DOI: 10.25442/hku.30587327.v1
title: Supporting data for "A comprehensive study of winter mass mortality of oysters in Deep Bay, Hong Kong"

description: Ocean has been providing food for humans for thousands of years, but until recently, aquaculture has surpassed captive fishery in production volume. Aquaculture has become an important part of human food security, feeding the ever-growing population that is demanding for healthy animal proteins. Bivalve aquaculture makes up a large proportion of the overall aquaculture production, with oysters contributing the largest proportion in the bivalve aquaculture. However, the sustainability of oyster aquaculture is threatened by mass morality events that occur globally across different oyster production regions and to different species. Deep Bay, the only oyster production area in Hong Kong unfortunately is also one of the affected regions. However, this serious threat to sustainable oyster aquaculture in Hong Kong has never been studied before.In this thesis, I will first confirm the presence of winter mass mortality via a combined survey and field study approach, monitoring mortality rate of oysters during winter period between December to April. Molecular and histopathology examinations were carried out in order to identify the main biological cause of winter mortality. With only a hydrozoan parasite that has never been confirmed to cause mass mortality in bivalves identified, a second experiment designed to understand the environmental stressors interactions with oysters was conducted. As salinity increases in Deep Bay due to lack of rainfall, it creates stress for the estuarian oyster species <i>Crassostrea hongkongensis</i> that is cultured in Deep Bay. The in-lab experiment that mimics winter high salinity and the extreme realistic warming temperature showed that oysters have strong resilience to single environmental stressors. However, when exposed to combined high salinity and high temperature, mass mortality could be caused within a short period of time. The addition of Vibrio infection to the environmentally weaken oyster did not show mass mortality, showing that the winter mortality occurring in Deep Bay is caused by a specific biological cause. Finally, a comprehensive field experiment was carried out in Deep Bay during winter period again but learning and improving from the first field study. Water sampling was included to examine the environmental biological factors, increased sampling and monitoring was practiced, the use of excluding primer during metabarcoding removed most of the host contamination that affected analysis. Most importantly, a short-term oyster mortality mitigation method was tested out using increased culturing depth was tested and two statistical models were created to predict oyster mortality rate in the coming week, one using publicly available air temperature data and one using water and oyster tissue samples. The air temperature plays a significant role in oyster mortality, and some organisms found in water, oyster gut, are important indicators of incoming mortality. In addition my study provides future directions to research and create both short term and long term mortality mitigation strategies.