Traffic on Southwest Airlines and AirTran fell in January, which was a mixed month for the airline industry.

Southwest joined Delta in reporting that traffic fell last month compared with January 2011, while American Airlines and its American Eagle short-haul affiliate both reported gains.

January is typically a slow month for travel, and most airlines reported that they trimmed passenger-carrying capacity from a year earlier, which helped push prices higher.

Southwest Airlines Co. said Tuesday that paying passengers flew 7.38 billion miles last month, down from 7.59 billion in January 2011. Southwest acquired AirTran in May 2011 but it added AirTran's January 2011 numbers to those of Southwest for year-ago comparison purposes.

Combined, the airlines reduced passenger-carrying capacity by 0.7 percent. Airlines can cut capacity by operating fewer flights.

The average flight was 74.3 percent full last month, compared with 75.8 percent a year earlier.

Southwest plans to eventually fold AirTran into its own brand and eliminate the AirTran name. It expects to begin repainting some AirTran planes in Southwest colors in the next couple months.

Chief Financial Officer Laura Wright said last week the company has completed the combination of back-office operations including finance, marketing, revenue management and purchasing. Pilots and flight attendants for the two airlines have ratified agreements to combine seniority lists, an important step for airline workers, while mechanics are still voting.

Wright said Southwest expects to hold 2012 capacity flat with last year even with the addition of new Boeing 737-800 aircraft that have more seats than the company's current 737s.

On Monday, AMR Corp.'s American Airlines reported that January traffic grew 1.4 percent, and its Eagle affiliate posted a 10.3 percent gain.

Delta Air Lines Inc. said that traffic fell 1.5 percent from a year ago, but it reported a 14.5 percent jump in a key figure, passenger revenue for each seat flown one mile. The revenue number indicated that airlines are charging higher prices than a year ago, largely by holding down the supply of seats.

Delta slashed January capacity by 4.3 percent, while American reduced it by 2.1 percent.

Southwest ended Tuesday down 11 cents at $9.64.