Tenacibaculosis Caused by <i>Tenacibaculum maritimum</i> Is Not Transmitted From Atlantic Salmon ( <i>Salmo salar</i> L.) to Canadian Chinook Salmon ( <i>Oncorhynchus tshawytscha</i> W.) in a Cohabitation Model

Canadian salmonid aquaculture provides a sustainable protein source; however, there are concerns that Atlantic salmon ( Salmo salar L.) mariculture reduces wild Pacific salmon survival through interspecific disease transfer. Tenacibaculosis, caused by species of Gram‐negative bacteria in the genus Tenacibaculum , has the potential to be transmitted interchangeably between farmed Atlantic salmon and wild Pacific salmon, though there is a lack of corroboration establishing transmission. To provide evidence for interspecific horizontal transmission of tenacibaculosis from Atlantic salmon to Pacific salmon, Atlantic salmon were bath‐exposed to an isolate of Tenacibaculum maritimum and cohabitated with naïve Atlantic or Chinook salmon ( Oncorhynchus tshawytscha W.) for 25 days. Exposed and naïve cohabitant Atlantic salmon exhibited morbidity with multifocal superficial and ulcerative epidermal lesions with intralesional T. maritimum (culture, histology, and qPCR). At 10 8 CFU mL −1 , exposed and naïve cohabitant Atlantic salmon had 43% and 60% mortality, respectively. Contrastingly, cohabitant Chinook salmon experienced no morbidity or mortality, despite successful culture of T. maritimum (10 8 CFU mL −1 n = 5/6 fish; 10 6 CFU mL −1 n = 0/6 fish) from skin swabs. These findings suggest that BC Chinook salmon do not develop clinical tenacibaculosis through interspecific horizontal transmission from farmed Atlantic salmon with mouthrot under the tested conditions and that the presence of T. maritimum alone is insufficient for disease. Further research needs to clarify the genetic differences between hosts and pathogens in different geographical locations, and investigate additional T. maritimum isolates, alternative Tenacibaculum species, environmental variables, and temporal scales that could lead to clinical tenacibaculosis in Chinook salmon.