An Amazon river dolphin swims in the Airao River in Amazonas state, Brazil, in this November 2005 photo released by Instituto Nacional de Pesquisas da Amazonia (INPA). Researchers say Amazon river dolphins are increasingly being slaughtered and used as fish bait in Brazil and the practice threatens to wipe out local populations of the mammals, which have already died off in other parts of the world.

Rio de Janeiro ? The bright pink color gives them a striking appearance in the muddy jungle waters. That Amazon river dolphins are also gentle and curious makes them easy targets for nets and harpoons as they swim fearlessly up to fishing boats.

Now, their carcasses are showing up in record numbers on riverbanks, their flesh torn away for fishing bait, causing researchers to warn of a growing threat to a species that has already disappeared in other parts of the world.

“The population of the river dolphins will collapse if these fishermen are not stopped from killing them,” said Vera da Silva, the top aquatic mammals expert at the government’s Institute of Amazonian Research. “We’ve been studying an area of 27,000 acres for 17 years, and of late the population is dropping 7 percent each year.”

That translates to about 1,500 dolphins killed annually in the part of the Mamiraua Reserve of the western Amazon where da Silva studies the mammals.

Da Silva said researchers first began finding dolphin carcasses along riverbanks around the year 2000. They were obviously killed by human hands: sliced open and quartered, with their flesh cut away.

The killings are becoming more common, researchers and environmental agents say. Even the government acknowledges that there is a problem. It’s already illegal to kill the dolphins without government permission — as with all wild animals in the Amazon. But little is being done to stop it.

Less than five agents are tasked with protecting wildlife in a jungle region covering the western two-thirds of Amazonas state, which is more than twice the size of Texas, according to the Brazilian Institute of Environment and Renewable Natural Resources (Ibama), the enforcement arm of the Environment Ministry.

“It’s a matter of priority, and right now the government is focusing on deforestation,” said Ibama’s Andrey Silva. “The killings of these dolphins exists — it’s a fact.”

The dolphins are attractive to anglers for their fatty flesh that is a highly effective bait for catching a type of catfish called piracatinga.

Consumption in neighboring Colombia is driving the slaughter. Some 884 tons of the fish came from Brazil in 2007, according to the Colombian Institute for Rural Development. That jumped to 1,430 tons in 2008 and spiked to 2,153 tons in 2009.

Simple economics exacerbates the problem: Killing dolphins is free, and their meat is valuable. Using the flesh from one carcass, fishermen can catch up to 1,100 pounds of piracatinga. According to da Silva and other researchers, they can sell the catfish for 50 cents per kilogram, translating into $550 for just a few nights’ work — about double Brazil’s monthly minimum wage.

“It’s attracting a lot of poor people to this region to kill the dolphins and make easy money,” said Antonio Miguel Migueis, a dolphin researcher with the Federal University of Western Para since 2005.

So far it’s impossible to quantify the exact impact fisherman are having on the river dolphins — little research has been done to study the killings or even the overall population of the dolphins in the Amazon.