The volume of air freight moving through L.A.-area airports has been slowing down, as importers who diverted their goods to planes to avoid the massive congestion at Southern California ports last year have returned to the slower but cheaper ocean carriers.

For the first five months of this year, cargo traffic dropped 10 percent from a year ago at Ontario International Airport, 7.5 percent at Long Beach Airport and 4.3 percent at Bob Hope Airport in Burbank.

Traffic at Los Angeles International Airport increased 1.7 percent for the five-month period, but even that represented a slowdown from the 3 percent gain a year earlier.

“You’re seeing a return to ocean on those shipments that were most sensitive to the surge in cargo congestion and cargo delays associated with last summer’s peak season problem,” said Ezra Finkin, legislative deputy for the Waterfront Coalition, a retailers’ trade group.

Last year’s diversions added significant costs for shippers. Sending a 40-foot container of goods to L.A.-Long Beach from Hong Kong by air costs $50,000 to $60,000, compared with $2,500 to $3,000 by water. Travel time is a day by air compared with 11 to 14 days by water.

“The ultimate cost is that you’re not servicing your customer,” said Steve Harrington, West Coast distribution manager for the Allegan, Mich. generic over-the-counter drug maker Perrigo Co.