State officials looking into a massive menhaden die-off in the Shinnecock Canal have concluded the fish suffocated in the waterway’s confined locks, and they are moving to a cleanup phase.

Jim Gilmore, chief of the Department of Environmental Conservation’s marine resources section, said tests of water on either side of the canal showed healthy oxygen levels and no toxins that could have otherwise caused the event.

“It’s very simple. The fish got caught in the lock and suffocated,” he said at a DEC marine resources advisory council meeting Tuesday.

Fisheries regulators have seen swelling populations of the baitfish, also known as bunker, he said, including up to the Maine coast.

Die-offs have been more typical on Long Island in the Peconic Bay and River, in the spring. The problem was so pronounced in 2015 that regulators this year expanded the quotas to 30,000 pounds a day from the previous 6,000 and state and local governments subsidized a bigger catch.

“It’s just this really large population increase,” Gilmore said.

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Efforts now are centering on containing the damage caused by dead fish washing up on beaches and sinking to the bottom of Shinnecock Bay, he said, and trucks are taking the fish to approved landfills for composting.

“The problem is there is quite a lot of fish in Shinnecock Bay,” Gilmore said. “We’ll be monitoring it.”

Gilmore has also been in touch with Suffolk County, which owns and operates the canal, to see how a buildup of fish in the lock system can be avoided. He said Monday’s die-off of tens of thousands of fish was the first he’d seen in the canal.

“It was just too many fish in the wrong place at the wrong time,” he said.