Understanding the correlation between rainbow trout (Oncorhynchus mykiss) and Atlantic cod (Gadus morhua) farming: A new perspective on freshwater and marine cold-water aquaculture

This thesis explored the biological and production-related connections between two emblematic cold-water aquaculture species: rainbow trout (Oncorhynchus mykiss), reared in small-scale farming systems in the Mediterranean hinterland of central Italy, and Atlantic cod (Gadus morhua), produced within an industrial North Atlantic aquaculture context. Although these species occupy contrasting ecological niches (freshwater vs marine environments), they share several physiological traits as well as common farming challenges related to nutrition, welfare, and health management. The research was conducted within the framework of two projects: the PRIMA-HORIZON EU project AgrI-fiSh and the Norwegian project FORCOD. AgrI-fiSh aimed to promote circular economy strategies in trout farming through the valorization of agricultural and animal by-products, while FORCOD focused on improving the biological, economic, and environmental sustainability of Atlantic cod aquaculture, a sector still constrained by significant welfare and health issues. The primary objective of the AgrI-fiSh trials, using brown (Salmo trutta fario) and rainbow trout, was to evaluate growth performance, welfare indicators, and gut health in fish fed three innovative diets formulated with locally sourced ingredients, including legume pasta discards, grape by- products (grape seed and pomace), rendered animal proteins (poultry by-products and hemoglobin), lactic acid bacteria, and phytogenic additives such as garlic powder. While the first two diets revealed critical formulation limitations, the final optimized diet, that is in the process of being patented, resulted in significant improvements in growth performance, intestinal morphology, antioxidant status, and otolith integrity. Beyond the biological outcomes, the study emphasized the socio-environmental relevance of utilizing Mediterranean raw materials, supporting the development of a short supply chain model and enhancing regional sustainability. Within the FORCOD project, Atlantic cod were fed diets incorporating crustacean shell meals derived from crab (Cancer pagurus) and shrimp (Pandalus borealis) processed residues, partially replacing plant-derived carbohydrates, in a short digestibility trial. Growth, feed intake, nutrient digestibility, distal intestinal dry matter, mineral content, gastrointestinal morphology, digestive enzyme activity, bile salts, and gut gene expression were evaluated. Diet had no significant effect on feed intake or growth, but fish fed the crab shell meal showed higher amino acid digestibility, increased bile salts, and enhanced brush border enzyme activity in the pyloric caeca. In addition, the most innovative aspect of the study was the mapping of gut gene transcripts in relation to gastrointestinal morphology and digestive capacity in farmed Atlantic cod. Overall, this comparative approach could improve the understanding of nutritional and physiological adaptations in freshwater and marine cold-water species, highlighting how circular economy principles could support more sustainable and welfare-oriented aquaculture systems.