FEEDING LIVE YEAST AND WATER TEMPERATURE IMPACTS BLOOD AND GUT PHYSIOLOGY OF RAINBOW TROUT
Introduction
Alternative protein sources are needed to replace the unsustainable use of fishmeal in diets for farmed fish. Single cell proteins, such as yeast, contain similar amino acid profile as fishmeal and studies have shown yeast can replace up to 40% of fishmeal in heat-extruded diets for rainbow trout (Oncorhynchus mykiss) without reductions in growth performance (Hauptman et al., 2014, Huyben et al., 2017). However, high levels of yeast fed to trout can result in increased nucleic acid metabolism that may impair red blood cell function (Huyben et al., 2016). High inclusion of dietary yeast in diets is typically inactivated by heat-extrusion, while effects of live yeast at high inclusions are unknown. In addition, elevated water temperature can increase fish growth, but this may induce intestinal inflammation and dysfunction. The objective of this study was to investigate the effects of feeding live yeast and water temperature on blood and gut physiology of rainbow trout.
Materials and methods
Rainbow trout (129g) were fed either cold-pelleted fishmeal or yeast-based diet and reared in water temperatures of either 11oC or 18oC for 6 weeks. In the yeast diet, 40% of fishmeal protein was replaced with the yeast Saccharomyces cerevisiae (Jästbolaget, Solna, Sweden). Fish were fed at 1.5% bodyweight per day and weighed before and after the experiment. Afterwards, blood from the caudal vein was collected from three fish per tank (n=12) and analysed for pH, haematocrit, haemoglobin, red blood cell counts and these values were used to calculate red blood cell indices. Plasma cortisol was analysed using an enzyme linked immunosorbent assay (ELISA). Also, mucosa from the proximal and distal intestine was collected and RNA was extracted, equally diluted and cDNA was synthesized. Quantitative PCR with SYBRGreen mastermix was performed using primers that targeted pro-inflammatory genes: tumor necrosis factor (TNFα), interferron-gamma (IFNγ) and interleukins (IL) -1β, -8 and -17, anti-inflammatory genes: transforming growth factor beta (TGFβ) and interleukin 10 (IL10), and heat shock protein (HSP) genes: 70 and 90. Relative expression of each target gene was calculated based on expression of the reference gene β-actin.
Results
For growth, no significant difference in specific growth rate, weight gain and feed conversion ratio were found (p>0.05), although fish fed yeast in cold water had the lowest growth. For blood, fish in warm water had significantly lower pH and increased haemoglobin and mean corpuscular haemoglobin concentration. Plasma cortisol was 2-3 fold higher in fish in warm water, but not significantly different. For gene expression, in the proximal intestine, no significant differences were found for IFNγ, TNFα, TGFβ, IL8 and IL17 (Figure 1), which generally showed reduced expression in fish reared in warm water, especially when fed yeast. No significant difference existed for gene expression in the distal intestine.
Discussion and conclusion
Fish growth was expected to increase with temperature, but the similarities between fish at 11 and 18ͦC indicate that these temperatures were outside their optimal growth temperature (Jobling, 1995). Reduced pH and increased hemoglobin in the blood in fish reared in warm water can be attributed to a normal physiological response to increase oxygen demand. Metabolic rate increases with temperature and more haemoglobin is needed to transport oxygen and low pH increases the off-loading of CO2 in the blood (Jobling, 1995). Increased plasma cortisol and reduced expression of pro-inflammatory and anti-inflammatory genes in fish reared in warm water (Figure 1) indicates the temperature increase induced a stress response that may have suppressed the innate immune system. This may explain reduced disease resistance in previous studies (Segner et al., 2012) and demonstrates the negative effects of elevated water temperature on the health of rainbow trout. The further reduction of cytokine expression in fish fed yeast in warm water is still unclear and requires more research.
References
Hauptman, B.S., Barrows, F.T., Block, S.S., Gaylord, T.G., Paterson, J.A., Rawles, S.D., Sealey, W.M., 2014. Evaluation of grain distillers dried yeast as a fish meal substitute in practical-type diets of juvenile rainbow trout, Oncorhynchus mykiss. Aquaculture. 432, 7-14.
Huyben, D., Vidakovic, A., Nyman, A., Langeland, M., Lundh, T., Kiessling, A., 2016. Effects of dietary yeast inclusion and acute stress on post-prandial whole blood profiles of dorsal aorta-cannulated rainbow trout. Fish Physiol. Biochem., 1-14.
Huyben, D., Nyman, A., Vidaković, A., Passoth, V., Moccia, R., Kiessling, A., Dicksved, J., Lundh, T., 2017. Effects of dietary inclusion of the yeasts Saccharomyces cerevisiae and Wickerhamomyces anomalus on gut microbiota of rainbow trout. Aquaculture. 473, 528-537.
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Segner, H., Sundh, H., Buchmann, K., Douxfils, J., Sundell, K.S., Mathieu, C., Ruane, N., Jutfelt, F., Toften, H., Vaughan, L., 2012. Health of farmed fish: its relation to fish welfare and its utility as welfare indicator. Fish Physiol. Biochem. 38, 85-105.
