The effect of emersion and UV solar radiation on Pacific oyster Crassostrea gigas survival and pathogen development in the intertidal environment.

Pacific oysters Crassostrea gigas are cultured intertidally in estuaries and bays and are prone to disease outbreaks associated with pathogens (ostreid herpesvirus-1 microVar (OsHV-1 mVar) and Vibrio bacteria), which can decimate stocks. The holding height on the intertidal exposes C. gigas to various biotic and abiotic factors for e.g. UV radiation which has poorly known impacts on oyster survival and pathogen development. A 5-month field trial was carried out at two shore heights (2 m vertical differential) to investigate young C. gigas (spat) survival and pathogen development. UV exposure, tidal height and air temperature were recorded. 1000 oyster spat were divided equally between two shore heights (HS and LS) and sampled weekly. Mortality was counted in situ and pathogen screening for V. aestuarianus and OsHV-1 mVar was carried out by PCR and qPCR on (n = 570) C. gigas. Results indicate that higher oyster mortality was positively associated with emersion duration, including UV exposure (>2.4 kJ/m2) and high air temperatures (>21 oC). Pathogen partitioning was observed i.e. a higher prevalence of OsHV-1 mVar in HS C. gigas while a higher prevalence of V. aestuarianus was detected in LS C. gigas, also reflected in infection intensity. This study shows that oyster spat mortality and pathogen infection levels are influenced by the growing height on the shore above Chart Datum. Data are supported by and contrasted to previous studies. These findings are especially relevant to global coastal aquaculture sites experiencing summer mass mortality episodes and outbreaks of Vibrio and/or herpesvirus related diseases.