Maine reached full employment at the end of 2015. Too bad that’s not as good as it sounds.

According to the Bureau of Labor Statistics, Maine’s unemployment rate dropped below 4 percent in 2015, and the news was even more impressive in the state’s three metropolitan regions, especially Portland, which was one of only 25 metro areas in the nation with unemployment under 3 percent.

That sounds great, and it’s encouraging to think that we are finally emerging from the Great Recession.

But the rate doesn’t tell the whole story. Maine has a slow-growing population, and it is aging quickly. The low unemployment rate has as much to do with people leaving the workforce as it does with an expanding economy, and a tight labor market will discourage future growth and opportunity. Maine policymakers should not be satisfied with the low unemployment rate — they should take steps to get more people in the workforce, giving businesses the fuel they need to start up and grow.

It’s important to remember what the employment rate measures and what it does not.

The rate tells you how many people in the workforce are out of a job and looking for a new one. It does not count people who lost their full-time jobs and can find only part-time work, and it doesn’t measure people who have given up looking for work.

The unemployment rate isn’t the only thing that’s shrinking. So is the workforce.

There were 676,627 employed and unemployed Mainers in the workforce last December. One year earlier, there were 693,676 people. In December 2007, just at the start of the recession, there were 701,498 people in the workforce, nearly 25,000 more people than there were at the end of last year. Most members of the baby boom generation — roughly 411,000 Mainers — are expected to leave the workforce by 2032. If every Mainer under the age of 20 sticks around and joins the workforce, there will be a shortfall of 109,000 workers.

Economic development efforts in Maine have usually been focused on attracting employers, but they should also be centered on ways of attracting workers from other parts of the country, and from other countries. Immigration by younger people is the only way that Maine can avoid the demographic winter that lies ahead.

And we have to make better use of the people who already live here. Young people need job training and counseling to enter the workforce. Healthy, active seniors are a key source of labor in an aging state. Many seniors are already employed, and many more would appreciate the income and connection that come with a part-time job.

Maine needs to do everything possible to bring more people into the workforce. Full employment will be something to celebrate when it’s a measure of an expanding job market, not a shrinking labor supply.