Reproduction, condition and mortality of the Pacific oyster Crassostrea gigas (Thunberg) in Sonora, Mexico

Starting in 1997, mortality outbreaks in cultured oysters Crassostrea gigas have been reported in northwestern Mexico. Previous studies have shown that massive die-offs result from multi-factor processes related to reproduction of the species. We studied the reproductive cycle and the condition index of cultured oysters in the coastal lagoon of El Soldado, Sonora, as well as the relationship of the life cycle with environmental parameters. We used oocyte diameter to determine reproductive stages and the condition index to describe the physiological state of oysters. Additionally, the temperature, salinity, seston and chlorophyll a were recorded at the study site. The results showed that C. gigas began accelerated reproductive activity in March under the influence of high temperature and increased concentrations of food. No spawning events were recorded and gametes were reabsorbed within the gonad in September and October. The results showed a period of nutrient storage during autumn-winter and another period of gamete production in spring-summer. A mortality event occurred at the end of winter, tied to significant increases in temperature and availability of food and in accelerated reproductive activity and high condition index. These conditions were very similar to those reported in other countries during summer die-offs of C. gigas.