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Diets fortified with 30 percent fermented and green rooibos extract significantly enhanced larval growth
The incorporation of medicinal herbs like garlic, ginger and moringa into finfish diets has garnered attention for their potential to boost immune responses and disease resistance, presenting a natural alternative to chemical treatments. Tea polyphenols are attracting considerable interest as natural functional ingredients to improve aquatic animal welfare. Research reviews indicate that phenolic acids can enhance growth, feed utilization, immune responses, and disease resistance through their antioxidant activity and the modulation of biological pathways. Their adoption provides an environmentally sound alternative to synthetic additives, offering a vital opportunity to boost aquaculture productivity and contribute to food security.
Rooibos tea, derived from the rooibos plant (Aspalathus linearis) native to South Africa, has much potential for inclusion in both human and animal diets due to its rich composition of bioactive compounds. The health-promoting properties of rooibos tea are also largely attributed to its diverse array of antioxidant compounds – including flavonoids, such as aspalathin, nothofagin, orientin, isoorientin, quercetin, rutin, and luteolin. These compounds contribute to the tea’s notable antioxidant activity, which can combat oxidative stress by neutralizing free radicals, thus preventing cellular damage and reducing the risk of chronic diseases.
The inclusion of rooibos in animal diets could offer several advantages, including improved antioxidant potential, enhanced immune function, and reduced inflammation. Studies have explored the effects of rooibos supplementation on livestock, poultry, and aquaculture species, with promising results observed in terms of growth performance, disease resistance, and product quality. In aquaculture, rooibos supplementation has the potential to enhance the immune response of fish, making them more resistant to pathogens.
This article – summarized from the original publication (Okuthe, G. et al. 2025. Functional Feed for Tilapia: Exploring the Benefits of Aspalathus linearis Tea Extract. Biology 2025, 14(7), 778) – reports on research to evaluate the potential of A. linearis tea extract as a functional ingredient in O. mossambicus feed, seeking to contribute to the development of sustainable and efficient aquaculture practices.
Study setup
All experiments were performed at the Freshwater Biology Laboratory at Walter Sisulu University. Larvae of Mozambique tilapia (Oreochromis mossambicus) – a widely introduced and cultured species around the world – were sourced from a registered supplier (Aquaculture Innovations, Grahamstown, South Africa). Green and fermented rooibos tea was sourced from the manufacturer (Rooibos Ltd., Clanwilliam, Cape Town, Western Cape Province, South Africa).
Rooibos tea has a rich phenolic composition and established health benefits. Depending on processing methods, rooibos tea is commercially available as fermented rooibos tea (FRT), characterized by its reddish-brown hue and distinct phytochemical composition, or unfermented green rooibos tea (GRT), which retains a greenish color. Key compounds identified in FRT include tannins, specific flavonoids (notably aspalathin and aspalalinin), and various phenolic acids. Aspalathin stands out as the primary flavonoid in GRT and is also a significant component of FRT water extracts.
Three distinct dietary treatments were formulated and administered to the experimental groups. The first, designated as the Control (CBD) group, received a commercial basal tilapia diet (Avi Feed Co., Johannesburg, South Africa) to which no rooibos tea extract (0.0 percent w/v) was added. The second treatment, the FRT Diet, consisted of the same basal diet supplemented with 30 percent (w/v) fermented rooibos tea extract. Finally, the GRT Diet represented the third treatment, comprising the basal diet fortified with 30 percent (w/v) green rooibos tea extract.
Larval fish were acclimatized for two weeks before the onset of the experiments and fed a commercial tilapia diet twice daily. Before feeding trials, fish were starved for 24 h, size-sorted by hand, and randomly stocked into each recirculating aquaculture system (RAS). A total of nine hundred fish (initial weight of 0.54 ± 0.08 g and length of 2.22 ± 0.01 cm) were randomly distributed into the RAS in triplicate at a hundred fish per tank. Each diet was assigned to triplicate tanks of fish, and fish were hand-fed to satiation three times daily for eight weeks at 2 percent body weight. At the end of eight weeks, samples were withdrawn to measure different growth parameters.
For detailed information on the experimental design and diets; animal husbandry; and sample and data collection and analyses, refer to the original publication.
