A spokesman for the nonprofit St. Lawrence Seaway Management Corp said that once closed, it would take about 20 hours to reopen the Seaway.
Canada's Conservative government could move to legislate the union back to work as it did during strikes at Air Canada and Canada Post earlier this year.
"Our government is concerned that a strike is being considered and we are taking this situation very seriously," Labour Minister Lisa Raitt said in a statement.
She warned that a work stoppage on the Seaway could hurt the country's economic recovery.
The Seaway stretches more than 374 miles (600 km) from Montreal to Lake Erie, and forms part of the 2,340-mile Great Lakes St. Lawrence Seaway shipping system. Eight to 10 vessels move through the Seaway each day, carrying 40 million tonnes of cargo a year.
Seaway Communications Officer Andrew Bogora said a strike is not inevitable.
"Negotiations are continuing in earnest at the table, and to our knowledge the tone is still positive. We're still optimistic of reaching a resolution without a labor interruption," he said.
The parties have been negotiating since May, and Canadian Auto Workers National President Ken Lewenza said the major outstanding issues are wages, health care co-pays, and outside contracting.
Lewenza expressed frustration with Raitt's position.
"There's no question that the Conservative government is going to use their majority to put the balance of power in the hands of an employer," he said.
Lewenza said he was optimistic an agreement can be reached.
"Ninety eight percent of collective agreements are resolved without a strike, and 99 percent of them go down at the last hour," he said. "I'm confident that we can get a collective agreement." (Reuters)
