The Bronx River was considered an “open sewer” for years, in large part because of the large amounts of industrial waste dumped in the New York City waterway in the 19th and 20th centuries. Dolphins were among the several species that the pollution had driven out — but last week, for the first time in years, they made a return. 

“It’s true — dolphins were spotted in the Bronx River this week,” the New York City Department of Parks & Recreation tweeted Thursday along with a video of at least two dolphins swimming in the water. “This is great news — it shows that the decades-long effort to restore the river as a healthy habitat is working.”

The Bronx River Alliance said that the dolphins were spotted at Starlight Park, an area in the Bronx that has previously unused developed land and has since been restored with vegetation, picnic areas, playgrounds and more. According to the BBC, this is the first time dolphins have made their way up to the Bronx in five years.

Two dolphins were also spotted in the Whale Creek Tributary last week next to Greenpoint, Brooklyn’s Newtown Creek Wastewater Treatment Plant. 

The parks department said the dolphins may have “naturally found” their way back to the waterway in the search of fish. In 2018, the department released 400 adult alewife, a type of herring, into the river as part of its efforts to help improve the water ecology. In a video released then, the department explained that the river is the only one that is freshwater in New York City, and that the hope is that the newly released fish will use the area to lay eggs and that their offspring would do the same. 

The river became highly polluted in the 19th and 20th centuries, according to the American Museum of Natural History, when it was used for the disposal of industrial waste, including fertilizers and oil. That pollution resulted in the loss of many plants and animals. 

But groups have been working for decades to restore the waterway. The Bronx Zoo stopped discharging more than 200,000 gallons of animal waste into the river in 2001 and the New York Botanical Garden stopped discharging thousands of gallons of pollutants into the water in 2002, according to the Bronx River Alliance. 

Though not often seen in the Bronx River, marine mammals are frequent visitors to the waters around the Big Apple. From spring to autumn every year, bottlenose dolphins visit local waterways to feed. 

As marine animals make their presence known in the area, the state’s Department of Environmental Conservation recently warned that people should avoid coming into close contact. The Marine Mammal Protect Act requires that people avoid touching, feeding, disturbing or harassing marine mammals, and says that those who do so are subject to fines and jail time.