Dolphins have been spotted in the Bronx River after years of pollution and industrial waste, sparking hope across environmentalists and city residents alike.

Two dolphins were unknowingly spotted in the Bronx River and recorded on video by an onlooker on Jan. 19. The sighting was later confirmed by the New York City Department of Parks & Recreation.

“It’s true—dolphins were spotted in the Bronx River this week! This is great news—it shows that the decades-long effort to restore the river as a healthy habitat is working. We believe these dolphins naturally found their way to the river in search of fish,” tweeted the NYC DPR.

The NYC DPR reported the sighting to be a result of its restoration process focused on combatting the heavy pollution from the 19th and 20th centuries in the Bronx River.

“It became polluted during the 19th and 20th centuries because it was used as a natural sewer to dispose of industrial wastes like fertilizers (nitrogen and phosphorous), PCBs, and oil,” according to the American Museum of Natural History.

When fertilizers are disposed into ocean bodies, they create excess nitrogen. This excess can lead to an increase in plants that use more oxygen, depleting available oxygen within those bodies.

Numerous projects by researchers, alliances and volunteers were put into place to counteract the effects of industrial waste on the Bronx River.

One pilot project from 2017 studied how a marine ribbed mussel could aid in cleaning efforts.

“As it eats, a single mussel can filter up to 20 gallons per day, remove excess nitrogen both by assimilating it into their shells and tissues and burying it in the sediment as waste,” reported WIRED.

These mussels, by removing excess nitrogen, made the water in the Bronx River cleaner for the pilot project and were then put into place by the NYC DPR in 2018.

Most of the fish and plants originally found in the Bronx River were replaced with invasive species.

“These waters once ran silver in the spring as alewife herring made their way back to their home river to spawn. Sadly, these waters were fouled and disregarded over time,” according to the Bronx River Alliance.

The NYC DPR released 400 adult herring into the Bronx River in 2018. The idea for releasing the fish was to create a family in which the herring would reproduce and reinhabit the river every season, attracting other marine wildlife to the river.

After five years the NYC DPR is seeing the effects of its restoration projects — with the dolphins now visiting the Bronx River. This new development is confirmation to the Bronx River Alliance and ecologists that their efforts were successful.

With the dolphins now visiting the estuary for possible fish it means the reimplementation of an ecosystem.

“Bottlenose dolphins can thrive in many environments and feed on a variety of prey, such as fish, squid, and crustaceans (e.g., crabs and shrimp),” according to NOAA Fisheries.

If more dolphins visit the Bronx River in the coming years, it could mean the ultimate restoration of the estuary.