A strike has shut down all shipping on the St. Lawrence Seaway, interrupting exports of grain and other goods from Canada and the United States via the Great Lakes to the rest the world.

Around 360 workers in Ontario and Quebec with Unifor, Canada’s largest private-sector union, walked out early Sunday in a dispute over wages with the St. Lawrence Seaway Management Corp. The strike has shut down 13 locks between Lake Erie and Montreal, bottling up ships in the Great Lakes and preventing more ships from coming in.

The St. Lawrence Seaway and Great Lakes are part of a system of locks, canals, rivers and lakes that stretches more than 2,300 miles (3,700 kilometers) from the Atlantic Ocean to the western tip of Lake Superior in Minnesota and Wisconsin. It carried over $12 billion (nearly $17 billion Canadian) worth of cargo last year. Ships that travel it include oceangoing “salties” and “lakers” that stick to the lakes.

It’s the first time that a strike has shut down the vital shipping artery since 1968. Before the union walked out early Sunday, it said they were still “1,000 nautical miles apart on wages” despite several months of negotiations.