The arapaima fish, native to the Amazon River basin, can weigh as much as 400 pounds (180 kilograms).

This story was updated at 11:00 am ET on Aug. 15.

Measuring 10 feet (3 meters) long and weighing in at more than 400 pounds (180 kilograms), it's hard to imagine that the arapaima, the largest fish in the Amazon River basin, could ever go missing. But these huge fish are quickly disappearing from Brazilian waterways, according to a new study.

A recent survey of fishing communities in the state of Amazonas, Brazil, found that the arapaima is already extinct in some parts of the Amazon basin. In other parts of the Amazon, its numbers are rapidly dwindling.

However, the researchers also uncovered some good news: In communities where arapaima fishing is regulated, the species is actually thriving, giving the researchers hope that conservation of the species is still possible. [Photos of the Largest Fish on Earth]

Commonly known as pirarucu, arapaima (Arapaima gigas) are the largest freshwater fish in South America. They possess an uncommon quality for fishes— the ability to breathe air. This feat is made possible by a primitive lung, which arapaima possess in conjunction with a gill system that allows them to breathe underwater. The fish developed this function because they typically live in oxygen-poor waterways, according to the Tennessee Aquarium, which is home to several arapaima.

But while this supplemental breathing technique helps the fish survive in its native habitat, it also makes the arapaima much easier to catch, according to the researchers.

"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 Caroline Arantes, a doctoral student in wildlife and fisheries science at Texas A&M University in College Station, who helped conduct the study.

Fishy policies

Of the five known species of arapaima, three have not been observed in the wild in decades, according to study co-author Donald Stewart, a professor with the State University of New York at Syracuse's College of Environmental Science. Stewart said that all five species dominated fisheries in the Amazon just a century ago.

A commercially important species, arapaima are traditionally fished by local Amazonian communities, a practice that's largely unregulated, the researchers said. To find out how this lack of regulation might be affecting the giant fish, the researchers interviewed local fishers operating within a 650-square-mile (1,683 square kilometers) floodplain in northwestern Brazil.

In 19 percent of the 81 communities surveyed, the arapaima was found to be already extinct. And the giant fish's numbers are depleted, or approaching extinction, in 57 percent of the communities surveyed. In 17 percent of the communities, the fish were deemed "overexploited," according to the researchers.

"Fishers continue to harvest arapaima regardless of low population densities," said study leader Leandro Castello, an assistant professor of fisheries at Virginia Tech's College of Natural Resources and Environment, in Blacksburg.

But the blame for the arapaima's dwindling numbers doesn't just fall on local fishing communities. Policymakers in Brazil may also be responsible, the researchers suggest. Government officials in the region tend to follow a "bioeconomic" line of thinking, which may have doomed the arapaima, the researchers said. [Amazon Expedition: An Album]

"Bioeconomic thinking has predicted that scarcity would drive up fishing costs, which would increase price and help save depleted species," Castello said. "If that prediction were true, extinctions induced by fishing would not exist, but that is not what has happened."

Fishing down

What is happening in the Amazon River basin is in line with something Castello and his colleagues call the "fishing-down" theory. This idea helps explain how large, high-value, easy-to-catch fish — such as the arapaima — can be fished to extinction.

In communities where arapaima are scarce, local fishers stop hunting the fish in traditional ways, such as with a harpoon. However, this doesn't mean fishers aren't killing arapaima; they're simply killing them in a different way.

These fishers use gill nets to harvest smaller fish, including juvenile arapaima. While local fishers don't necessarily catch the smaller arapaima on purpose, by "fishing down" they still end up killing the fish and further depleting the arapaima population.

But there is a bright side to this sad fish tale, according to study co-author David McGrath, a researcher with the Earth Innovation Institute in San Francisco. In communities that have implemented fishing rules, such as imposing a minimum capture size for arapaima and restricting the use of gill nets, the density of arapaima is 100 times higher than in places where no such rules exist.

"These communities are preventing further arapaima extinctions," McGrath said.

Unfortunately, only 27 percent of the communities surveyed have management rules in place for fishing arapaima. One community that does manage these fish, Ilha de São Miguel, banned the use of gill nets two decades ago. It now has the highest arapaima densities in the region, the researchers found.

But regulations like those implemented by the community of Ilha de São Miguel are not common in floodplain regions, Castello said. These areas, he explained, suffer from widespread illegal fishing, a fact that he worries could lead to fishing-induced extinctions for other Amazonian species.

Fixing the situation

Part of the problem, Castello said, is a lack of economic alternatives for the fishers who survive on the commercial trade of threatened fish species. But the researchers said their findings demonstrate that it's possible to save the arapaima from extinction without jeopardizing local food supplies.

"Fisheries productivity in Ilha de São Miguel is also the highest in the study area," Castello said. "Cast nets are allowed because they are much more selective, yet they yield abundant fishes for local consumption, so food security for the community is not compromised."

This bodes well for both fish and fishermen, said the researchers, who believe that spreading the fishing practices of Ilha de São Miguel to other areas of the Amazon could bring this unique species of fish back from the brink.

"Many previously overexploited arapaima populations are now booming due to good management," Castello said. "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."

The results of the study were published online today (Aug. 13) in the journal Aquatic Conservation: Freshwater and Marine Ecosystems.

Editor's Note: This story was updated to reflect that the arapaima's ability to breathe air is uncommon, but not unique.

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