NEW YORK >> U.S. airlines continue to collect record fees from passengers who check suitcases or make changes to their reservations. In the first three months of this year, airlines took in $1.6 billion in such fees, up 7.4 percent from the same period last year.

That’s the highest amount for the first quarter since bag fees started in 2008, according to data from the Bureau of Transportation Statistics. January through March is traditionally the slowest period for air travel.

The fees climbed, in part, because more passengers flew at the start of the year — 3.2 percent more seats were filled. The airlines also increased some fees and forced more passengers to pay them.

While the standard fee for the first checked suitcase remains $25, fees have been going up for the second or third checked bag or for overweight luggage. And more passengers are now subject to those fees, which can be as high as $200 per bag. For instance, United Airlines recently reduced the number of bags some elite frequent fliers can check for free.

And JetBlue Airways announced in November that it will soon start charging for checked baggage for its cheapest fares, although it has to disclose the amount of the fee and when it would apply to passengers.

All of these add-on charges, along with falling jet fuel prices, helped the largest 26 U.S. airlines post a combined $3.1 billion in profits during the first quarter. Their net profit margin reached 8 percent, a significant jump from the 1.3 percent margin the year before.

During the past 12 months, the airlines took in $3.6 billion in bag fees and another $3 billion in reservation change fees, typically $200 for domestic tickets and as high as $1,000 for an international itinerary.