Each year, we analyse the health status of fishes in the North Sea and Baltic Sea during research cruises with RV “Walther Herwig III”. The diseases we record are acting as indicators of the impact of environmental stressors on the susceptibility of fish against pathogens and toxic effects of contaminants. Our major target fish species are dab, flounder and cod, and, based on internationally standardized methods, we record onboard externally visible infectious and non-infectious diseases, parasites and liver tumours. In addition, we take tissue samples for subsequent microscopic analysis of cellular liver changes. The studies are part of Thünen Institut’s integrated marine environmental monitoring and assessment.