BANGOR – Passenger traffic at Bangor International Airport was down 4.7 percent the first five months of 2007, compared with the same period a year ago, and officials said this week they believe it will be increasingly difficult to compete with other airports that have attracted low-cost carriers.

In May alone, BIA recorded a 16.2 percent decrease in passenger traffic, dropping from 118,389 travelers during the same month in 2006 to 112,154 this year.

Delta Air Lines, the airport’s largest carrier, serving a third of the Bangor market, experienced a 33-percent decline in May passenger traffic compared with last year. At the same time, Portland International Jetport reached an all-time monthly boarding record in May, flying 135,375 people, an increase of 32 percent over May 2006.

“Portland right now is experiencing tremendous success and is really an anomaly in New England,” said J. Brian O’Neill, assistant airport director at Manchester-Boston Regional Airport, which competes with Portland and Bangor.

Low-cost airline JetBlue moved into Portland last May, and AirTran Airways began nonstop flights to Baltimore and Orlando, Fla., in early June. AirTran will fly the Orlando route only on Saturdays until November, when greater numbers of people head south, said Greg Hughes, marketing manager for the Portland airport.

“The last three out of five months were all-time records,” Hughes said. “We are looking at an awesome year coming up.”

The influx of low-cost carriers “has a rippling effect on mainline carriers, too,” Hughes said. Price is a determining factor for many travelers, and the new carriers have made Portland more competitive overall, he said.

“We want to take from Logan [in Boston] and Manchester, but we want Bangor to be as healthy as we want to be,” Hughes said. “We are not in competition with Bangor.”

June statistics were unavailable on Tuesday.

“Obviously we are disappointed with the passenger numbers,” said BIA Director Rebecca Hupp. “Unfortunately, we don’t set the schedule or the pricing.”

Most airports are struggling to recover from a devastating year in 2006, during which many airlines filed for bankruptcy protection and restructured, said Manchester’s O’Neill.

“It is a very fickle industry, and airports like Bangor and Manchester are truly held hostage to [airline] corporate decisions,” O’Neill said. “Airlines make decisions that dramatically impact the airport’s ability to provide services and flights that our passengers need.”

At BIA, Delta pulled its 50-seat passenger jets from the Bangor-Boston route in April and replaced them with 19-seat turboprop aircraft. The smaller planes, paired with an additional trip daily, were considered the best match for the Bangor market, said Betsy Talton, spokeswoman for Delta.

That switch could have been a factor in May’s 33-percent passenger decline, she said.

In April, Hupp reported to the city of Bangor, which owns BIA, that airlines using the airport had decreased capacity, or the number of available seats for a route, by 10 percent when compared with April 2006.

From January to April 2007, capacity was down 18 percent, she said.

A reduction in the number of flights is a national trend, Hupp said.

Despite the trends, Delta’s flight to and from Atlanta is doing very well, she said.

The Manchester airport’s passenger counts peaked in 2005, when 4.4 million travelers boarded, but in 2006 only 3.9 million passed through the airport gates, O’Neill said. Before 2006, the last time Manchester saw an annual decline was in 1995, according to the airport’s Web site.

In order to address Bangor’s disappointing numbers, Hupp said, the airport has been working with focus groups to look at advertising strategies and how to promote the facility in the region. The airport also will look into partnering with groups that promote the state, such as area Chambers of Commerce, visitors’ bureaus and tourism boards.

While trying to improve air service, BIA wants to “encourage existing carriers to add capacity and try to attract a new carrier on a new route,” Hupp said.