Winter mass mortality syndrome in Hong Kong oyster (Crassostrea hongkongensis): Causes and impacts

Cultivation of Hong Kong oyster species (Crassostrea hongkongensis, also known as Magallana hongkongensis) in estuarine areas of South China including Hong Kong contributes to almost a quarter of global edible oyster production. Recently, many growers and scientists observed that mass mortality of this species in winter months, i.e., at the time of their harvest. However, the characteristics and patterns of aforementioned winter mass mortality syndrome (WMS), as well as the potential underlying causes and impacts are yet to be evaluated. In this study, WMS was first described and examined within Hong Kong's cultural areas by employing a combination of interviews, field observations, histological examinations, and molecular analyses to identify the potential pathogen responsible for WMS in this region. Significant mortality in field experiment confirmed the presence of WMS and matched with results from interviews with oyster growers. Eugymnanthea japonica is identified as one of the contributing factors to winter mortality in Deep Bay. Nevertheless, it cannot be definitively established as the primary and sole cause of WMS. Our results and observations suggest that multiple environmental stressors driven primarily by high salinity stress combined with infection of specific parasite, serves as the main cause of this WMS. This study highlighted the knowledge gaps warranting future investigation and proposed potential short-term mitigation strategies to tackle WMS in the region.