Lake freighters, or lakers, are bulk carrier vessels that operate on the Great Lakes of North America. These vessels are traditionally called boats, although classified as ships. Since the late 19th century, lakers have carried bulk cargoes of materials such as limestone, iron ore, grain, coal, or salt from the mines and fields of the upper Great Lakes to the populous industrial areas farther east. The 63 commercial ports handled 173 million tons of cargo in 2006. Because of winter ice on the lakes, the navigation season is not usually year-round. The Soo Locks and Welland Canal close from mid-January to late March, when most boats are laid up for maintenance. Crew members spend these months ashore. Depending on their application, lakers may also be referred to by their types, such as oreboats or ironboats (primarily for iron ore), straight deckers (no self-unloading gear), bulkers (carry bulk cargo), sternenders (all cabins aft), self unloaders (with self-unloading gear), longboats (for their slender appearance), or lakeboats, among others. In the mid-20th century, 300 lakers worked the lakes, but by the early 21st century, there were fewer than 140 active lakers. , which sank in 1975, became widely known as the most recent and largest major vessel to be wrecked on the Great Lakes. Lakers vs. salties By way of the Saint Lawrence Seaway, smaller lakers have access to the Atlantic Ocean, and some ocean-going vessels have access to the lakes. Visiting ocean-going vessels are called "salties". Many modern ocean-going vessels are too large for the relatively small locks on the Saint Lawrence Seaway, so large salties cannot travel farther inland than Montreal, Quebec. Because one of the Soo Locks is larger than any Seaway lock, salties that can pass through the Seaway may travel anywhere in the Great Lakes. Conversely, the largest lakers are confined to the upper lakes (Superior, Michigan, Huron, Erie) because they are too large to use the Seaway locks, beginning at the Welland Canal that bypasses the Niagara River. Because of their deeper draft and the lower buoyancy of fresh water, salties may accept partial loads on the Great Lakes, "topping off" upon exiting the Seaway. Cargo Lakers are generally bulk carriers; that is, they carry cargoes of rock, ore, salt or grain in large contiguous holds, not packed in containers. The earlier ships required rail cars unloading on ore docks and unloading machinery at the receiving docks,