The Shinnecock Canal in Hampton Bays became the scene of a massive fish die-off Monday morning, with tens of thousands of menhaden — more commonly known as bunker — clogging the water surface for hundreds of yards.

Authorities, including state Department of Environmental Conservation investigators, responded to the scene Monday.

Regional DEC spokesman Bill Fonda said staff investigators were looking into the cause, but that initial indications were the die-off was “probably due to the usual reasons. . . . We don’t see anything so far to indicate it’s chemical” or from a pollutant.

“Typically, what happens with these fish kills is that a large number of fish become trapped in a confined area and dissolved oxygen levels go way down, leading to suffocation,” Fonda said.

Bunker often are preyed upon by a variety of predatory fish, such as striped bass, weakfish and bluefish, as well as by birds.

The fish are subject to massive die-offs on Long Island, especially in the fall — after they are chased by predators into creeks and inlets where oxygen levels are not sufficient to sustain the number of fish.

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The problem was so pronounced last year that federal, state and local officials this year expanded fishermen’s ability to harvest bunker to five times the normal levels.

As a result, there was no significant die-off in the river this spring and summer, officials said.

Southampton Supervisor Jay Schneiderman said he planned to declare a local state of emergency to help with the cleanup as dead fish shift from the surface of the canal to the Shinnecock Bay and begin to inundate bay beaches.

“It looked like a million dead fish on the surface of the canal,” Schneiderman said. “As soon as the tidal gates opened, that pushed a lot of the fish from the canal into the Shinnecock Bay. All the embankments are covered in fish. There are just fish everywhere. We hoped they’d get pushed out to sea.”

Schneiderman said some local boaters have been netting some of the fish to use and sell for bait, but he said he’s been in contact with a local bayman to do the work on a larger scale.

He’s coordinating with the town’s highway, municipal works and parks departments to supply trucks and equipment to take fish to a Hampton Bays composting facility, if possible.

Paris Hodges, a member of the Shinnecock Indian Nation, said he saw the wall-to-wall bunker swarm about 10 a.m. Monday as he was passing the canal in a car.

“It was just crazy,” he said. “I’ve never seen so many fish. It was like a nuclear fallout. They just kept floating by, and piling up on the beach.”