NEW YORK (CBSNewYork) – The New York area used to be home to millions of oysters, but overfishing and pollution led to their decline.

Now one nonprofit is stepping up to bring them back, reports CBSN New York’s Nina Kapur.

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Though they may be little, oysters have a big job.

“Oysters do everything, they filter and provide a habitat for other organisms,” said Dr. Alliosn Fitzgerald.

As “nature’s filter,” one oyster alone can clean up to 50 gallons of water a day.

In mass numbers, their reefs create a solid barrier between land and sea, protecting coastlines from erosion.

“In their daily life they’re helping us clean the water, pulling pollutants out, pulling extra sediment, and cleaning the water – but also by living there in close association with each other as a reef, they attract other organisms as a nursery or refuge from predators or to feed,” said Fitzgerald.

Volunteers and staff with the nonprofit NY/NJ Baykeeper group hit the Bronx River several times a week from May to November.

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There they study existing oysters and place dead oyster shells in the reef, giving new ones a base to latch onto.

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“We inventory and measure the oyster population thats on the reef as well as the other fish and other types of animals that are living off the reef or using it as their home,” said volunteer Meghan Magee.

By doing so, the nonprofit is ultimately keeping the Bronx river clean.

The water makes for a perfect home. It’s brackish, a combination of salt and fresh water. Although it’s muddy, slippery and uneven out there, volunteers say it’s rewarding work.

“I want to leave this world a better place than when I got it,” said Magee.

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So far, the nonprofit has restored more than 7 million oysters across New York and New Jersey.