SOMERVILLE, MASS. (WHDH) - Thousands of dead fish have been seen bobbing on the surface of the Mystic River in Somerville and Revere, leading many who live in the area to cover their noses.

“You want to put your mask on just to tolerate the smell. It’s just terrible,” said Alma Kennedy, who is visiting the area from North Carolina.

Wildlife experts say the fish are Atlantic Menhaden who were likely chased to a nearby dam by a predator and were left with nowhere to go.

“The school is large enough that they came in, they couldn’t find their way out, and while they were en mass, they used up the oxygen enough that they weren’t able to live in there,” said Mystic River Watershed Association Executive Director Patrick Herron. “We had a die-off of these fish.”

The die-off began a couple of days ago with dead fish washing ashore between Assembly Row and Everett. A snow removal service has been called in to help clean up the mess by the Encore Boston Harbor casino.

While the sight and stench of death is disturbing and hard on the stomach, Herron said it’s a natural phenomenon and even a positive sign of marine life.

“In some ways, the presence of these fish, while it is upsetting to see them on the surface, it’s also a hallmark of the fact that there is a healthy population of Menhaden in the sea,” he said.

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