Seasonal settlement of native west coast oyster, Ostrea conchaphila, larvae in two southern California estuaries

Declines in populations of the native west coast oyster, Ostrea conchaphila, Carpenter, 1857, have piqued recent interest in restoring its populations. Since local population persistence is influenced by larval settlement, information about the magnitude and timing of settlement may provide valuable contributions to restoration efforts.  Previous literature based on an anomalous open coast population in La Jolla, CA found that settlement of O. conchaphila occurred once seawater reached 16°C and ceased once temperatures fell below that point. To observe variation in larval settlement over seasons within the more common estuarine habitat in southern California, we placed ceramic tiles in two locations within Upper Newport Bay, Newport, CA and in two locations within Aqua Hedionda Lagoon, Carlsbad, CA.  Tiles were collected and oyster settlers counted during spring tides to pinpoint peaks in settlement. Tidbit™ temperature loggers simultaneously measured temperatures throughout the time span of the study. There was significant seasonal variation in settlement, with significantly greater settlement occurring during June 2005 and June 2006 within both estuaries. Settlement within Aqua Hedionda Lagoon ranged from 0 oysters/m² to 223.6 ± 49 oysters/m². Settlement within Upper Newport Bay ranged from 0 oysters/m² to 845 ± 248 oysters/m². Contrary to previous findings, we did not observe a universal temperature trigger controlling the initiation and termination of oyster settlement, nor any significant correlations linking water temperature with peaks in settlement.