Premium Atlantic salmon diets have contained up to 30% fishmeal (FM) and 20% fish oil (FO). Sustainability concerns over raw materials such as those derived from forage fish has led to continuous research and refinement of aquafeeds. Despite the growth of novel ingredients (such as insect meals, algal meals and single cell proteins) as FM/FO replacements, maintaining a low Fish-in : Fish-out ratio (FiFo) for salmon aquaculture largely depends on the more readily available plant derived raw materials. However, the combination of low levels of marine ingredients with high levels of plant derived raw materials can lead to poor gut health and welfare.
Functional feed additives such as probiotics, prebiotics, enzymes, and phytobiotic compounds have been incorporated in aquaculture diets for their ability to improve intestinal health and enhance disease resistance. Particularly, cell wall components of Saccharomyces cerevisiae (rich in β-1,3 and -1,6-glucans and mannan oligosaccharides) have been demonstrated to confer immunomodulatory effects in fish. These benefits are at least partially induced by improvements of intestinal health. However, despite these reported benefits, many knowledge gaps exist with the exact mode of action of yeast cell wall derivatives. Therefore, an experiment was conducted to investigate the efficacy of whole cell walls and highly purified β-glucans derived from brewer’s yeast on the intestinal health of Atlantic salmon fed low marine content (15% FM, 8% FO) diets.
A total of 120 parr (ca. 20g) were randomly assigned into six experimental units (20 fish per tank) and fed either 1] control (no yeast additives), 2] PβG (0.02% Leiber® Beta-S) or 3] WYCW (0.2% Biolex® MB40) treatments for 4 weeks. All treatment groups were fed to the same percentage of biomass (av. 1.5% per day). At the end of the experiment, distal intestine and skin tissues were collected from 5 fish per tank (N = 10 per treatment) for histology, electron microscopy and gene expression analyses according to protocols described elsewhere (Merrifield et al., 2009; Rawling et al., 2021).
Results and Discussion
At the end of the experiment, there were no significant differences in zootechnical performance (weight gain, SGR and FCR) between fish fed the different diets. Histological appraisal (Figure 2) revealed that fish fed the WYCW treatment had a 39% increase (P = 0.0422) in goblet cell abundance in the distal intestine and that the PβG treatment-fed fish had a 49% increase (P = 0.0459) in goblet cell abundance in the skin when compared to the control group.
In addition, electron microscopy analysis (Figure 3) of the distal intestine revealed significantly different microvilli morphometrics. Fish fed the PβG treatment had 20% longer (P < 0.0001) and 34% denser (P = 0.0001) microvilli compared to the control group. Fish fed the WYCW treatment had 25% denser microvilli arrangement (P = 0.0056) than the control group.
Targeted gene expression analysis of immunomodulatory and barrier function genes is on-going, however, the available data show that both functional feed additives demonstrated the potential to enhance the epithelial barriers studied.
Merrifield, D. L., Dimitroglou, A., Bradley, G., Baker, R. T. M. & Davies, S. J. (2009) ’Soybean meal alters autochthonous microbial populations, microvilli morphology and compromises intestinal enterocyte integrity of rainbow trout, Oncorhynchus mykiss (Walbaum)’. Journal of Fish Diseases, 32 (9), pp. 755-766.
Rawling, M., Leclercq, E., Foey, A., Castex, M. & Merrifield, D. (2021) ’A novel dietary multi-strain yeast fraction modulates intestinal toll-like-receptor signalling and mucosal responses of rainbow trout (Oncorhynchus mykiss)’. PLOS ONE, 16 (1), pp. e0245021.
Urán, P. A., Gonçalves, A. A., Taverne-Thiele, J. J., Schrama, J. W., Verreth, J. A. J. & Rombout, J. H. W. M. (2008) ’Soybean meal induces intestinal inflammation in common carp (Cyprinus carpio L.)’. Fish & Shellfish Immunology, 25 (6), pp. 751-760.