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Pacific oyster

The Pacific oyster (Crassostrea gigas) is a non-indigenous species introduced to B.C. for aquaculture purposes. Its native range is from Sakhalin Island and coastal Russia through Japan to Kyushu, China, Korea, Southeast Asia and Pakistan. They have been introduced and have established populations in many countries worldwide.

The Pacific oyster was introduced extensively on the west coast of North America in the early 1900s, and was first brought into B.C. in 1912 or 1913. Small-scale introductions continued and large-scale importation of seed oysters began in 1925. Successful reproduction was reported in Ladysmith Harbour in 1925, 1926 and 1932, followed by successful dispersal beyond the harbour in 1936. Widespread reproductive success was reported in 1942, 1958 and 1961 resulting in the establishment of Pacific oysters throughout the Strait of Georgia. They were transplanted to the west coast of Vancouver Island (Esperanza Inlet; Barkley, Clayoquot and Kyuquot Sounds) in 1937 and they are now established in suitable habitats on the west coast of Vancouver Island south of Brooks Peninsula. There is also confirmed reproductive success of Pacific oysters in Skidegate Inlet, Haida Gwaii and reported occurrence of natural-set Pacific oysters from Tasu Sound on the west coast of Haida Gwaii.

Pacific oysters are protandric hermaphrodites, initially spawning as males and then may become females during the winter season. They are broadcast spawners with a pelagic larval period of 3-4 weeks depending on temperature. Their natural distribution in B.C. is limited to locations with warmer water temperatures that are required to stimulate gonadal development, spawning and the metamorphosis of larvae. Although spawning can occur at temperatures between 16-34°C and salinities ranging from 10-42 ppt., temperatures of 20-25°C and salinities of 35 ppt. are considered optimal. However, the range of Pacific oysters can be expanded by manual introduction to microhabitats. Adults are sessile and the only exchange between sites is through larval transport or human intervention. Adults grow relatively quickly in the first few years after settlement and growth slows with maturity and senescence.

Longevity and age structure of populations are not documented due to difficulties in establishing aging methods and criteria. Methods for aging Pacific oysters have been tested on Pacific oysters in China, but these methods still need to be tested for the Pacific oysters in B.C. Both the literature and local knowledge suggest that Pacific oysters can live for decades.

Pacific oyster populations in B.C. generally occur in mid to high intertidal zones on hard substrates but can vary depending on the environmental conditions of the site. Fishers have noted that Pacific oysters are lower in the intertidal zone on the west coast of Vancouver Island. A preferred settlement substrate is oyster shell, and large aggregations form if populations are not disturbed. Under appropriate conditions they can form reefs on gravel banks at the tidal mouths of small streams. Harvestable populations of Pacific oysters may be present on bedrock walls and outcrops where successful larval recruitment occurs on a regular basis.

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