Thousands of dead fish washed up in the Mystic River in Everett and Somerville this week, blanketing the muddy shorelines and emanating a stench that customers and employees at a nearby Costco store said could be whiffed from the parking lot.
But city and state officials, as well as local environmentalists, said Thursday that while the sight was alarming, the fish had died due to natural causes.
Patrick Herron, executive director of the Mystic River Watershed Association, said the Department of Environmental Protection and state Division of Fisheries and Wildlife were investigating what he called a “die-off” that left tens of thousands of menhaden — or, as they’re more commonly known, “pogies” — in the area.
Herron said the likely explanation is that the fish were driven into the shallow, warmer waters by a predator. Once in that area of the river, the fish depleted the oxygen supply and ultimately perished, he said.
“The die-off started seemingly on Monday, but on Tuesday afternoon into Wednesday, large numbers of the fish were showing up dead,” he said. “It appears they are in the high thousands, if not in the tens of thousands.”
According to the state Executive Office of Energy and Environmental Affairs, water samples recently taken by MassDEP investigators did not show any problems with oil or hazardous materials, indicating the fish kill was not pollution-related. The water was around 83 degrees Wednesday, they said.
“This kill of primarily Menhaden was likely due to natural causes such as aggregations for spawning and/or predator avoidance, localized oxygen depletion, and rising water temperatures,” according to a statement from the agency.
The toothless fish can reach a length of up to 12 inches. Many creatures feed on them, including tuna, striped bass, swordfish, and bluefish, according to the state’s website.
Tom Philbin, a spokesman for the City of Everett, said a number of residents sent photos and called to alert the city about the fish — and the bad smell.
A Costco employee and manager also reached out to city officials about the stench, he said.
In terms of cleanup, the next steps are still being determined, Philbin said.
Freelance photographer Katy Rogers said she caught wind of the dead fish in Everett on Wednesday, and quickly grabbed her cameras to go down and check it out. When she got there, she said, she was surprised by just how many fish had washed ashore.
Rogers posted a picture of the white-bellied pogies scattered up and down the banks of the river to her business’s Facebook page, where residents commented that they could smell the fish from the Costco lot.
“I’ve never seen anything like that,” she said. “When I first got the call, I was thinking, ‘This isn’t going to be a big deal,’ because I get calls all the time. But there was nowhere to step without stepping on a dead fish. It was disgusting.”