Responses to Moritella viscosa in skin and spleen of Atlantic salmon

Bacterial pathogen Moritella viscosa, the causative agent of winter ulcer, causes heavy losses in Atlantic salmon aquaculture. The study compared responses in salmon reared under normal condition (G100) and fish exposed to hypoxia - 60% saturation of dissolved oxygen - at early life (G60). G60 showed lower survival after challenge. Analyses were performed in the most affected tissues: skin and spleen The M. viscosa challenge trial was performed at the Aquaculture Research Station in Tromsø, Norway. Atlantic salmon (AquaGen strain) was smoltified by the following light regime: 6 weeks of 18 h darkness and 6 h light (winter), followed by 6 weeks of 24 h light (summer). After smoltification the fish, mean weight 80 gr, was transferred to seawater in 900 l circular tanks, 325 fish per tank at 10oC and acclimated for 5 days before bath challenge (stagnant oxygenated water, density 58 kg/m3, duration 1 hour, bacterial dose 10e7 cfu/ml) with M. viscosa (LFI5006/2). Samples were collected before challenge, at day 0, and at the end of the challenge experiment. Mortality was registered for 36 days and all dead fish were scored for wounds. Verification of M. viscosa as cause of death was done by isolation of the pathogen from head kidney samples of a selected number of fish on blood agar with 2% NaCl. At termination all surviving fish from the two tanks were scored for wounds, and 30 fish from each tank were sampled for blood (plasma, erythrocytes), gill, skin, head kidney and spleen on RNAlater.