This project, building on already by MSc./Drs. Johan Aerts as part of his doctoral research conducted and related patent application, place a comprehensive biological validation on cortisol scales as valid biomarker for chronic stress in fish. It was chosen for carp (Cyprimus carpio) because of the already obtain positive results for this type of fish, its function as a model organism as well as its importance in aquaculture. Additionally, the foundation will be laid for the future development of a biosensor to analyze large amounts of sample on-site (in the aquaculture business in wildlife as well as in public aquariums and fish kept as laboratory animals) in a quick, simple and inexpensive way . A single sample site would make it possible to quantify chronic stress in fish and contribute in this way to optimize breeding, monitoring and keeping fish. Besides the benefits to growers and fish this is crucial for an informed decision strategy on animal welfare by the government