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

Abstract

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 125kg/m3) at a specific water flow rate of 0.6l/kg/min. Microscopic examination of fluorescence stained whole-mount skin samples demonstrated differences in epithelial cell morphology with increased spacing between epithelial cells at 50kg/m3 and 125kg/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 125kg/m3, while only matrix metalloproteinase 9 and claudin 10 had increased transcription at a density of 100kg/m3. Together, these results suggest structural alterations in the skin epithelium at densities ≥100kg/m3.In the specific water flow experiment, four different water flow levels were established (0.2, 0.3, 0.4 and 0.5l/kg/min) while the fish density was kept constant at 75kg/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 ≤0.3l/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. Statement of relevanceThis paper is of high importance for the industry as it investigates the effects of fish density and specific water flow on skin properties in semi-closed-containment-systems in Atlantic salmon post-smolt production. Increased fish densities and reduced specific water flow can increase production profitability. Thus, biological limits related to these two factors are important to identify to avoid reduced animal welfare.