Abstract
The family Flavobacteriaceae comprises multiple bacterial species, many of which are responsible for serious diseases in wild and cultured fish populations worldwide. These diseases often result in widespread economic and ecological losses.1 The three best-characterized Flavobacterium species, F. columnare (etiologic agent of columnaris disease), F. psychrophilum (etiologic agent of bacterial coldwater disease), and F. branchiophilum (etiologic agent of bacterial gill disease) commonly infect salmonids.1,2,3,4 Members of this family have also been recognized as emergent fish pathogens, including F. johnsoniae-like isolates, F. spartansii, F. chilense, F. araucananum, and F. oncorhynchi.1,2
Chinook salmon (Oncorhynchus tshawytscha) are a keystone species of the Pacific Northwest. They migrate as juveniles from high mountain streams to the Pacific Ocean where they mature, and back upstream as adults to spawn in their native freshwater streams.5 At least seventeen distinct runs of Chinook salmon are recognized in California. The fall-run Chinook migrate upstream as adults from July through December and spawn from early October through late December.5
In order to gain a better understanding of the prevalence of Flavobacterium spp. in this population, 100 fall-run Chinook salmon (46 males and 54 females) spawned at the Nimbus Fish Hatchery on the American River in the fall of 2017 were analyzed. Approximately 30% of fish presented with gross skin or fin lesions. Posterior kidney samples (∼50 mg of tissue) were collected and homogenized in 1X phosphate buffered saline prior to inoculation into tryptic soy agar supplemented with 5% sheep blood (blood agar), tryptone yeast extract salts (TYES) agar, and modified Shieh agar plates supplemented with Tobramycin. Bacteria were cultured from 31 out of the 100-sampled fish. Specifically, 59, 68, and 41 isolates were recovered from blood, TYES, and modified Shieh agars, respectively. Amplification and sequence of the 16S rRNA gene6 identified the organisms as Aeromonas sobria (n=3), A. veronii (n=2), unspecified Aeromonas sp. (n=5), Flavobacterium gilvium (n=2), F. columnare (n=6), F. succinicans (n=1), unspecified Flavobacterium sp. (n=3), Yersinia ruckeri (n=1), unspecified Comamonas species (n=1), Lactococcus lactis (n=1), unspecified Acidovorax species (n=1), Enterococcus mundtii (n=1), Chryseobacterium indologenes (n=4), Acinetobacter tjenbergiae (n=1), unspecified Microbacterium sp. (n=1) as well as uncultured bacterium (n=3). Flavobacterium columnare isolates were typed as genomovar I/II. Future research will focus on investigating the virulence and role as primary or opportunistic pathogen of these organisms. This information can be used to generate baselines of pathogen prevalence for use in the future to evaluate health risks to this keystone species that is already challenged by environmental (drought, climate change, ocean acidification) and anthropogenic (fishing, dams) factors.7
Acknowledgements
The authors wish to thank the staff at Nimbus Fish Hatchery and the people of California for access to these fish.
* Presenting author
+ Student presenter
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