For the first time in recent decades, U.S. East Coast ports handled more loaded containers in the first three months of the year than their West Coast counterparts, demonstrating the profound and lingering shift in trade during and now in the wake of the West Coast labor crisis.

Numbers released by PIERS, a sister product of JOC.com within IHS Maritime & Trade, show loaded container traffic jumped 9 percent year-over-year on the East Coast in the first quarter of 2015, boosting Atlantic ports' market share to 48 percent.

West Coast traffic, meanwhile, dropped 12 percent in the first three months of the year, hitting 44 percent. Gulf Coast traffic also saw a boost in laden container traffic, up 8 percent in the first quarter, snagging an 8.3 percent share of the market.

Over the past five years, East Coast ports averaged 43 percent of U.S. container market share. Things began to change in October 2014, though, as labor negotiations between port operators and union workers began to heat up on the West Coast, spurring congestion and work slowdowns at America's busiest ports.

Between October and February, West Coast ports steadily lost their market share to counterparts in the east, as well as rival ports in Canada and Mexico. Diverted imports have been a substantial boost to business on the East Coast, spurring traffic at ports in Virginia, South Carolina, Georgia, Florida, as well as on the Gulf Coast in Texas.

East Coast ports grew their share of the U.S. import market to 44 percent between January and April this year, up roughly 5 percent since the end of last year, according to PIERS data.  Meanwhile, West Coast ports' share of U.S. container imports shrank from 54 percent to 49 percent in the first quarter.

By April, East Coast container imports were at near parity with West Coast traffic, but it was still uncertain how much of the west-to-east diversions would prove permanent. The fate of those diversions remains uncertain even today, as the threat of congestion continues to loom over Pacific Coast ports and added traffic on the Atlantic Coast has now sent waves of congestion to those shores as well.