Two of the world's biggest freshwater fish are in big trouble, come reports from scientists in North and South America.

First up, the genetically distinct Kootenai River population of white sturgeon (Acipenser transmontanus), North America's largest freshwater fish. This massive monster has been known to reach almost six meters in length and weigh half a metric ton, but its size hasn't offered it any protection. In fact, it has made it more attractive, and the species has historically been heavily overfished.

The problem in the Kootenai River isn't overfishing, although it is man-made: Montana's Libby Dam, built in 1974. The dam prevents the river from the very flooding that used to tell the sturgeon it was time to spawn. Before the dam was built, an estimated 10,000 white sturgeon lived in the river. Now, just 500 remain, and they have not spawned in the wild in 35 years. Oops.

Despite the lack of wild spawning, the fish have not died out, and that's also thanks to human intervention. The Kootenai Tribe of Idaho periodically restocks the river with farm-raised sturgeon.

The U.S. Fish and Wildlife Service has been trying to save the Kootenai River white sturgeon from extinction for years by adjusting the amount of water that flows through Libby Dam, but last Thursday they announced that all of their recent attempts have failed. They'll keep trying, though, and will send even more water through the dam this year. But they can't send too much or they'll flood local towns.

Amazonian arapaima

Meanwhile, in South America, another of the world's largest freshwater fishes—in fact, the largest species with scales—is also in danger of extinction, if it even still exists. A paper in the December issue of the Journal of Applied Ichthyology reports that the giant Amazonian arapaima (Arapaima gigas) are threatened by weak and unenforced fishing regulations in Brazil, despite the species's protected status under the Convention on International Trade of Endangered Species of Wild Fauna and Flora (CITES).

Arapaima can reach more than four meters and weigh more than 180 kilograms. The fish actually comes to the surface to breathe, leaving it vulnerable to fishing with spears and nets.

Part of the problem with preserving the arapaima is that it has never really been studied, until now. Authors Leandro Castello and Donald Stewart examined several arapaima samples in museums and found that only one of them was actually the Arapaima gigas. "Our new analyses indicate that there are at least four species of arapaima," Castello told BBC News. "So, until further field surveys of appropriate areas are completed, we will not know if Arapaima gigas is extinct or still swimming about."

Castello and Stewart recommend increased monitoring and tighter controls over harvests to protect the multiple arapaima species from extinction.

And tuna, too

As long as we're talking about giant fish, let's not forget the endangered bluefin tuna (Thunnus thynnus), which also remains heavily overfished, and as a result commands incredibly high prices on the open market. Last week, a single, 232-kilogram tuna sold for an all-time high of $175,000 to two Japanese restauranteurs—60 percent higher than last year's record. That breaks down to $21.38 an ounce—almost three dollars more per ounce than the cost of silver. (And by the time it reaches the dining table as sushi, it will be more like the cost of gold.)

But even at those prices, bluefin won't be on the menu (or maybe anywhere) for very long.

Images: Arapaima via Wikipedia