The Semiahmoo belonged to a group of tribes called the Straits Salish, a division of the Coast Salish. The Straits Salish have been set off from their neighbours on the basis of language and their most important subsistence activity – reef–net fishing for the yearly runs of salmon. These tribes spoke slightly differing dialects of the same Coast Salish language which were unintelligible to speakers of neighbouring languages – the Halkomelem, the Nootka, the Puget Sound, the Nooksack language groups – if they spoke only their own language. The Straits Salish were distinguished by their annual round of subsistence activity based on the yearly runs of salmon; the most important of which was the sockeye run to the Fraser River.
They took these sockeye runs in reef–nets set in salt water channels off the southern shore of Vancouver Island and in the Gulf and San Juan Islands. This fishing technique contrasts with those used by neighbours both to the north and to the south, fishing in streams with small mobile nets or with weirs and traps. Associated with reef–netting was...a great stress on the private ownership of the fishing locations.
To the Straits Salish belong the tribes Sooke, Songish, and Saanich of southeastern Vancouver Island, and the Semiahmoo, Lummi, and Samish of the mainland to the east. The territories extended from the tip of Vancouver Island, across the Gulf and San Juan Islands to the coastal fringes of northwest Washington and southwestern British Columbia. They occupied a continuous area across the present International Boundary. Suttles, Post contact culture Among the Lummi Indians