Atlantic salmon (1.2 million tons) and rainbow trout (70 thousand tons) are the main aquaculture production in Norway. Growth is one of the most important traits. Fish may not be able to maintain high growth when the rearing conditions are not optimal. I n the grow-out phase, the fish are reared under natural conditions in net cages in the sea with variation in environmental conditions both within and between years. A fish whose growth performance is changed in response to variations in macro-environments , e.g., different locations, can be defined as having low stability of growth and being macro-environmental sensitive. In contrast, micro-environmental sensitivity refers to the variation within a macro-environment, leading to size variation within a grou p of fish of the same age.The project STABLEFISH focuses on the possibility to increase stability of growth by reducing the environmental sensitivities through selective breeding. First, genetic variation in macro and micro environmental sensitivity will be estimated. Second, the possibility to select against macro environmental sensitivity will be investigated using genomic information. Finally, Smoltified fish are transferred to the sea which is one of the major environmental changes in their life, lead ing to considerable genetic change in micro-environmental sensitivity. Genetic change in micro-environmental sensitivity in different life stages will be studied. Stability of growth performance in farmed fish, being well adapted to multiple environments and variable environmental conditions increases animal welfare and reduces fish mortality and losses. Being able to create more uniformly performing fish stocks will allow harvesting a larger proportion at an appropriate body size. There will be less comp etitive interaction among the animals, contributing to more efficient feeding and better fish welfare. This will improve consumer acceptance of farmed fish and contribute to increases in aquaculture industry profit.