Shipping on the St Lawrence Seaway between Montreal and Lake Erie stopped on Sunday after 361 workers in the Unifor labour union in Ontario and Quebec walked off the job.
The strike began jsut after midnight, as contract negotiations with the St Lawrence Seaway Management Corp (SLSMC) broke down.
Business interest groups want the federal government to intervene to reopen shipping on this vital artery as soon as possible.
Talks continued after the union had served a 72-hour strike notice on Wednesday, but failed to reach an agreement.