Bioactive properties of filamentous fungus Paecilomyces variotii KCL-24: New insights into modulation of Atlantic salmon health and welfare after seawater transfer

We assessed the potential impact of Filamentous fungus Paecilomyces variotii KCL-24 on health and welfare of Atlantic salmon (Salmo salar) post-smolts. Fish were fed a control diet (CD) or one of three experimental diets where P. variotii replaced crude protein at 5 % (PV5), 10 % (PV10) or 20 % (PV20) for five weeks in seawater (SW). Prior to SW transfer, fish were fed these same diets for four weeks in freshwater (FW). No significant differences in fish performance was detected between dietary groups. However, fish fed PV10 and PV20 showed an increase in intestinal simple fold length. By contrast, PV5 decreased vacuole size, while PV20 reduced Goblet cell (GC) density but increased vacuole count. Moreover, fish fed PV5 showed a higher level of Muc-like proteins in the skin mucus and the upregulation of anti-inflammatory cytokine il10 in the head kidney. Interestingly, fish fed PV20 had higher total immunoglobulin M (IgM) (in serum and skin mucus) and specific IgM against Vibrio anguillarum (in serum), as well as a lower serum creatine kinase. Also, the prevalence of mild to moderate welfare disorders (e.g., fin damage, skin haemorrhage and scale loss) were prevalent in fish fed PV20. Overall, the results suggest that moderate P. variotii KCL-24 inclusion can modulate the immune response of Atlantic salmon post-smolts during an early stage after SW transfer, but high inclusion level (PV20) may have a deleterious effect on fish welfare durng SW phase.