Diseases pose an ongoing threat to aquaculture, fisheries and conservation of marine species, and determination of risk factors of disease is crucial for management. our objective was to decipher the effects of host, pathogen and environmental factors on disease-induced mortality of Pacific oysters (Crassostrea gigas) across a latitudinal gradient. We deployed young and adult oysters at 13 sites in france and we monitored survival, pathogens and environmental parameters. the young oysters came from either the wild collection or the hatchery while the adults were from the wild only. We then used Cox regression models to investigate the effect of latitude, site, environmental factors and origin on mortality risk and to extrapolate this mortality risk to the distribution limits of the species in europe. We found that seawater temperature, food level, sea level atmospheric pressure, rainfall and wind speed were associated with mortality risk. Their effect on hatchery oysters was generally higher than on wild animals, probably reflecting that hatchery oysters were free of Ostreid herpesvirus 1 (OsHV-1) whereas those from the wild were asymptomatic carriers. the risk factors involved in young and adult oyster mortalities were different, reflecting distinct diseases. Mortality risk increases from 0 to 90% with decreasing latitude for young hatchery oysters, but not for young wild oysters or adults. Mortality risk was higher in wild oysters than in hatchery ones at latitude > 47.6°N while this was the opposite at lower latitude. Therefore, latitudinal gradient alters disease-induced mortality risk but interacts with the initial health status of the host and the pathogen involved. practically, we suggest that mortality can be mitigated by using hatchery oysters in north and wild collected oysters in the south.

Surface water samples of size-selected seston (0.7-20 lm) were collected from April 2013 to September 2013 at three similar coarse-sand benthic habitats. Additionally, seston sampling was performed at a fixed location throughout a complete tidal cycle (2014). A combination of fatty acid (FA), isotope, and flow cytometry analyses were used to determine the quality and quantity of nano-and pico-sized particulate organic matter (POM). High variability was found between fatty acid replicate samples. Similar temporal patterns were observed at two sheltered sites, while the exposed site displayed less pronounced seasonal changes. Lower concentrations of 16C and 18C polyunsaturated fatty acids were found during low tide sampling. Globally, POM was dominated by picoeukaryotes, with concentrations exceeding 50,000 cells mL 21 , and (16:4x3 1 18:3x3)/Rx3 is proposed as novel biomarker of picoeukaryotes in this region.