Yersinia ruckeri , the causative agent of enteric redmouth disease (ERM), is a common pathogen that causes substantial economic losses in salmon id aquaculture . This study evaluated the efficacy a thymol-based blend – Prototype TCS – in reducing mortality on juvenile rainbow trout in following immersion challenge with Yersinia ruckeri . Two practical diets were manufactured to contain 2000 ppm of Prototype TCS and no additive (Control) and fed to 320 trouts divided in 16 tanks (n =8). After four weeks of feeding, the f ish were challenged with Y. ruckeri. Fish fed diets containing Prototype TCS effectively reduced post-challenge mortality by 15.6% and hazard ratio by 29.3% (P<0.05 -Figure 1) . Additionally, f ish fed Prototype TCS had a significantly lower prevalence (P<0.05) of pathological signs in survivors, supporting enhanced disease robustness. Our results demonstrated that including Prototype TCS in rainbow trout diets significantly increased resistance against ERM. This additive could therefore serve as promising alternatives to reduce antibiotic reliance and support more sustainable aquaculture practices.