Yersiniosis caused by Yersinia ruckeri and pasteurellosis caused by “Pasteurella atlantica gv. salmonicida” are bacterial diseases presenting substantial welfare challenges for Atlantic salmon in Norway. In recent years, these diseases have proliferated, emerging as concurrent epidemics. There are knowledge gaps related to the increase of these diseases. However, disease outbreaks are often linked to stressful situations like transfer to sea or non-medicinal delousing. Hence, there is a possibility that the widespread use of non-medical delousing in recent years might have increased the vulnerability of salmon to infectious disease that extend beyond the immediate physical harm to their skin and mucus layer. This study aims to investigate microbial endocrinology as an underlying mechanism for exploring changes in virulence triggered by stress, which may explain whether exposure to stress-hormones have contributed to the increase of yersiniosis and pasteurellosis in Norwegian aquaculture. In this study it was found that the stress-hormone noradrenaline influences the growth of Y. ruckeri and “P. atlantica”. Noradrenaline was shown to act as a growth stimulator for these bacterial pathogens, suggesting a potential link between stress-induced catecholamine release and the increased outbreaks of these diseases in Norwegian aquaculture. The findings indicate that noradrenaline concentrations ranging from 150 to 200 µM significantly enhance Y. ruckeri growth in minimal media designed to mimic the host environment, and “P. atlantica” growth in a more nutrient rich media. Moreover, the study suggests that the presence of stress hormones may contribute to the activation of virulence factors in bacteria, such as motility, potentially leading to more severe infections. Masteroppgave i fiskehelse FISK399 MAMN-FISK