Union membership on Long Island hasn’t recovered from losses sustained during the Great Recession, as young job seekers leave the Island and aging union workers retire, according to a new study from Hofstra University timed for Labor Day.

Nassau and Suffolk counties had 302,868 union members, on average, during the 2014-16 period of the study. The number was calculated using data from the Current Population Survey, a labor report produced by the Census Bureau and the Bureau of Labor Statistics. Union ranks fell from an average of 317,450 members between 2004 and 2006, before the sharp recession that lasted from December 2007 to June 2009.

“Long Island has a slowly aging population,” said Gregory DeFreitas, a Hofstra economics professor and director of the university’s Center for the Study of Labor and Democracy. He co-authored the study with Bhaswati Sengupta, formerly of Hofstra and now an assistant professor at Iona College in New Rochelle. “The retiring Baby Boomers are an increasingly large population share in comparison to the rest of the country and especially New York City. That causes problems for unions.”

The number of union members on Long Island increased slightly from a recession low of 300,924.

The graying of Long Island’s unions is reflected in statistics from the study. There were more than 14,600 union workers here 65 and older in the 2014-16 period, up from more than 8,100 in 2004-06.

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The number of union members ages 25 to 34 fell to about 48,300 from more than 64,000.

The shortage of young workers isn’t just a problem for unions, DeFreitas said. Spurred largely by Long Island’s lack of affordable rental housing and minimal public transit options, young people are seeking job opportunities elsewhere.

“For the future economic health for Long Island, it’s increasingly well recognized that steps have to be taken to reverse the aging of the population and the labor force,” he said. “We really depend on retaining young people and recruiting young people to grow our economy.”

Workers ages 25 to 34 were only 16 percent of the Long Island workforce in 2014-16.

They constituted about 30 percent of New York City’s workforce, making them the largest age group, the study found. The city was a magnet for young workers.

While Island unions face demographic obstacles, market conditions also pose challenges.

In construction, a Long Island industry with high union density, pressures from clients have had an impact on building firms and unions.

Plainview-based EW Howell Construction Group, a 125-year-old firm, relied on union workers exclusively for most of its history. About 10 years ago, however, demand rose for lower-cost labor from private-sector clients, leading the company to use more nonunion sources of labor for projects.

“It’s just a response to the economic pressure on projects today really driven by the owner’s requirements to get the price to something that’s economically feasible to them,” said Howard Rowland, chief executive of EW Howell.

“It was a trend we were not really anxious to follow, but economic pressures have pushed us and everybody else in that direction,” he said.

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While the firm now uses a mix of union and nonunion workers, Rowland said the company maintains good relations with unions.

Union officials said they realized that Long Island and their organizations need more young workers.

Investment in infrastructure and affordable housing would help retain the next generation of union workers, said Roger Clayman, executive director of the Long Island Federation of Labor.

“If we were to do these projects in a way where there are good union jobs created in the process, we make more livable communities and more opportunities for our millennials to live and work,” he said. “Every industry that’s here on Long Island grows and prospers when there’s an adequate supply of workers.”

The Hofstra study found that union membership was slightly higher in the city — nearly 876,000, up from 856,300 before the recession — and the number of members ages 25-34 grew as well.

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Of Long Island’s roughly 1.25 million workers, 24.5 percent are in a union, according to the report, down from 25.3 percent in 2004-06.

But Long Island and New York remain more heavily unionized than the nation. Union members represented 23.6 percent of the workforce in the state in the most recent period in the study.

Union membership in the United States fell from an average of 15.5 million in 2004-06 to 14.6 million in 2014-16, with the union percentage of the total U.S. workforce at 12 percent in that recent period, about half the rate on Long Island.

“As of last year, New York State is the only state in the country that has over 20 percent of the workforce who are union members,” DeFreitas said. “If you go back into the Sixties, the majority of states had over one-fifth of their workers in unions.”

Unlike New York City and the nation, Long Island has more union members in the public sector than in the private.

More than 70 percent of the 237,152 public-sector jobs here are unionized, while union members make up only 13.5 percent of private-sector jobs. The union totals come to 168,274 members in the public sector and 134,594 in the private sector.

Median wages for union members in hourly full-time nonmanagerial working-class occupations are higher than their nonunion equivalents on Long Island, the study found: $25.27 and $16.83, respectively, though factors such as work experience and specific industry play a part in those differentials.

“There’s a sizable union advantage in pay still,” DeFreitas said. “Unions have maintained that post-recession.”

Costs remain a concern for employers working with unions, said Martin F. Scheinman, a Port Washington-based mediator and arbitrator.

In many of the cases he works on, “the employer is saying, ‘The paradigm doesn’t work for me vis-à-vis my competitor,’ ” said Scheinman, who oversaw the resolution in July of the strike by unionized workers of beer distributor Clare Rose, based in East Yaphank.

Scheinman, who has worked on labor disputes throughout New York, said that for a lot of employers the recession had long-lasting effects, making them reconsider the cost of defined pension benefit packages, a longtime staple of union membership.

“If you’re a company that’s doing well but not doing as well as you think you should do, you say, ‘I’ve been disadvantaged by the terms and conditions I provide my workers,’ ” he said. “And the workforce is saying in all of these instances, ‘Stop trying to take away things that were a part of our deal.’ ”

For management of the Northwell Health network, working cooperatively with its roughly 27,000 union workers is a priority, said Joseph Moscola, senior vice president and head of human resources for the health care provider.

The health system, the largest in New York State, works with 33 different bargaining units representing different unions throughout New York, making “open dialogue” and consistent communication key.

“Time is spent on developing a trusting relationship,” Moscola said.

Two of the longest strikes on Long Island in the past decade occurred this year.

In April, 130 workers from Teamsters Local 812 at Clare Rose struck over proposed wage cuts and the replacement of a pension plan with a 401(k) retirement plan. For more than 80 days striking workers picketed the firm’s two Island facilities and called for a boycott of its products.

The strike was one of only three work stoppages on Long Island to exceed 60 days in the last 10 years, according to the Federal Mediation & Conciliation Service, a federal agency that serves both unions and employers.

Sixteen Teamsters Local 807 workers at Alside Supply Center in Old Bethpage went on strike April 1 over proposed changes to health care plans and removal of pensions, a spokesman for Teamsters Joint Council 16 said. The strike against the Ohio-based company ended Aug. 28, and workers have since found work at other Teamster-represented companies, the spokesman said.

Long Island’s recent strike activity stands in contrast to the nation’s. The frequency of work stoppages — strikes and lockouts — across the country has fallen sharply since the late 1970s.

Last year, there were 97 work stoppages in the country, the lowest number since the federal mediation service began keeping records in 1948. The record number was 3,111 in 1977, according to the service.

One factor behind the strikes on Long Island is the region’s relatively healthy economic conditions, DeFreitas said.

Unemployment on Long Island was 4.5 percent in July, the state Labor Department said. So striking workers have less fear of not finding other work, DeFreitas said.

“Usually strikes happen and last longer during good economic times, because workers aren’t scared stiff they can’t get another job,” he said. “If you don’t have good job prospects and you’re scared for your job, then you’re more hesitant to join in a strike.”