Geographic distribution and intertidal population status for the Olympia oyster, Ostrea conchaphila, from Alaska to Baja

Despite the recent interest by ecologists and government agencies to restore historical populations of the Olympia oyster, Ostrea conchaphila (Carpenter 1857), focus has been limited to projects located at the north of this species’ range with little or no attention to southern California and Baja populations. In addition, historical information on the status of natural populations across the range has been mainly qualitative. Thus, there is no comprehensive information on the current status of natural populations. We conducted the first large scale quantitative survey of remnant populations of O. conchaphila.  We surveyed intertidal populations at 24 historical sites during spring and summer 2005 and summer 2006, established presence/absence and provided data on densities and percent cover. Results indicated that average maximal densities range from 0.0 to 36.7 oysters/0.25 m². Even though densities were low at most sites in southern California, intertidal populations were present at all bays and estuaries south of Morro Bay, and multiple size classes of oysters were present suggesting regular recruitment. In northern sites, representation of multiple size classes was less common and intertidal populations were more often absent, such as at Netarts Bay, OR, Willapa Bay, WA and Grays Harbor, WA, though there was evidence for the presence of subtidal populations. Ostrea conchaphila was absent from intertidal sites at the northern range endpoint at Sitka, AK. This study represents the first comprehensive biogeographic survey of intertidal populations of the Olympia oyster, Ostrea conchaphila, and identifies sites in southern California as suitable locations for future restoration projects.