Since then, scientists there have discovered more than 200 dead Guiana dolphins, or Sotalia guianensis, a quarter of what was the world’s largest concentration of the species. The deaths, caused by respiratory and nervous system failures linked to a virus, have subsided, but scientists are working to unravel the mystery behind them.

How, they ask, did a virus that might ordinarily have claimed a handful of dolphins end up killing scores of them? And does part of the answer, scientists and local residents ask, lie in the bay itself, at once a testament to Brazil’s economic power and a portent of environmental risk.

The Guiana dolphin is considered a «sentinel» because the mammal is a top predator. «When something is wrong with them, that indicates the whole ecosystem is fractured,» said Mariana Alonso, a biologist at the Biophysics Institute at the Federal University of Rio de Janeiro.

Once a sleepy fishing area with white sand beaches and tiny hill-shaped islands, Sepetiba Bay, 60 kilometers west of downtown Rio, became one of the principal gateways for Brazilian exports over the past generation. In 2017, 39 million tons of iron ore and other commodities shipped from there.

The wooden fishing boats that crisscross the bay now weave around massive merchant ships loaded with iron and steel. Though people still swim in its waters, four ports and a constellation of chemical, steel and manufacturing plants have risen on its shores. One of world’s most prominent iron ore producers, Vale, occupies a new terminal in an old fishing spot on nearby Guaiba Island.

«The number of industries and ventures along Sepetiba Bay has been growing exponentially in recent years,» Dr. Alonso said. «What that generates is a greater concentration of pollutants in the seafloor and in the food chain.»

Scientists have attributed the rash of dolphin deaths to morbillivirus, an airborne virus from the family that causes measles in humans. The effects of the virus — rash, fever, respiratory infection, disorientation — suggest an agonizing death. Dying dolphins were seen swimming sideways and alone. Some carcasses had ugly deformations, and blood dripping from their eyes.

Sergio Hirochi, 49, a fisherman who was born in the area, said he had seen the bay’s environmental decline.

«From here, I see how much mineral waste winds up in the ocean,» said Mr. Hirochi. «The Bay of Sepetiba is an estuary, a nursery of species. And when you destroy it, you destroy marine life.»

Manufacturing plants arise in a fishing village.