The biggest fish of the Amazon River basin, a giant known as the arapaima that can grow to 10 feet long and weigh more than 400 pounds, has seen its numbers declining and has gone extinct in some areas of the region, researchers say.

Although the giant fish was abundant in the Amazon a century ago, three of its five known species have not been seen for decades, they say.

Researchers making fish counts and surveying fishing communities in more than 650 square miles of Amazon floodplain area found arapaima populations to be extinct in 19 percent of communities and depleted -- approaching extinction -- in 57 percent.

Arapaimas, commonly known in the region as pirarucu, are the biggest freshwater fish found in South America.

Unique among fish species, they can breath both air above water, thanks to a primitive type of lung, and underwater with a gill arrangement, which lets them survive in waterways that are oxygen-poor.

Although beneficial for their survival, the unique breathing capabilities make them easier to capture, the researchers said.

"Arapaima spawn on the edges of floodplain forests and come to the surface to breathe every 5 to 15 minutes, when they are easily located and harpooned by fishers using homemade canoes," said study participant Caroline Arantes, a doctoral student in fisheries and wildlife science at Texas A&M.

In some fishing communities, the fish are being overexploited, the researchers said.

"Fishers continue to harvest arapaima regardless of low population densities," study leader Leandro Castello of the College of Natural Resources and Environment at Virginia Tech said.

When fewer large mature arapaima are available, fishers resort to gill nets in an effort to gather smaller fish species, which leads to the unintentional harvesting of juvenile arapaima, putting further pressure on the remaining populations, he said.

There is some good news, the researchers said; in communities setting rules including minimum capture size and controlling the use of gill-nets, arapaima numbers are rebounding.

In Ilha de Sao Miguel, which banned gill-net use 20 years ago, arapaima density is the highest levels in the regions, they said.

Adoption of measures like those practiced in Ilha de Sao Miguel could pull this unique fish species back from the edge of extinction, scientists say.

"Many previously overexploited arapaima populations are now booming due to good management," Castello says. "The time has come to apply fishers' ecological knowledge to assess populations, document practices and trends, and solve fisheries problems through user participation in management and conservation."