Does moving water change how readily young Chinook salmon acquire a bacterial infection?

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Flavobacterium columnare is a common pathogen of Chinook Salmon (Oncorhynchus tshawytscha) in the Klamath River. Elevated water temperatures cause fish to congregate in thermal refugia and are associated with columnaris disease. As an initial step toward simulating riverine exposure, researchers compared a flowing-water F. columnare challenge system with the standard static-bath challenge.

In this controlled laboratory challenge comparison, juvenile Chinook Salmon were exposed to 103 CFU/mL of F. columnare for 20 h, either in an aerated static bath or in a recirculation swim chamber set at one body length per second. Before the challenge, fish were held at a constant 20°C or exposed to short-term temperature fluctuations reaching a maximum of 24°C. Mucus and gill samples were collected at the end of the 20-h challenge and from fish held for up to 96 h after the challenge. Samples were tested for F. columnare using quantitative PCR and conventional plate culture.

In the static-bath challenge groups, F. columnare was detected in asymptomatic Chinook Salmon (38%) and moribund Chinook Salmon (29%). In the flowing-water challenge groups, it was detected in only one asymptomatic fish (4%) and one moribund fish (4%). Prechallenge temperature conditions had no effect on infection. Other yellow-pigmented bacteria were isolated from the fish, particularly in the static-bath challenge, but were not associated with morbidity and were not amplified by the F. columnare quantitative PCR assay.

The researchers concluded that F. columnare transmission among juvenile Chinook Salmon was low under flowing-water conditions simulating an early-summer thermal refuge: 20°C, a flow of one body length per second, and a 20-h exposure to 103 CFU/mL. They concluded that the flowing-water system is suitable for examining how environmental factors in riverine exposures—water velocity, temperature, host density, exposure duration, and bacterial concentration—affect F. columnare transmission to juvenile Chinook Salmon.