** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** MUSCLE/FILLET IS THE MOST VALUABLE PRODUCT OF FISH. IMPROVING FILLET YIELD AND QUALITY CAN ENHANCE THE EFFICIENCY OF AQUACULTURE PRODUCTION AND CUSTOMER SATISFACTION AND HAVE SIGNIFICANT ECONOMIC BENEFITS.CLASSICAL BREEDING HAS LIMITED SUCCESS IN IMPROVING FILLET YIELD AND REQUIRES SEVERAL GENERATIONS. THE AQUACULTURE INDUSTRY IS EAGER TO ADOPT INNOVATIVE GENETIC SELECTION METHODS TO INCREASE THE EFFICIENCY OF GENETIC IMPROVEMENT, I.E., INCREASING THE ACCURACY OF BREEDING VALUES AND REDUCING GENERATION INTERVALS, CONSEQUENTLY INCREASING GENETIC GAIN. GUT MICROBIOTA HAVE RECENTLY SHOWN A ROLE IN MAINTAINING AND REGULATING HOST PHYSIOLOGY, INCLUDING MUSCLE GAIN AND FUNCTIONS IN MAMMALS. HOWEVER, HOST GENETICS' ROLE ON THE GUT MICROBIOTA-MUSCLE AXIS REMAINS TO BE DISCOVERED IN AGRI-ANIMAL SPECIES, INCLUDING FISH.A RECENT STUDY IN OUR LABORATORY SHOWED THAT FILLET YIELD GENETIC LINES PRODUCED AT USDA/ARS/NCCCWA ARE PREDICTIVE OF THE GUT MICROBIOME COMPOSITION IN RAINBOW TROUT, INDICATING THE PRESENCE OF HERITABLE FILLET YIELD-ASSOCIATED MICROBIOME COMPONENTS.IN THE PROPOSED STUDY, WE WILL ASSESS NOVEL APPROACHES TO IMPROVE FILLET YIELD IN TROUT BREEDING PROGRAMS AT NCCCWA AND ONE OF THE WORLD'S LEADING COMMERCIAL PRODUCERS OF RAINBOW TROUT EGGS. WE WILL USE HOST GENOTYPES ALONG WITH HIGH-THROUGHPUT GUT MICROBIOME/METAGENOMICS DATA IN MICROBIOME GENOME-WIDE ASSOCIATION STUDIES TO 1) IDENTIFY GENE VARIANTS ASSOCIATED WITH THE MICROBIOMEAND DETECT HERITABLE MICROBIAL GROUPS ASSOCIATED WITH IMPROVED FILLET YIELD THAT COULD BE USED AS MARKERS TO DEVELOP IMPROVED FISH FOR BREEDING PURPOSES, 2) ASSESS THE FEASIBILITY OF IMPROVING THE GENETIC PREDICTION ACCURACY AND GENOTYPING COST EFFICIENCY WHEN THE MICROBIOME DATA AND THE GENOMIC RELATIONSHIP INFORMATION ARE INTEGRATED INTO GENOMICS-METAGENOMICS ANALYSES.ADDITIONALLY, WE PLAN TO ASSESS THE EFFECTS OF MODULATING THE GUT MICROBIAL COMPOSITION IN FAVORING BENEFICIAL MICROBIAL FLORA TO INCREASE FILLET YIELD VIA EARLY-LIFE FECAL TRANSPLANTATION.