Aquaculture remains highly dependent on fishmeal protein and its supply contingent on the exploitation of wild fisheries. Any reduction in its utilization by farmed fish would reduce the ecological burden of aquaculture and improve its sustainability. For the intensive farming of fish species, the provision of alternative ingredients that reduce dependence on fishmeal protein yields yet another significant environmental benefit: decreasing the dietary protein nitrogen catabolism and consequently effluent ammonia produced by fish farms. This could mitigate the impact of this industry on the ecosystems which are particularly vital in Portugal since most fish farms are installed in transitional waters.</p><p>Glycerol, a by-product of biodiesel, is an abundant and widely available material. It has been successfully used as an alternative ingredient for the swine and poultry industry. However, its potential as an alternative ingredient for fish feed has been generally overlooked. This project will investigate the metabolic utilization of dietary glycerol and its effect on fish performance, digestibility and ammonia excretion, which will allow a better understanding of the underlying biochemistry of glycerol utilization, particularly in carnivorous fish species. In liver, the excess of dietary amino acids appears to be largely directed to the intermediary metabolism and the synthesis of glucose, converting their nitrogen moieties into ammonia. In carnivorous fish, gluconeogenic catabolism of amino acids accounts for essentially all endogenous glucose production. The provisions of an alternative non-nitrogenous substrate such as glycerol in commercial diets, could either directly or indirectly attenuate amino acid utilization.</p><p>The environmental and economic benefits of feeding carnivorous fish glycerol-supplemented diets are clear. Fishmeal could be partially replaced by a less expensive, non-nitrogenous ingredient readily available on the global market, therefore reducing feed costs, a major contributor to the operational budget of any fish farm.