Long Island’s population dipped slightly between 2015 and 2016, mainly because of large numbers of people who left Suffolk County — twice the number that left Nassau County — according to new population estimates the U.S. Census Bureau released Thursday.

Suffolk saw its population go from 1,497,903 in July 2015 to 1,492,583 in July 2016 — a decline of 5,320 people. Conversely, Nassau’s population went from 1,359,702 to 1,361,500 in that same time span — an increase of 1,798 people.

Long Island’s total 2016 population was 2,854,083, declining by 3,522 from the prior year, or 0.1 percent, said Jan Vink, a researcher for Cornell University’s Program on Applied Demographics, which is an affiliate of the Federal-State Cooperative on Population Estimates.

According to the census estimates, 11,278 people — the bureau calls them “domestic migrants” — left Suffolk for other parts of the nation, while 5,090 domestic migrants left Nassau. International migration into Long Island was similar for both counties, at about 4,300 for each for the one-year period.

“I would say domestic migration is the biggest problem” for Suffolk, Vink said. He added that in earlier decades, Suffolk’s birthrate would compensate for the domestic migration losses. Though Suffolk still has more births than deaths, “the number of births have been decreasing,” he said.

Vink speculated people leaving New York City for the suburbs might be drawn to neighboring Nassau, giving that county a boost over Suffolk.

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Domestic migration trends on Long Island and in the state fit a pattern that is seen throughout the Northeast and Midwest, said demographer William Frey, a senior fellow at the Brookings Institution’s Metropolitan Policy Program in Washington, D.C.

“Suffolk’s decline is clearly due to . . . domestic migration,” Frey said. “But it is not an isolated phenomenon. The four biggest New York City boroughs, as well as most other suburban counties in the metro area . . . sustained bigger out-migration in 2015-2016.”

In Vink’s analysis of the census population estimates, in which he combined New York State’s 62 counties into the state’s 10 economic development regions, only two — New York City and the Mid-Hudson Valley — showed population gains between 2015 and 2016.

New York City saw a 0.2 percent increase, or 21,171 more people, and Mid-Hudson Valley had a 0.1 percent increase, or 3,342 additional people.

New York State, according to the bureau’s estimates released last December, saw its population drop by about 0.01 percent, or nearly 1,900, to 19,745,289 for 2016.

Frey said that the greater New York metropolitan area, including some counties in New Jersey, saw about 200,000 people leave for other parts of the nation, a total that he said was “bigger than any year since before the [2007-2009] recession.”

The out-migration that slowed during the recession and several years since, has begun to pick up again, Frey said, and the southern and western Sunbelt states are reaping the gains.

And while Frey said the five New York City boroughs saw an overall population gain between 2015 and 2016, “it does have the biggest out-migration of the 100 largest” metro areas, with 121,411 people who left.

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Frey said while birthrates remain a factor, international migration was key to the region’s population growth.

“The only route of growth is international migration. That’s been the case for New York for 25 years,” Frey said. “That’s not a change.”