AMONG TOP DISEASE CHALLENGES FOR FARMED TROUT ARE PATHOGENIC FLAVOBACTERIA, PARTICULARLY FLAVOBACTERIUM PSYCHROPHILUM (FP). UNFORTUNATELY, FEW FDA-APPROVED TREATMENT OPTIONS EXIST, AND EFFECTIVE VACCINES ARE CURRENTLY UNAVAILABLE IN THE USA. OUR PREVIOUS USDA-FUNDED STUDIES DOCUMENTED SUBSTANTIAL FP DIVERSITY AND LINKED CERTAIN STRAINS TO DISEASE OUTBREAKS IN US TROUT FARMS, SOME OF WHICH ALSO RESISTED APPROVED ANTIMICROBIALS (OXYTETRACYCLINE, SULFONAMIDES AND FLORFENICOL), AND THAT MAY POSSESS HEIGHTENED TRANSMISSIBILITY OWING TO BIOFILM FORMATION OR ENVIRONMENTAL PERSISTENCE. WE ALSO IDENTIFIED MULTIPLE FP-VARIANTS THAT, DESPITE CAUSING DISEASE, WERE FP BY SEQUENCE ANALYSIS BUT COULD NOT BE CHARACTERIZED USING STANDARD ASSAYS, INCLUDING ROUTINE DIAGNOSTIC TESTS AND POOR GROWTH ON TYPICAL MEDIA. OUR OVERARCHING GOAL IS TO ACCOUNT FOR THIS DIVERSITY AND REDUCE FP-INDUCED TROUT LOSSES BY IMPROVING FLAVOBACTERIAL DIAGNOSTIC CAPACITY,MODIFYING MANAGEMENT PRACTICES TO ACCOUNT FOR STRAIN-BASED PERSISTENCE AND TRANSMISSION STRATEGIES, AND EXPLORING NOVEL VACCINATION TACTICS BY: 1) DEVELOPING RAPID/SENSITIVE MOLECULAR AND CULTURE-BASED DIAGNOSTICS TO DETECT ALL FP-VARIANTS; 2) ELUCIDATING HOW FP-STRAIN VARIATION IN BIOFILM-FORMING CAPACITY, ENVIRONMENTAL PERSISTENCE, AND TRANSMISSION CONTRIBUTE TO DISEASE ASSOCIATED LOSSES, AND MODIFY MANAGEMENT ACCORDINGLY; 3) EVALUATE NON-VIRULENT NON-FP FLAVOBACTERIA AS VACCINE CANDIDATES AGAINST FP. THIS RESEARCH WILL IMPROVE FARMED-TROUT PRODUCTION/PROFITABILITY IN SHORT-TERM VIA: A) IMPROVED DISEASE DIAGNOSIS/TIMELY INTERVENTION; B) REDUCED DISEASE LOSSES THROUGH TARGETED MANAGEMENT. PRODUCTION/PROFITABILITY WILL IMPROVE MEDIUM-TERM VIA POTENTIAL VACCINE DEVELOPMENT AND REDUCING ANTIBIOTIC USE. THIS PROPOSAL ADDRESSES CRITICAL DISEASE ISSUES IMPACTING COMMERCIAL AQUACULTURE SPECIES PRIORITY OF USDA NIFA SPECIAL RESEARCH GRANTS PROGRAM, GOALS 2&AMP;4 OF 2018-2022 USDA STRATEGIC GOALS, AND THE ANIMAL HEALTH AND PRODUCTION AFRI PRIORITY AREA (2008 FARM BILL).