Strait of Canso of, 27 km long, 3 km wide, 60 m deep, is a deep, narrow channel separating the Nova Scotian mainland and Cape Breton Island. Formerly this waterway provided free communication between the waters of St Georges Bay in the Gulf of St Lawrence and the Atlantic Ocean. However, in the early 1950s, the Canso Causeway was built to carry rail and highway traffic between the mainland and the Island. The completion of the causeway in 1955 created a year-round ice-free port for the major towns along the strait, Port Hawkesbury and Mulgrave, and helped establish the area as an important pulpwood centre. But the closing of the strait has also cut off the supply of gulf-spawned lobster larvae to the Atlantic side. This is thought to be one of the major factors responsible for the precipitous decline of the lobster fishery on the southeastern Atlantic coast since the late 1950s.