Reduced water quality associated with higher stocking density disturbs the intestinal barrier functions of Atlantic salmon (Salmo salar L.)

The stocking density of fish in aquaculture is of major importance as it may have profound effects on water quality resulting in impact on fish health and possibly affect the external barriers that protect against pathogens There are many husbandry conditions, including stocking density, that may affect the primary protective barriers, i.e. the skin, intestine and gills, against invading pathogens and other harmful substances. It is well known that increased fish density will lead to decreased dissolved oxygen (DO) levels and affect other water quality parameters such as carbon dioxide, pH and ammonia. It is not known if such changes in the rearing environment affect the intestinal primary barriers of Atlantic salmon. Groups of Atlantic salmon post-smolts were kept for 57 days in tanks supplied with seawater at a constant flow at stocking densities of 10, 30, 50 and 70 kg m(-3); reduced water quality was associated with higher stocking density. Repeated sampling for plasma cortisol and water cortisol release rate indicate that the highest stocking density elicited a primary stress response in the fish, which decreased with time. The physical intestinal barrier was assessed using paracellular permeability measurements, i.e. transepithelial electrical resistance (TER) and diffusion rate of C-14-mannitol, in combination with the translocation rate of heat-inactivated Aeromonas salmonicida. The physical intestinal barrier decreased with increasing density, both when measured as decreased transepithelial electrical resistance and as elevated paracellular permeability for C-14-mannitol. As this was observed at a time point when no differences could be seen in plasma cortisol or cortisol release rate, it suggests that intestinal paracellular permeability can be a useful marker for chronic stress in salmon. The status of the intestinal immune system was assessed as degree of neutrophil infiltration as well as the mRNA expression of the pro-inflammatory cytokines, interferon gamma (IFN gamma), interleukin (IL) 1 beta and tumor necrosis factor (TNF) alpha, anti-inflammatory cytokines, IL-10 and transforming growth factor beta (TGF beta) and other immune related-genes, IL-8 and the inhibitor of the transcription factor nuclear factor kappa B (I kappa B). The intestinal immune system was affected at the highest stocking density as observed by a decreased expression of IFN gamma in parallel with higher infiltration of neutrophils. In conclusion, high stocking density associated with reduced water quality is chronically stressful to the fish as it elicits a primary stress response as well as a weakened physical and disturbed immunological primary barrier.