Production of the global salmonid aquaculture industry now exceeds 2.4 megatons per annum. Major European producers expect to expand their outputs between 30-50% over the next five years. Ambitions for expansion on this scale create major concerns around fish welfare, ecological impact and the sustainability of salmon feed components. Nutrition lies at the heart of the issue. In the wild, Atlantic salmon are specialized carnivores. In aquaculture, in a move away from the unsustainable use of wild fish protein and oil, proteins of plant origin now constitute the majority (>60%) of their diet. Associated digestive abnormalities are common. In addition, plant-based diets may affect the rate at which nutrients are absorbed and the associated growth rate of salmon, which determines how quickly they grow in marine cages. Rapid marine growth is desirable since it permits more extensive fallowing of coastal aquaculture sites, which reduces the impact of farmed fish on the marine environment (pathogen transfer, nutrient pollution). Finally, while wild fish protein can be replaced by plant protein in salmon diets, oils cannot. Key omega-3 fatty acids must be sourced from the marine environment; a significant burden on wild fisheries. Ensuring the efficient assimilation of fatty acid components from salmonid diets is of paramount importance to safeguard the sustainable exploitation of marine resources. Salmon energetic phenotypes are composites of several interlinked traits: metabolic rate, body fat content, growth, energy harvest from food, energy economy in times of starvation. These traits underpin concerns around salmon nutrition. Significant energetic variation exists in both wild and farmed salmon with multiple possible drivers - both genetic and environmental. Importantly a wealth of new data indicates a role for intestinal microbiota - the bacteria that live in the guts of all vertebrates - in determining host energy metabolism. Understanding how gut bacteria influence Atlantic salmon energetics is thus fundamental to understanding their role in nutrition. This is the principal aim of this project. To achieve this we will establish the influence of gut bacteria on the energetics of salmon living in salt and freshwater, in both wild and aquaculture settings. First, in a unique experimental river system established in Burrishoole, Mayo (Marine Institute/University College Cork, ROI) we will track introduced juvenile salmon through their freshwater lifecycle, measuring both metabolic and gut microbiome variation. This 'wild' cohort will include wild and released farmed fish to establish whether differences between gut microbiota contribute to the poor performance of farmed juveniles in the wild. Secondly, we will undertake parallel freshwater experiments in simulated aquaculture conditions in the laboratory at the University of Glasgow (UoG),UK. Finally, in association with Marine Harvest, we will carry out corresponding experiments on saltwater phase pre-adult salmon in Norway. Alongside experiments with live fish, we propose to harness bioengineering expertise at the School of Engineering, UoG and biotechnological expertise via industry partner Alltech to build an artificial salmon gut system. Via the transfer and maintenance of gut bacteria from metabolically different fish from farmed and wild settings into our gut model we aim to establish how bacterial fermentation underpins differences in energy harvest from feed. Once these 'artificial' bacterial communities are established, a final exploratory phase of the project will involve their transplantation into laboratory reared juvenile salmon to evaluate their potential impact on host metabolism. Understanding how salmon gut bacteria change energetic phenotypes will open new avenues to improve fish health, nutrition and productivity. A model Atlantic salmon gut in a world class UK bio-engineering laboratory puts in place an invaluable tool for salmon aquaculture in the UK