Long Island notched small population gains from 2014 to 2015 on the strength of growth in Nassau County, while Suffolk County’s population dipped — largely the result of people leaving for other parts of the nation, according to U.S. Census Bureau estimates released Thursday.

Nassau’s population grew by 2,677 people between July 1, 2014, and July 1, 2015. Suffolk’s population, however, declined by 755 people during the period.

Nassau’s 2015 population was estimated at 1,361,350 and Suffolk’s at 1,501,587, the bureau’s data show.

Peter Borsella, a Census Bureau demographer, said Suffolk’s population decrease represented 0.1 percent of the county’s total population. “Technically, it’s a decline, but it’s pretty much flat,” he said.

Both counties registered more births than deaths — what the bureau calls a natural increase — that were of similar size, at about 3,000 people.

However, the bureau estimated that more than twice as many people left Suffolk County for other parts of the nation than left Nassau during the one-year time period. Those losses from Suffolk were estimated at 9,522 people, compared with 4,471 in Nassau.

International migration into Nassau of more than 4,900 people overcame the outflow of residents to other parts of the country. That was not the case in Suffolk, which had about 5,300 international migrants.

While it was unclear why more people were leaving Suffolk than Nassau, experts noted that the New York City metropolitan region and the Northeast overall have long experienced significant domestic migration losses.

Census data show such losses for both Nassau and Suffolk were far higher from 2000 until the middle of that decade, but slowed during and after the 18-month recession that began in 2007 and officially ended in June 2009.

“The whole metro area is showing net out-migration,” said William Frey, a demographer who is a senior fellow in the Brookings Institution’s Metropolitan Policy Program. “It has to do with people moving to the Sunbelt” for jobs, more affordable housing and warmer climes, he said.

With the recession “easing up,” Frey said his analysis of the data show “there’s this movement out of New York again. It seems to be affecting more of the outer suburban counties than the inner suburban counties.” He placed Suffolk among the outer suburban communities, along with Putnam and Dutchess counties.

Jan Vink, a researcher with Cornell University’s Program on Applied Demographics, said in an email that multiple factors affect whether people move in or out of an area. He cited, for example, the availability of or lack of jobs, affordable housing, the “age structure” of an area’s population, and a perceived “risk of events like flooding.”

While Vink said he couldn’t comment on what factors might be applicable to Long Island, he noted that he found it “striking” that Suffolk had shown a decrease in the number of births since 2000.

“More analysis is needed to say something about why that is,” Vink said. The Cornell program is an affiliate of the Federal-State Cooperative on Population Estimates.

The metropolitan area encompassing New York City, Long Island and portions of New Jersey and Pennsylvania was by far the most populous metro area in the country in 2015, with nearly 20.2 million people.

The bureau’s estimates showed strong gains among counties in Texas, Florida and the coastal areas of the Carolinas. Last year, the bureau said, four Texas metropolitan areas collectively had an increase of more people — 412,000 — than any other state except for Texas itself, which added 490,000 people.

CORRECTION: An earlier version of this story misspelled Peter Borsella’s name.

Population estimates

2015 population

Nassau: 1,361,350

Suffolk: 1,501,587

2014 population

Nassau: 1,358,673

Suffolk: 1,502,342

Source: U.S. Census Bureau

Latest Videos