Over the past recent years, there has been a growing interest in applying environmental DNA (eDNA) in various research fields like pollution response, air quality monitoring and invasive species detection. Aquaculture and in particular recirculated aquaculture system (RAS) is a promising food sector relying mostly on monospecific production. Consequently, outbreaks caused by various pathogens (e.g. bacteria, parasite, fungi, virus) represent a serious challenge to animal welfare and economic losses. The present study is part of the EU project RASOPTA which aims to increase knowledge regarding water quality, off-flavor, pathogen detection in RAS and develop a biomonitoring tool based on water sampling. In the present experiment, we investigate the sensitivity of water eDNA and eRNA to predict the onset of enteric redmouth disease (ERM) caused by Yersinia ruckeri, in rainbow trout. One week prior to infection, half of fish were subjected to unpredictable chronic stress to evaluate the effect of stress on fish immune response and mortality rates in relation with eDNA and eRNA levels of the bacteria. The fish were either mock-infected or expose to low (1E5 CFU/mL) or high (1E7 CFU/mL) dose of bacteria by bath infection. Fish spleens were sampled to evaluate immune response and bacterial load. Clinical signs were equally recorded to monitor disease occurrence. Water and fish were sampled regularly before the start of mortality and after the mortality had ceased. Results from the experiment will be compared and discussed to bring forward the optimal methods between eDNA and eRNA to evaluate occurrence of enteric redmouth disease in rainbow trout. Outcome from this study will contribute to extend knowledge which will provide a basis to develop technologies to predict and prevent outbreaks in RAS.