PORTLAND — Ocean temperatures off the northeast United States reached a record high for the first half of 2012, the National Oceanic and Atmospheric Administration said Tuesday.

The average sea surface temperature for the waters over the Northeast continental shelf, from North Carolina to Canada, was a little over 50.5 degrees from January through June, breaking the previous record of 50.45 degrees for the same six months set in 1951. The average temperature for those months during the past three decades has been about 48 degrees.

The rising temperatures impact virtually all ocean life, said Kevin Friedland, a scientist who works in Narragansett, R.I., for NOAA’s Northeast Fisheries Science Center.

Atlantic cod and other commercially valuable fish, for instance, have been shifting northeast from their historical distribution centers in recent years because of warming waters. Warm water was blamed for lobsters shedding their shells a month or more earlier than usual in Maine waters this past spring, leading to a strong early harvest that created havoc within the industry.

As cold-water species move north in search of colder waters, warmer-water species will do the same as their waters warm up as well, Friedland said. The rising ocean temperatures could also affect the biological clocks of many species, which spawn at certain times based on environmental signals such as water temperature.

“There’s going to be climate winner and losers,” Friedland said. “If it’s too warm for a species, it opens up a niche for other species that are tolerant of warmer waters.”

NOAA has been collecting ocean water temperature readings along the Northeast coast using ship-board measurements since 1854, and by satellite since 1982, Friedland said.

For the first six months of the year, above-average temperatures were found at all ocean depths, from the bottom to the surface, and throughout the range from Cape Hatteras to the Canadian Maritimes. They were also found beyond the continental shelf to the Gulf Stream, the warm ocean current that flows from the Gulf of Mexico along the East Coast beyond the shelf.

In some nearshore locations, such as the Chesapeake and Delaware bays in the Mid-Atlantic, the average temperature for was about 11 degrees above the historical average at the surface and 9 degrees above average at the ocean bottom.

Scientists say two possible factors to explain the water temperature rise are the above-normal air temperatures across the region last winter and the warming of the Labrador Current. The Labrador Current begins in the Labrador Sea, between eastern Canada and Greenland, and brings cooler water to the Gulf of Maine and points south along the eastern seaboard.

Some water temperature variations historically have also been attributed to long-term naturally occurring water oscillations.

Glen Gawarkiewicz, a scientist of physical oceanography at the Woods Hole Oceanographic Institution in Falmouth, Mass., was on a scientific cruise in May off North Carolina where the water temperatures were 9 degrees warmer than a cruise in the same location in 1996.

During the May cruise, scientists were unable to find bluefish and butterfish they were looking for, but instead found warmer-water fish, including amberjack and blue runners, that normally wouldn’t have been there, Gawarkiewicz said.

It’s unclear exactly why the ocean temperatures are rising as they are, he said. Perhaps more importantly, it’s also unclear if the rise is long-term or a short-term anomaly.

“That’s the $64,000 question,” he said.

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