Effects of chronic sub-lethal nitrite exposure at high water chloride concentration on Atlantic salmon (Salmo salar, Linnaeus 1758) parr

The present study examined the protective effects of water chloride (Cl-) towards nitrite toxicity in Atlantic salmon parr during 84-day long nitrite exposure. Effects on growth, histology, blood indices and gene expression were studied at a fixed nominal Cl- concentration of 200 mg/L and at several water nitrite concentrations (0, 0.5, 2, 5 and 10 mg/L NO2--N). The specific growth rate was significantly reduced during the first three weeks at a Cl:NO2--N ratio of 21:1, suggesting the activation of coping mechanisms at the later stages of the experiment. No significant effect of nitrite on gill histology and mortality was found. Nitrite accumulated in plasma; however, a Cl:NO2--N ratio of 104:1 or higher prevented nitrite entry. The concentration of NO2--N in plasma was significantly reduced at the end of the study, supporting the hypothesis of coping mechanisms. Cystic fibrosis transmembrane conductance regulator (cftr)-1 showed a significant up-regulation at highest nitrite concentration on day 22, and in three of the highest exposure groups at the end of the experiment. Our findings suggest that a Cl:NO2--N ratio above 104:1 should be maintained through episodes of nitrite accumulation in water during the production of Atlantic salmon parr.