Management has accused the International Longshore and Warehouse Union (ILWU) of orchestrating some slowdowns to bolster its leverage at the bargaining table in talks that have been under way for eight months.
"The ILWU slowdowns and the resulting operational environment are no longer sustainable," said PMA spokesman Steve Getzug in a statement.
"The PMA has a sense of urgency to resolve these contract talks and get our ports moving again," he added.
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Union officials point instead to factors such as a shortage of tractor-trailer chassis used for hauling cargo from ports to warehouses and other management errors as causing the backups.
"Longshore workers are ready, willing and able to clear the backlog created by the industry's poor decisions," ILWU President Bob McEllrath said in a statement.
"The employer is making nonsensical moves like cutting back on shifts at a critical time, creating gridlock in a cynical attempt to turn public opinion against workers," he added.
A federal mediator is currently involved in the negotiations between the association and the union, which represents some 20,000 dockworkers. The workers' latest contract expired on June 30.