FARGO (AP) - North Dakota's senior population is poised to swell by more than 55,000 by 2020, a new report says.

The demographic "tidal wave" will cause a projected loss of earnings from wages and salaries approaching $1 billion in 2020, said Richard Rathge, director of the North Dakota State Data Center and lead author of the report on the economic implications of the state's graying population.

"This is just one year, and this is not even the peak year," Rathge said.

The leading edge of the baby boom generation reached age 60 two years ago, and the number of those reaching retirement age by 2020 likely will swell by 55,785, or roughly the population of Bismarck.

That year, the number of seniors age 65 or older - almost 150,000 - will exceed the number of those in the prime working-age population, ages 35 to 54, a group projected to drop from 183,435 in 2000 to 146,717 in 2020.

The reduction in wage and salary earners has financial implications for state government, the reports said. Federal income tax figures from 2004 indicated that 72 percent of the tax liability was paid by wage and salary earners.

Seniors paid about 15 percent of North Dakota's $252.6 million total tax liability in 2005 and comprised 15 percent of the state's tax filers.

Despite the shrinking segment of workers in the prime age group, the economic projections predict North Dakota will experience an increase in total earnings of almost $570 million by 2020, with a much larger portion coming from retirement investments and Social Security.

Rathge said the shift to a more elderly population will descend rapidly in North Dakota. From 2000 to 2005, the increase was a modest 3.8 percent. But that will escalate to an increase of 17.5 percent from 2015 to 2020, he said.

"It's like this big tidal wave, it's moving forward," Rathge said of aging baby boomers, those born from 1946 to 1964. "And we've had calm seas for decades."

The report, which combines population projections with economic models developed by economists at North Dakota State University, is intended to provoke thought and discussion.

"It's intended as a wake-up call to say, 'Let's think about this,'" Rathge said.