As the nation has modernized, the threats confronting the river dolphins have multiplied. Mercury from illegal gold mines, fertilizer from farms and industrial waste from factories have polluted the Irrawaddy. Increased ship traffic has harried the dolphins, and collisions are often fatal. Overfishing has devastated food sources, and the dolphins can get trapped in fixed gillnets and drown. Several are believed to have been electrocuted by fishermen illegally using car batteries to try to stun fish.

In 2012, the Wildlife Conservation Society reported finding 77 dolphins in the Irrawaddy. A 2016 survey found only 65. Since 2006, Myanmar’s government has established a special dolphin protective zone stretching about 50 miles north of Mandalay. But in the zone’s early years, there was little enforcement against electroshock fishing or pollution.

Recently, patrols have increased, and U Han Win, a researcher at Myanmar’s Department of Fisheries, says that dolphin conservation efforts have turned a corner. “The population decline of the dolphins would have been steeper if not for conservation,” he said. “Today, despite many challenges, the population is stable and doing better than it would have otherwise because of the conservation efforts.”

Mr. Thin Myu, the fisherman, said that the increased patrols had made a positive difference. “Because of the patrols, now there are fewer electric fishermen,” he said, “and so the dolphins are working more cooperatively with us.”

As he waited for the convoy of barges to rumble past his boat, Mr. Thin Myu patiently smoked a cheroot. After the convoy passed, he was again able to call the dolphins. He yelped at two he recognized, one by the distinctive white band around its neck, as if greeting old friends.

Image When the dolphins stopped being prey and became partners is not known, but U Thin Myu, 44, says he thinks it started in the time of his great-grandfather, who was a fisherman as well. Credit... Doug Clark

One of the dolphins turned upside down, lifted its tail out of the water and slapped it down hard to the right — signaling to Mr. Thin Myu the location of their prey. With a shot-putter’s twist, he cast his net. The white mesh billowed open and came down over the roiling water. Another dolphin swam behind the splashing fish, herding them into the net. As Mr. Thin Myu reeled it in, the dolphins plucked at escaping fish.