TACOMA — March was the busiest month since September at Puget Sound’s two largest ports, Tacoma and Seattle, new statistics show.

But don’t expect April’s figures to reflect the same 21 percent year-over-year increase in container traffic at the two ports, shipping officials say. That’s because March’s big increase was in large part the result of longshoreworkers’ clearing out an accumulated backlog of containers that jammed the two ports during a slowdown that began in late October and continued through late February.

That slowdown happened as the International Longshore and Warehouse Union and the Pacific Maritime Association, which represents shipping lines and terminals on the West Coast, deadlocked in negotiations for a new contract. The association said the union deliberately slowed operations at terminals from Bellingham to San Diego. The union blamed the decline in productivity on mismanagement of the terminals by shipping lines and their terminal operators.

The pace of work returned to normal after the two sides reached a tentative deal in late February. Union leadership has recommended ratification of that deal. The result of the union membership vote will be announced in late May.

The figures released in combined form by the Tacoma and Seattle ports show together they handled 361,951 20-foot-equivalent containers in March. That compares with 298,250 units in March 2014.