“People could not believe he could fish so many tons in one night,” said Nélio da Silva Campo, 32. “He became a professional at it.”

The fishermen soon discovered that the catfish was a potential bonanza. “In just two hours we would be making 100 reals,” Mr. Rego said, more than $60. “It was fast.”

Mr. Rego and other fishermen said they had stopped slaughtering dolphins about a year ago, fearing action by the authorities, and now used pig meat to catch the catfish instead. But two of their wives said the fishermen continued to kill dolphins, sometimes in front of their homes.

“I saw many die here,” said Silvia Rego de Santos, 31.

In Igarapé, veteran fishermen like Edilson Rocha, 58, recount stories of their battles with the pink dolphins. To the fishermen, the dolphins are abundant in the river and should not receive special environmental protection. They say they cause a nuisance by getting caught in their nets trying to feed.

“We don’t like him; we are his enemy,” said the burly Mr. Rocha, minutes after hoisting a stingray from the river with a lone fishing line. “I killed one when I was waiting for the fish to bite,” he continued. “He kept coming closer and the fish were leaving, so I harpooned the dolphin. I couldn’t stand it anymore.”