Growth, condition, and survival of the Pacific oyster Crassostrea gigas cultivated within and outside a subtropical lagoon

Mass die-offs of Pacific oysters have become a widespread concern in northwestern Mexico. Cultivation of the species occurs in shallow inland waters (lagoons, estuaries, and reservoirs) at high stocking densities and under elevated seasonal temperatures; all conditions that are potential causes for high die-offs. Cultivation outside inland waters may be an alternative to reduce the impact of possible stressors. This study compares growth, condition, and survival of oysters (Crassostrea gigas) cultivated at different densities within a subtropical lagoon and on the gulf side of Laguna La Cruz in the State of Sonora. Three densities were tested at each site using cultivation trays. Temperature, salinity, oxygen, pH, seston content, and chlorophyll a content were measured at sampling stations inside and outside the lagoon. Results showed that cumulated mortality was significantly lower for oysters cultivated outside the lagoon. Peak mortality occurred in November-December 2006 and March-May 2007, which coincided with mass die-offs in the region and with abrupt changes in temperature. High variations in temperature were related to water masses from the open ocean. Lagoon oysters were influenced by slightly higher mean daily temperatures and showed continuous shell growth and were significantly larger than oysters on the gulf side. However, lagoon oysters showed similar dry weights as gulf side oysters, indicating retarded somatic growth, probably from a relatively low food supply. Gulfside oysters had a higher condition index than oysters in the lagoon throughout the period of study. Results showed that oysters were sensitive to overstocking, since growth at high densities was less than at low densities at both sites. This suggested an advantage for cultivation at intermediate and low densities. Although C. gigas responded in a similar way to overstocking at both sites, they responded differently in relation to culture site. Diminished somatic growth, condition, and survival suggest that oysters in the lagoon experienced more stress than oysters at the gulfside site, probably from differences in water quality. Cultivation of these oysters outside of protected waters is an alternative that producers may try to obtain a better product and reduce mortality.