Introduction
Preventive health is a pillar of modern aquaculture that functional nutrition cement. Mannan-rich yeast parietal fractions (MOS) are safe and natural ingredients widely documented to conditionally promote health, growth and/or feed performance across a diversity of farmed animals. To date, no MOS dose-studies are reported in aquatic species limiting the optimal application of this biotechnology. In Atlantic salmon, a specific commercial MOS product (sMOS) was recently confirmed to enhance gut and skin barrier functions (Micallef et al., 2017; Leclercq et al., 2020). The aim of this study was to characterize the dose-response relationship between growth, mucosal health and sMOS dietary intake with the view to inform the strategic use of this ingredient.
Materials and Methods
The 13-week trial tested 4 dietary groups in quadruplicate using Atlantic salmon post-smolt (173 ±3 g; 200 fish / tank; 740 L flow-through tanks; 16 ppt, 16L:8D). The test diets consisted of a “European” post-smolt baseline recipe (17% FM, 17% FO; 45% prot, 23% lip) supplemented pre-extrusion with sMOS (Lallemand SAS, France) at 0, 2, 4 or 6 kg/T (Control, 2-, 4-, 6-sMOS) and hand-fed to apparent appetite 4 times daily across the trial.
The trial (Fig.1) comprised an ideal (no challenges), a chronic heat-stress and a recovery period following exposure to hydrogen-peroxide (H2O2), together mimicking demanding commercial-conditions. Growth and feed performance were assessed, as well as gut and skin barriers (histomorphometry, skin mucus level and functionality) across sampling points (Fig 1). Difference to Control group were assessed using an unpaired t-test, significance was accepted at P < 0.05, data are shown as mean ± SEM.
Results and Discussion
There was a significant positive effect of 2-sMOS on SGR over the ideal (+9.6%) and full trial’s duration (+7.2%). At higher doses, the growth-response to sMOS (Fig 2a) decreased but remained significant at 4 kg/T and numerically higher at 6 kg/T. Heat-stress significantly compromised SGR across diets (-22.1%) and cancelled the statistical benefit of sMOS. FCR showed a similar dose-response to sMOS and deterioration over the heat-stress period (+37.7% across diets).
In contrast to growth, the excretion of skin mucus overall increased with increasing sMOS intake (up to a significant +24.5% in 6-sMOS, Fig 3).
When considering mucus level per body-weight (BW) group and treatment (Fig 4), there was a significant negative linear relationship between skin mucus and BW in the Control (Fig 4); indicating weaker secretion in larger fish prioritizing for growth. In contrast, 2- and 4-sMOS appeared to remediate this apparent mucus deficiency in larger fish, while 6-sMOS further enhanced and standardized mucus excretion across BW-groups.
Further, an anecdotal reduction of mortality following H2O2-bath was recorded, from 3.47% in the Control and 2-sMOS groups to 1.39% in the 4- and 6-sMOS.
Modulation of nutritional markers, gut and skin cytoarchitecture as well as skin mucus properties in response to increasing dietary intake of sMOS will be presented.
Conclusion
Dietary sMOS elicited a growth and skin health benefit that is dose and body-size dependent. At low dose (2 kg/T), sMOS enhanced population growth and appeared to remediate the weak mucus excretion observed in faster growers. Higher intake (6 kg/t) further enhanced skin mucus excretion across body-sizes; without growth penalties but at the expense of the growth benefit observed at lower intake. This together suggests that the nutritional benefits achieved at low sMOS levels are increasingly traded towards health and resilience with increasing intake of this immune-modulatory compound.
These novel findings confirm the contribution of sMOS to overall farm performance and can be used to inform the strategic or seasonal use of this functional technology in support of targeted biomass build-up or mucosal robustness.
References
Micallef et al. 2017. Dietary yeast cell wall extract alters the proteome of the skin mucous barrier in Atlantic salmon: increased abundance and expression of a calreticulin-like protein. PLoS One 12, e0169075.
Leclercq et al., 2020. Dietary supplementation with a specific mannan-rich yeast parietal fraction enhances the gut and skin mucosal barriers of Atlantic salmon (Salmo salar) and reduces its susceptibility to sea lice (Lepeophtheirus salmonis). Aquaculture 495, 1-12.