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WARWICK, R.I. — They may have simply stayed too long.

Over the last several weeks, Atlantic menhaden have rolled up with the surf, dying and dead, on beaches in Rhode Island, Connecticut, and New York. Marine biologists say they aren’t exactly sure why.

In Rhode Island, people reported fish kills in the Providence River last month, and dozens of dead menhaden were found scattered along Gaspee Point in Warwick on Dec. 31 and early January.

Connecticut and New York officials were also investigating scores of dead menhaden in their waters last month. The New York State Department of Environmental Conservation sent samples to Stony Brook University’s Marine Animal Disease Laboratory and to Cornell University to analyze for a viral infection or other potential causes.

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The samples tested negative for any specific disease, despite the fish showing symptoms like irregular swimming behavior, New York DEC said in a statement Wednesday.

Mike Healey, spokesman for Rhode Island DEM, said they’re surmising that the menhaden stuck around too long and died from “cold shock” when the water temperature suddenly changed.

DEM has gotten reports of menhaden swimming in upper Narragansett Bay — an ordinary sight in other times of the year, but unusual over winter. The fish kills reported in the Providence River in December were, in most cases, reported in the morning after a sudden drop in temperature when the tide went out the previous evening.

Menhaden, which run in estuaries and coastal waters from northern Florida to Nova Scotia, are vulnerable to environmental changes, and they’re susceptible to “cold shock,” so die-offs aren’t uncommon.

Why didn’t they take the hint and leave before winter? No one really knows.

“It’s hard to say exactly why they’re stayed in the bay, but it seems like they’ve missed some sort of environmental cue,” Rhode Island DEM principal marine biologist Katie Rodrigue said in an e-mail. “Perhaps the timing of a temperature change was not ‘lined up’ with their internal clock, or maybe there was good food availability in the bay which made them stay later and then miss the opportunity to migrate out.”

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Amanda Milkovits can be reached at amanda.milkovits@globe.com. Follow her on Twitter @AmandaMilkovits.