Population Dynamics of Native and Invasive Oysters in the San Diego River Estuary

Although the initial introduction of the Pacific oyster, Magallana gigas (= Crassostrea gigas), to the west coast of North America happened roughly a century ago, it is only since the early 2000's that this invader has become an established and conspicuous species in many of Southern California's estuaries. In Southern California, this invasion is particularly relevant for the native Olympia oyster, Ostrea lurida. The Olympia oyster has both present-day and historical threats to its population due to invasive species, overfishing, and pollution. To address the local distribution and demography of these two species, sampling was conducted along the artificial rocky shore of the San Diego River Estuary. Transects were established in three different tidal zones, and quadrats along these transects were sampled quarterly for one year. Population densities, biomass, condition index, size-frequencies, and von Bertalanffy growth constants were assessed for the two species over this time period. For M. gigas, growth lines in the shell were also used as a tool to estimate age and growth. Ostrea lurida densities were greater than those of M. gigas, and these native oysters had greater numbers in the low intertidal and diminished as tidal height increased. Magallana gigas numbers also decreased with increasing tidal height, although less dramatically than O. lurida. Given that M. gigas is much larger, total biomass of M. gigas at the study site was greater than that of O. lurida. Von Bertalanffy growth rate constants (k) for both species were similar, indicating similar growth rates towards asymptotic size. Further, for M. gigas, size-at-age estimates from the von Bertalanffy growth model showed good agreement with estimates derived from analysis of shell growth lines. Examination of these lines also indicated that M. gigas growth rate decreased as tidal elevation increased.