittel

More than 16 feet long and weighing up to 1,100 pounds, Chinese sturgeons are among the world's largest freshwater fish. They're big and they're ancient. According to fossil records, they've been swimming China's Yangtze, Qiantang, Minjiang and Pearl Rivers since the time of the dinosaurs.

And now they're on the brink of oblivion, having disappeared from all of their former range except for small portions of the Yangtze.

Over the millennia, humans have sought out these freshwater leviathans not so much for their flesh as for the thousands of tiny black pearls that can be found within the adult females—in other words, caviar.

China began regulating sturgeon fishing in the 1970s, when the full breeding population had been whittled down to just 10,000 individuals. The move saved the species from extinction, but alas, in recent decades an even more existential threat has cropped up.

Dams. So many dams.

Chinese sturgeons are what's known as anadromous fish. Like salmon, they spend part of the year in the ocean and part of the year plying freshwater rivers and streams on the way to their ancestral breeding grounds. But unlike salmon, Chinese sturgeon don't die after spawning. Instead, after they mix up their DNA through an exchange of sperm and eggs in shallow waters upriver, they beat fin back to the sea. Under normal conditions, a Chinese sturgeon can live up to 20 years—spawning again and again and again.

Now, imagine you're a huge fish that's been swimming up a river for a decade and change, just like your anfishestors have for millions of years, and one day you run into a concrete wall.

That's what happened to Chinese sturgeon in 1981 when the Gezhouba Dam began operating on the upper reaches of the Yangtze River. The dam shortened the sturgeon's annual migration by 730 miles. Amazingly though, the fish still managed to breed. Well, somewhat. According to a study published in Current Biology this month, the reproductive output of the local population dropped by more than 75 percent after the dam was in place.

But the sturgeon swam on, making do with their new, shortened home range, since scientists at the time decided there was no good reason to install a device that would allow for fish passage.

Then in 2003, the Three Gorges Dam was stretched across the Yangtze, again with no fish passage device. And then in 2012, the Xiangjiaba Dam went up, followed by the Xiluodu Dam the very next year.

As each new structure divided the river into ever smaller sections, the Chinese sturgeon population flatlined. Their current annual rate of reproduction is now estimated at between 4.5 percent and zero.

The walls themselves aren't the only problem the dams bring for the fish. Dams create large reservoirs of water behind them that soak up heat from the sun. This creates layers of varying water temperatures within the river, similar to a really big lake, says Stephanie Januchowski-Hartley, an environmental biologist at Swansea University in Wales who studies the ecological impact of dams. (Januchowski-Hartley was not part of the study.)

When the dams discharge these reserves, they can have an enormous effect on water temperatures downstream. Numerous factors come into play, such as which depth layer of water is discharged and at what time of year, but in the end, these fluctuations can muck up the sturgeon's internal workings. (Oh, and climate change isn't helping matters, according to the paper's authors.)

"Not all fish like it hot," said Januchowski-Hartley. Not only can higher water temperatures stress out cold-water fish, but the drastic difference between what they're used to and what they're getting seems to affect the rate at which the fish's gonads mature.

Historically, the fish would have had a long, progressively colder swim in which their bodies slowly shifted into reproductive mode. But now their access to the river has been cut by so much that they're jumping right into mating without all the physiological foreplay they've evolved to require. And it just isn't working. According to the new paper, there may be just 156 mature fish left in all of the Yangtze River.

The Chinese government invested heavily in repopulating the Yangtze with more than nine million sturgeon fry, or juveniles, between 1983 and 2007, but scarcely any survived. The researchers refer to these efforts as "inadequate and unsustainable" because the government kept adding new fish but did nothing to enable those fish to reproduce.

What now?

No one expects that any of these dams are going to come down anytime soon. But there is still a modicum of hope for returning the fish's spawning habitat to a proper breeding temperature. For example, dam managers could selectively release water from the reservoir that is an agreeable temperature for life downstream, or perhaps churn up the standing water in a way that mixes the layers of different temperatures. Leaving tributaries that pour into the Yangtze undammed could also help keep the temperature steady. Studies have shown that in areas just below a confluence with an undammed tributary, sturgeon spawn better and insects are more prevalent, which suggests that naturally flowing tributaries can create pockets of suitable habitat for all kinds of wildlife.

All these strategies fall into creating what's called an environmental flow, says Januchowski-Hartley, but she has doubts they'll be enough to save the sturgeon over the long term.

But something needs to be done—and quick. If not, the study's authors predict the Chinese sturgeon will likely go extinct within the next 10 to 20 years. Which means that after 140 million years on this planet, the generation of Chinese sturgeon alive today would be the last.