An iconic freshwater fish of tropical South America, the arapaima is a massive, slender beast that can grow up to 10 feet (3 meters) long and weigh 440 pounds (200 kilograms). It is known as the pirarucu in Brazil and the paiche in the western Amazon, and is one of the largest freshwater fish in the world.

“These are nothing like the typical fishes that are caught here in the U.S., they are huge animals,” said Donald Stewart, a fisheries professor at the SUNY College of Environmental Science and Forestry (ESF) in Syracuse, New York.

SUNY ESF fisheries professor Donald Stewart examines the teeth of an arapaima skeleton. Photo courtesy of SUNY ESF

Stewart has just published a paper that shakes up nearly a century and a half of conventional wisdom on the arapaima. In the March issue of the journal Copeia, Stewart re-described a second species, and reported that there are at least four previously-described species of arapaima plying the rivers and flooded forests of the Amazon and other basins in South America. (See photos of megafishes.)

Stewart took Water Currents through the history of the re-discovered arapaima’s taxonomy. In 1829, a biologist published detailed drawings of an arapaima skeleton that was collected somewhere in the Brazilian Amazon (exact location unknown). In 1847, a French biologist asserted that the drawings represented a unique species and named it Arapaima agassizii after the biologist who published the original drawings. But in 1868, a British taxonomist disputed the view of the French one.

“He said there is only one species of arapaima [Arapaima gigas], and ever since, others have accepted his opinion,” Stewart explained.

“But I went back and started digging, and the drawings are really different, it’s a highly unusual fish,” he said, adding that there are several key anatomical differences between the skeleton and a “typical” arapaima.

Unfortunately, no one can actually take a look at that skeleton because it was destroyed in World War II, after a bomb hit the museum where it was housed. Further, the collector did not record a precise location.

But Stewart is convinced the drawings show a distinct species, and that there are several other different species of arapaima now living in South America, including Arapaima arapaima. In another paper set to be published in the near future, Stewart is describing a fifth species, the first new arapaima since 1847. “People have had the taxonomy wrong for 145 years, but that situation is not unusual for such large fishes. They are difficult to collect and expensive to store in museums,” Stewart noted.

Stewart added, “This is an icon of the Amazon; some people consider it a flagship species that we need to conserve, kind of like a tiger or an elephant.”

Fishing Pressure

Yet at the same time, the arapaima is one of the most important commercial fishes in South America. The animal’s large size means it provides a lot of meat, and a large individual can fetch around $150, a lot of money in the Amazon.

The arapaima is one of few fishes that must breathe air. Their swim bladder evolved into a primitive lung, while their gills provide at most about 20 to 30 percent of their oxygen needs, Stewart explained. This means they must pop their mouths out of the water every 15 minutes or so to catch a breath, when they emit a distinctive sound.

“Fishermen sit in canoes and wait. When they see it pop up, they stab it with a harpoon. It is easy to find and relatively easy to kill even though they are big, so they get over harvested very quickly,” said Stewart.

The arapaima may have tough scales, which scientists are studying for their ability to repel piranha bites, but they are still no match for a harpoon. Hunters themselves, arapaimas primarily eat other fish, although they occasionally take water birds.

Deforestation’s Toll

Stewart added that the arapaima is also threatened by deforestation, because the fish prefers to swim into flooded forests during the wet season to spawn and guard their young. The young are vulnerable to predators and need to find cover among the flooded vegetation.

Stewart said the arapaima is not particularly threatened by dams, since it tends to live in lowland areas, not in the highlands where dams are usually built, and it tends not to migrate very far.

Stewart did point out that there is a growing aquaculture industry for arapaima, which could result in genetic mixing of wild populations if managers aren’t careful. “People have been thinking it’s all one species so it doesn’t matter where you move them. If you introduce a cultured species, that potentially could lead to elimination of a local form that was depleted by overfishing,” he said.

“We need to figure out the taxonomy so we can know what to protect and what to eat, and so people won’t move them around in ways that cause new, unanticipated problems,” said Stewart. He and his graduate students have more field research planned—a challenge because of the vast areas involved—and he will also soon travel to Europe to examine more museum specimens.

Stewart’s research was supported, in part, by National Geographic Society and ESF.

Brian Clark Howard covers the environment for National Geographic. He previously served as an editor for TheDailyGreen.com and E/The Environmental Magazine, and has written for Popular Science, TheAtlantic.com, FastCompany.com, PopularMechanics.com, Yahoo!, MSN, and elsewhere. He is the co-author of six books, including Geothermal HVAC, Green Lighting, Build Your Own Small Wind Power System, and Rock Your Ugly Christmas Sweater.