Effect of hyperoxygenation and low water flow on the primary stress response and susceptibility of Atlantic salmon Salmo salar L. to experimental challenge with IPN virus

Intensive salmon smolt production normally includes reduced water flow and hyperoxygenation (added oxygen) of remaining water. There is little information on how different water quality parameters influence the fish health and the susceptibility to infectious diseases. The current experiment was carried out to evaluate if the combination of hyperoxygenation and reduced water flow (hyperoxic) can act as a chronic stressor to salmon in freshwater (FW) in such a way that it increases the susceptibility to IPN virus (IPNV) following seawater transfer. In FW, after 22 days of hyperoxic exposure plasma ion, TBARS and cortisol were measured. The cortisol levels were significantly ( p=0.011) higher in the hyperoxic group compared to controls maintained under normal oxygen saturation and water flow (normoxic), indicating chronic stress. Hyperoxygenation in FW caused decreased plasma [Cl.] compared to the normoxic group ( p=0.037), while [K+] tended to be higher in the hyperoxic group ( p=0.088). No significant differences were observed in plasma [Na+], total osmolality, TBARS or hematocrit, but there was a tendency towards a lower hct in the hyperoxic compared to the normoxic group. In SW the mortality was higher in the hyperoxic group challenged with IPNV (34%) compared to the normoxic group challenged with IPNV (20%) ( p=0.02), and no mortality was observed in the PBS injected fish. The challenged fish showed an overall increase in plasma cortisol day 8, 10, 12 and 14 post-challenge ( p=0.015, p=0.000, p=0.046 and p=0.022 respectively). After SW transfer and challenge, plasma [K+] was elevated in both challenged groups, but no consistent trends were found for plasma [Cl.], [Na+] or total osmolality during the SW phase. There were no significant differences in the gene expression level of IFN 1á, Mx and IL 1â prior to challenge, suggesting that the basic expression level of these genes were not affected by hyperoxygenation. IPNV was detected in kidney and pylorus, by immunohistochemistry, cell culture, and RT-PCR in head kidney. This experiment indicates that chronic stress induced by a combination of low water flow and hyperoxygenation increases the susceptibility to IPNV challenge.