ST LAWRENCE SEAWAY OPENS SHIPPING SEASON The St Lawrence Seaway said the first ship of the season passed through the St Lambert lock here this morning, officially opening the 2,300-mile-long waterway's 1987 shipping season. The seaway has said it expects little increase in freight levels this year from last year when it moved 37.6 mln tonnes of freight between Montreal and Lake Ontario and 41.6 mln tonnes on the Welland Canal, which links Lake Erie and Lake Ontario. The canal is scheduled to open tomorrow. Officials expect the waterway to lose nine to ten mln Canadian dlrs this year, about the same as the estimated deficit for fiscal 1986-87, which ends today.