spread from one area to another. The most significant introductions were to the Pacific Coast of the United States in the 1920s and to France in 1966. In most places, the Pacific oyster was introduced to replace the native oyster stocks which were seriously dwindling due to overfishing or disease. In addition, this species was introduced to create an industry that was previously not available at all in that area. In addition to intentional introductions, the Pacific oyster has spread through accidental introductions either through larvae in ballast water or on the hulls of ships. In some places in the world, though, it is considered by biosecurity, primary industry, and conservation departments and ministries to be an invasive species, where it is outcompeting native species, such as the Olympia oyster in Puget Sound, Washington; the rock oyster, Saccostrea commercialis, in the North Island of New Zealand; and the blue mussel, Mytilus edulis, in the Wadden Sea. Production techniques Numerous methods are used in the production of Pacific oysters. These techniques depend on factors such as the seed supply resources, the environmental conditions in the region, and the market product, i.e., whether the oysters are sold in a half shell, or shelled for meat extraction. Production can either be entirely sea-based or rely on hatcheries for seed supply. Seed supply Most of the global Pacific oyster spat supply comes from the wild, but some is now produced by hatchery methods. The seed from the wild can either be collected by the removal of seaweed from beaches or by hanging shell (cultch) in suspension from long lines in the open water. The movement towards hatchery-reared spat is important, as wild seed is susceptible to changeable environmental conditions, such as toxic algal blooms, which can halt the supply of seed from that region. In addition, several pests have been noted as considerable dangers to oyster seed. The Japanese oyster drill (Ocenebra inornata), flatworm (Koinostylochus ostreophagus), and parasitic copepod (Mytilicola orientalis) have been introduced accidentally to aquaculture areas, and have had serious impacts on oyster production, particularly in British Columbia and Europe. Broodstock Pacific oyster broodstocks in hatcheries are kept in optimum conditions so the production of large amounts of high quality eggs and sperm can be achieved. Pacific oyster females are very fecund, and individuals of 70–100 g live weight can produce 50–80 million eggs in a single spawn. Broodstock adults are held