Aided by strong investment results, multiemployer pension plan funding levels jumped last year, but many of the plans remain significantly underfunded, according to a new survey.

The Milliman survey — the first of its kind by the Seattle-based actuarial and benefits consulting firm — found that on average the plans were 81% funded in 2013, up from 72% in 2012.

In all, the analyzed plans had $473 billion in assets at year-end 2013, up from $414 billion a year earlier, while liabilities increased to $585 billion from $571 billion.

“On an aggregate basis, 2013’s strong market performance helped these plans return to funding levels similar to what they saw ahead of the global financial crisis,” Milliman said in a statement.

Still a hefty number of plans remain significantly underfunded. For example, at year-end 2013, 37% of the plans were less than 80% funded, an improvement compared to 2012, when 58% of the plans were at less than 80%, but substantially more compared to 2007 when 29% of the plans were less than 80% funded.

But many of the plans are becoming more mature, with a growing percentage of retirees collecting benefits and a shrinking percentage of active participants. For example, between 2002 and 2012, the number of active plan participants fell to 37% from 48%.

A shrinking active participant base means “there are relatively fewer participants on whose behalf contributions are being made into pension funds, with an ever-growing level of participants entitled to current or future benefits putting a significant strain on these funds,” the survey said.

The Milliman survey is based on its analysis of publicly available reports filed by nearly 1,300 multiemployer plans.

Multiemployer pension plans are maintained under collective bargaining agreements between employers and unions and typically cover employees in the same or similar industries. According to the Pension Benefit Guaranty Corp., the plans have about 10.4 million participants.

Milliman said it plans to update the analysis twice a year. It currently releases each month an analysis of funding levels of the 100 largest single-employer pension plans.