Impact of fish density and specific water flow on skin properties in Atlantic salmon (Salmo salar L.) post-smolts

Prolonged production of Atlantic salmon (Salmo salar) post-smolts in closed-containment systems has prompted research into biological requirements under higher production intensities. This study examined the effect of fish density and specific water flow on skin health in post post-smolts particularly focusing on epithelial cell morphology and gene expression. In the density experiment, post-smolts were kept at five different fish densities (25, 50, 75, 100 and 125 kg/m3) at a specific water flow rate of 0.6 l/kg/min. Microscopic examination of fluorescence stained whole-mount skin samples demonstrated differences in epithelial cell morphology with increased spacing between epithelial cells at 50 kg/m3 and 125 kg/m3. Gene expression analysis revealed increased transcription of mucin-like 2, cathepsins B, -D, -L, matrix metalloproteinase 9 and claudin 10 in fish reared at a density of 125 kg/m3, while only matrix metalloproteinase 9 and claudin 10 had increased transcription at a density of 100 kg/m3. Together, these results suggest structural alterations in the skin epithelium at densities ≥ 100 kg/m3. In the specific water flow experiment, four different water flow levels were established (0.2, 0.3, 0.4 and 0.5 l/kg/min) while the fish density was kept constant at 75 kg/m3. After eight weeks, transcription of mucin-like 2 and -5ac, inducible nitric oxide synthase, lysozyme and cathepsin B and -L increased in skin samples from fish reared in tanks with a specific water flow of ≤ 0.3 l/kg/min. Increased transcription of these genes implies activation of stress and immune responses in skin at low specific water flow. Results from this study suggests that skin is a sensitive organ for environmental changes, and suggests several molecular indicators that may be valuable in predicting the effects of varying rearing conditions on skin health. Further validation through long-term studies, combined with other health parameters is required for practical recommendations regarding critical fish density and water flow for optimal fish health and performance in semi-closed production systems