As labor disputes in U.S. West Coast shipping centers have hampered potato exports to JapanMcDonald's restaurants in the Asian nation are scrambling to ration their stock by offering only small-sized portions of fries. The international chain said it is importing more than 1,000 tons of frozen french fries by air, and rerouting another 1,600 tons through U.S. East Coast shipping ports.

"Unfortunately, without this sales restriction step, we would run the danger of running out of fries at some of our stores around the end of the year or beginning of the new year," McDonald's Japan spokeswoman Kokoro Toyama told Reuters Monday. She declined to comment on the impact to the company's earnings.

At least 3,100 stores in Japan will be affected by McDonald's french fry shortage, and while the company has no restrictions on how many small-sized portions customers can buy, it is unclear when it will resume sales of its medium- and large-sized portions. Meals that come with medium-sized fries will be 50 yen (42 cents) cheaper, according to Kotaku.

Ongoing labor disputes since October between some 20,000 dockworkers and the Pacific Maritime Association have resulted in container ports experiencing severe delays along the U.S. West Coast. John Toaspern, chief marketing officer for the U.S. Potato Board, said last week that the rate of frozen french fry shipments to Japan has roughly doubled from two to four weeks due to shipping port delays.

Japan is currently the United States' biggest Asian customer for frozen potato products, importing $336 million worth last year. The U.S. Department of Agriculture's Foreign Agricultural Service reports that the United States accounts for approximately 80 percent of Japan's frozen french fry imports. The agency also predicts that a potential contraction in Japanese domestic potato production over the next decade will mean a big opportunity for U.S. potato exports.