Research Document - 1999/150

Status of the Olympia Oyster, Ostrea conchaphila, in Canada.

By G.E. Gillespie

Abstract

The Olympia oyster, Ostrea conchaphila (= Ostrea lurida), is the native oyster of the west coast of North America. They are relatively small oysters, generally found attached to hard substrate or loose on soft substrate in singles or small groups. They are generally distributed from southeast Alaska to Panama, occurring discontinuously in appropriate habitats such as estuaries, lagoons, bays, tidal flats or attached to pilings or floating structures. Maximum reported size is 90 mm diameter, although most individuals are less than 60 mm. Maximum age is unknown, but could be > 10 years. In British Columbia, first maturity is generally achieved one year after settlement. Olympias are larviparous, protandrous, alternating hermaphrodites. Individuals mature first as males, then alternate between male and female phases throughout their lifetime. Fecundity is approximately 250,00-300,000 larvae per spawning. Larvae are retained in the parental mantle cavity for approximately 2 weeks, then are planktonic for 2-4 more weeks. Larvae settle preferentially on the undersurface of hard substrates. In British Columbia, brooding occurs from mid-May to July and settlement occurs from July to September. Dispersal is limited to the planktonic larval phase, once set the adults are sessile. Three to four years growth are required to reach 35-45 mm in size, and little growth occurs after 5 years.

Olympia oysters were commercially fished in British Columbia, Washington, Oregon and California beginning in the mid-1800's. Natural aggregations were overharvested, and largely depleted by the 1930's. The west coast oyster industry unsuccessfully attempted introduction of Atlantic oysters, Crassostrea virginica, and is currently dependent on introduced Pacific oysters, Crassostrea gigas. Olympia oysters are not commercially fished in British Columbia, and likely hold little recreational value because of their small size.

Olympia oyster distribution is limited by specialized habitat requirements, and relatively low fecundity and dispersal. Olympias are vulnerable to temperature extremes, and are not resistant to harvests on a commercial scale. Habitats which once supported large aggregations in Georgia Strait no longer do, in part due to historic overharvests and environmental stresses, and because development of large oyster reefs may require centuries without disturbance. Small relict populations survive at low tide levels and under floating structures. Olympias are locally common at sites on the west coast of Vancouver Island, and little information exists on populations in Johnstone Strait or in the Central Coast. They do not occur in the Queen Charlotte Islands.

Olympia oysters are not likely facing imminent danger of extinction or extirpation in Canada. Limiting factors have led to significant reductions to population levels in the past. From the limited data currently available, the author recommends a status of Special Concern is appropriate.

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