Thomas P. Loch

Objective Globally, flavobacteria (family Flavobacteriaceae and Weeksellaceae) are leading causes of disease‐related losses in fish‐farms and hatcheries. One route flavobacteria gain access to aquaculture facilities is via source water. Ultraviolet (UV) light treatment of source water has been effective in reducing the risk of disease outbreaks caused by nonflavobacteria; however, the UV dose required to inactivate flavobacteria has been understudied. The primary objective of this study was to examine the efficacy of UV light treatments for reducing the viability of fish‐pathogenic and fish‐associated Flavobacterium and Chryseobacterium species in a planktonic form. Methods Sixty‐five flavobacterial isolates belonging to ten Flavobacterium spp. and Chryseobacterium spp. were exposed to a low (25 mJ/cm 2 ) and high (126 mJ/cm 2 ) dose of UV light via a collimating beam apparatus under in vitro conditions, after which treatment efficacy was determined via culture. Result All assayed flavobacteria were reduced by an average of ~1000‐fold or ~100,000‐fold at the low and high UV doses, respectively; however, substantial differences in reduction at the same UV dose were noted among isolates of the same flavobacterial species, including F. psychrophilum , F. columnare , and F. oreochromis . In addition, F. psychrophilum multilocus sequence typing variants ST10 and ST78, which are two of the most widespread variants in the United States of America, were among the least susceptible to ultraviolet light. Conclusion Overall, results demonstrate that viable flavobacteria can be reduced substantially by ultraviolet doses of 25–126 mJ/cm 2 , suggesting such treatments represent a promising tool for minimizing flavobacterial loads in hatcheries and aquaculture facilities, thereby enhancing biosecurity and reducing the risk of epizootics.