Meanwhile, the International Union of Conservation of Nature lists the Ganges river dolphin in India as endangered, and the Irrawaddy river dolphin in Bangladesh as vulnerable.
Scientists believe river dolphins likely arrived in the Amazon during the Middle Miocene era 16 million years ago, when ocean levels were high around the world, and the sea inundated what is now lush rain forest.
For centuries they have been revered by locals and protected by myth. According to one tale, the dolphins transform into handsome men and leave the water at night, seducing and impregnating local women before returning to the river. Many simply consider it bad luck to kill them, given their supposed magical attributes.
But today, the quick payoff is trumping legend and superstition.
"Killing the dolphins is a fast and easy way for the fisherman to make money. It costs nothing but time," Vera da Silva said. "It's ugly because these dolphins have a folkloric value in the Amazon, and all that is disappearing for the sake of using them as bait."