Before European contact with Northwestern North America their influences were noticed among the Straits Salish people. In 1790 estimates of population placed the Semiahmoo at 300. About 1782 the west coast experienced its first smallpox epidemic, and losses everywhere were heavy. This was nearly ten years before the first Spanish explorers ventured into the strait. European influences were also seen in the few trade goods which reached the straits from Interior Indians and Vancouver Island peoples. To the Semiahmoo, European trade goods probably became available through other Indian groups after 1785.
In the pre–white contact period the Semiahmoo people were dominantly a fishing, especially reef–fishing, oriented community. Land hunting and trapping were not important. The cultivation of plants was not part of their culture. Their culture also lacked clan organization, political organization, or religious organization. Households were autonomous units, and life within them was at the subsistence food gathering level. Inter-tribal warfare, while not unknown, did not dominate life or influence it unduly.