Abstract:The oyster Crassostrea hongkongensis is the main aquaculture species along south China coast and has an important economic value. The Maowei Bay adjacent to Qinzhou of Guangxi Province has been the largest natural oyster seed field, and oysters originating from this area are named after the place (Qinzhou oyster). The estuary of the Dafeng River is a major aquaculture site for Qinzhou oyster in Guangxi Province. Due to the promotion of the deepwater-raft-hanging-culture mode, massive death of oysters has occurred frequently in spring since 2008 and resulted in the rapid drop in the amount of oyster seeds in the Maowei Bay. In this study we explored the reproductive cycle of oyster gonad development and investigated the causes of the massive death, using one-year old oysters from the estuary of Dafeng River (HH, whose larva came from the Maowei Bay). We developed a method for rapid identifying the developmental stage of oyster gonad with the combination of naked-eye observation, biopsy and histological analysis. We also compared the gonad development of two-year old oysters from inside (HH) and outside (XF) of the estuary of the Dafeng River and from the Maowei Bay (MS). We found that Qinzhou oysters had only one breeding cycle each year which can be divided into 4 gonad developmental stages: the undifferentiated stage from January to March, the differentiation stage in April when the gender became distinguishable, the mature and discharging stage from May to October when the oysters spawned repeatedly, and the resting stage from November to December during which the gonads atrophied and disappeared. Gonad development of XF oysters started half a month earlier than HH oysters and one month earlier than MS oysters. In 2014 it was observed that the gonad of XF oysters developed into mature stage in April. The variation might be caused by differences in temperature, salinity and food abundance. Moreover, in overly hot and dry weather the gonad discharge was inhibited probably by high salinity, which may explain the massive death in the high salinity area. Therefore, we recommend close monitoring on the gonad development and seawater salinity in spring. The oysters should be harvested or transported to the inner estuary immediately after they mature, so as to avoid the massive death caused by scarce rainfall
