As aquaculture continues its decade-long growth, ensuring the supply of sustainable alternative feed ingredients is an urgent need. A promising new protein source is single cell proteins notably yeast, due to its high protein content with suitable amino acid profile, probiotic properties, and the capacity of production via valorising agro-industrial wastes. This study aimed to assess the effect of dietary replacement of plant-based ingredients with yeast (Candida utilis) on stress response, plasma metabolite, and lipid metabolism in hybrid African catfish.
Fish (77.7±0.25 g) were allocated between 3 isoenergetic and isonitrogenous diet treatments in freshwater RAS. All diets contained 5% of fishmeal alongside plant-based ingredients (PBI) which were partially substituted with yeast in the other diets: Diet 1 (control, 0% yeast); Diet 2 (10% of yeast, 12% PBI replacement); Diet 3 (20% of yeast, 25% PBI replacement). At the end of the trial (10 weeks), some fish from each group were subjected to acute challenge test (ACT) by increasing density. Liver and intestine histology was performed. Plasma cortisol, ions and metabolites related to energy, stress responses and lipid metabolism were quantified. Lipidomic analysis was carried out in the liver and gene expression of brain plasticity measured in the telencephalon.
Growth performance (final body weight approx. 421 g) was unaffected by the diets. ACT had limited effects, as for cortisol and plasma ions. Since catfish are notoriously rustic to the environment, it is likely that the stressor type, duration, or intensity was insufficient to trigger a marked stress response. Alternatively, this may suggest replacing PBIs with yeast has little apparent welfare effects, pointing to the suitability of this novel ingredient. Conversely, metabolites related to lipid metabolism showed significant changes among diets. Triglycerides were significantly higher in Diet 1, probably due to the higher content of grain and cereals. TAG and LDL plasma concentrations significantly decreased while HDL increased with larger inclusion of yeast in the diet, pointing to significant effects on lipid metabolism and transport mechanisms. Elevation in HDL may signal improved cardiovascular performance compared to the control diets. In line, since elevated plasma creatinine levels are linked to impaired kidney functions, the lower creatinine level in the fish on yeast diets could indicate improvements to these processes, perhaps due to a reduction in anti-nutritional factors associated with soy.
All in all, this study did not only show no negative effect of replacing PBI with yeast but was associated with some improved parameters. However, changes in lipid metabolism highlights the need of further studies targeting the liver lipid content, compositional profiles and associated molecular mechanisms, that are currently being analysed. Also, we are currently finalizing histology analysis to explore any potential effects on liver and gut health, in terms of vacuolization lipids or (improving) signs of irritation and inflammation, respectively. Additionally, we are working on data analysis of expression of brain plasticity markers to evaluate potential effects of these diets on fish stress resilience and welfare. All these will be included into an updated version of the abstract to be presented at the conference.
Acknowledgments: This project has been funded by EU Horizon 2020 - iFishIENCi (818036). Most analysis performed in the FBA high throughput state of the art lab (SacLab), NORCE, Bergen, Norway.