Flavobacterium psychrophilum is a pathogen causing bacterial coldwater disease in salmonid hatcheries worldwide. This study evaluated the feeding of four commercial diets to rainbow trout Oncorhynchus mykiss at a hatchery with endemic bacterial coldwater disease in three sequential feed trials, beginning at 28 days after initial feeding and lasting for a total duration of 83 days. Two Bio-Oregon (Longview, Washington, USA) diets, Bio-Vita and BioPro2, and two Skretting (Toele, Utah, USA) diets, Classic Trout and Protec, were used. While the overall mortality rates were the same among all the diets, there were differences in the timing at which the mortality occurred. Significant differences in tank ending weight, weight gain, and feed conversion ratio only occurred in the third and final trial, with the Classic Trout diet producing the poorest results. Overall results, with all of the trials combined, indicated tanks receiving the Bio-Vita and BioPro2 diets had significantly higher weight gain than tanks receiving Classic Trout. Additionally, feed conversion ratios were significantly elevated in the Classic Trout treatment group compared to the other three dietary treatments. With few impacts on overall mortality among all diets tested, the faster growth obtained from the Bio-Oregon diets may lead to more rapid development of acquired immunity with possible implications for bacterial coldwater disease management beyond the duration of this study.
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