The present study explored alternative strategies to improve the health status and growth metabolism in cultured juvenile fish. In this context the effects of alginic acid administration on the welfare of rainbow trout juveniles were investigated. Both the hepatic gene expression of heat shock protein 70 and the cortisol plasma levels were evaluated in order to estimate the possible beneficial role of alginic acid in improving animal well-being. At the same time the body weight was measured and was correlated with the hepatic expression of insulin-like growth factor I, myostatin, and two isoforms of the thyroid receptor. Finally, improvement of the immune system was investigated by monitoring the expression of innate immune-related genes, such as interleukin1β, interleukin8 and toll-like receptor3, in the spleen. The data obtained demonstrated an improvement in the welfare of rainbow trout juveniles, revealed by growth and innate immune response enhancement by alginic acid administration. These results may be of great importance for the further development of more sustainable aquaculture practices, and can be considered as a biotechnological application to produce a safer product for human health.