Thousands of fish die near Assembly Square, Encore

Expert: Overpowering odor will linger until bodies sink

SOMERVILLE, Mass. — For the Menhaden, the area where the Mystic River meets the Atlantic Ocean might as well be the Bermuda Triangle. Three times since 2018, large schools of those fish have ended up dead in the waters between Everett and Somerville. The latest fish kill happened this week.

Patrick Herron, executive director of the Mystic River Watershed Association, first noticed the school, alive, a week ago when he was out on the water.

“At that time, we had some concerns what could possibly happen here,” Herron said. “At least in the last five years, we’ve seen at least two other events here where this large school will come in here and have a hard time finding their way out.”

It’s not known why the Menhaden have trouble returning to the open sea. But, once trapped near the dam, the results are devastating. Herron said it’s likely the thousands upon thousands of fish crowding the space consumed available oxygen, causing the school to suffocate.

But Rob Moir of the Ocean River Institute, a conservationist group based in Cambridge, believes it’s also possible the Menhaden swam into an oceanic ‘dead zone,’ that is, a section of water devoid or with an extremely low concentration of oxygen. And the cause of that is climate change, he said.

“Algae has been blooming in the water because it’s been hot and sunny,” Moir said.

Not only that, he added, the algae has had nutrients to feed on from run-off pollution: “Septic and sewage, agriculture, and from people putting fertilizer on lawns.”

Herron said, in such an intensely urban area, it’s always possible some pollutants could have made their way into the water. But he notes there was no heavy rain event to trigger runoff.

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“Our own water quality data that tracks over time shows that water quality is actually quite good in this area,” Herron added.

How the Menhaden wind up in such dire straits is somewhat of a mystery, as well. But, they are considered one of the most important species in the oceanic food chain with everything from whales to striped bass making meals out of those oily fish.

It’s possible, Moir and Herron said, that striped bass may have been in pursuit of this particular Menhaden school.

Throughout the day, visitors to the park adjacent to the fish kill stopped to snap shots of the thousands of silver bodies. Most floated motionless, but a few thrashed so violently they leapt out of the water in their final throes.

On the rocky shore, flies covered the desiccated bodies of the predeceased. The odor was overpowering.

“Pretty shocking, a bit overwhelming to see all these fish here dead and washed up on the shoreline,” said Leon Armand, a Boston resident visiting the adjacent park.

“Seeing all these fish out here – and this is a special place for me – and to see it kind of polluted like this, and to see this happening. It’s extremely disheartening,” added his companion, Sana Bosquet.

As for what will happen with the fish? The answer is: nothing. After a few more days of bacterial decay – and, yes, odor-emanation – Herron said the bodies will sink, pretty much all at once. And then the tide will take them out to sea.