Results and discussion
All three experimental diets exhibited very similar proximate compositions, indicating that the inclusion of FRT and GRT extracts did not drastically alter their fundamental nutritional makeup compared to the control diet. These findings suggest that while the extracts did not substantially change overall macronutrient and mineral percentages, they likely introduced other beneficial qualities, such as improved digestibility, altered nutrient bioavailability, or the presence of bioactive compounds. Further studies are needed to understand the specific impact of FRT and GRT on the availability and digestibility of these nutrients, as well as the presence of other beneficial compounds not captured by proximate analysis.
Fish are known to modify feed intake to satisfy their energy demand. In this study, the inclusion of FRT and GRT extracts in the basal diet (CBD) improved feed utilization and growth in larval fish, which may indicate efficient nutrient uptake, resulting in an increased organo-somatic index (VSI percent). The GRT-supplemented diet appeared more suitable for growth and feed conversion ratio (FCR) than the FRT diet. This suggests that adding rooibos tea extracts, especially GRT, may influence feeding behavior in larval fish, favoring enhanced growth performance.
Tilapia generally accepts a wide variety of plant-based diets. The observed improvements in growth performance and feed utilization are consistent with previous studies demonstrating the beneficial effects of plant extracts, such as green tea, as well as medicinal and other plant extracts, in various aquaculture species, including tilapia. Such inclusions have been shown to improve feed utilization, nutrient absorption, hematological parameters, and immune response. The significantly improved FCR in the rooibos extract groups compared to the control group clearly indicates that fish consuming rooibos-supplemented diets were more efficient at converting feed into biomass.
This enhanced efficiency is a key indicator of improved digestive health and metabolic performance, likely mediated by the properties of the rooibos extracts. While the rooibos groups showed superior weight gain, the control group exhibited the highest condition factor. The condition factor measures a fish’s robustness relative to its length. This observation, while appearing counterintuitive, suggests that while rooibos promoted efficient linear growth and overall biomass accumulation, the control fish might have accumulated more adipose tissue relative to muscle or, conversely, the rooibos extracts encouraged leaner muscle mass gain.
The dietary inclusion of A. linearis extract, as demonstrated in this study, can enhance the immune responses of Mozambique tilapia through various mechanisms. This plant extract is rich in bioactive compounds that can stimulate both innate and adaptive immune responses, thereby improving the overall health and resilience of the fish in several ways:
- Enhanced Phagocytic Activity: Rooibos extract can enhance the activity of phagocytes, which are crucial for the innate immune response and pathogen clearance. Dietary supplements like rooibos extract can enhance the phagocytic index in tilapia, similar to xanthones from mangosteen. Various dietary supplements positively influence tilapia immune responses, including phagocyte activity, suggesting rooibos extract’s similar role in enhancing innate immunity. Natural extracts can improve overall health and disease resistance in tilapia, indicating rooibos’s broader potential in aquaculture.
- Modulation of Immunomodulatory Cytokines: The extracts may promote the expression of immunomodulatory cytokines, such as interleukin-1 beta and interferon-gamma, which are vital for orchestrating immune responses.
- Increased Leukocyte Counts: The inclusion of functional ingredients like rooibos may lead to increased leukocyte counts, which is essential for effective phagocytosis.
- Reduction in Oxidative Stress: Compounds like aspalathin and nothofagin in rooibos exhibit strong antioxidant properties, reducing oxidative stress and inflammation in fish, which can enhance immune function.
- Improved Antioxidant Enzymes: The extract may elevate levels of antioxidant enzymes, such as catalase and glutathione peroxidase, further supporting immune health.
Future investigations should integrate biochemical analyses of liver enzymes, lipid profiles, and molecular markers of oxidative stress and inflammation to provide a more comprehensive understanding of these hepatic changes.
Perspectives
This study focused on the effects of rooibos tea extracts (FRT and GRT) on growth performance in larval O. mossambicus. The findings collectively demonstrate improvements in growth performance, feed utilization, and genoprotective effects, alongside a nuanced impact on organ histopathology. The observed superior weight gain and feed conversion ratios, particularly with green rooibos tea, suggest that the benefits extend beyond basic macronutrient content, and are likely attributable to the bioactive polyphenolic compounds, such as protocatechuic acid, which may enhance palatability, gut health, and nutrient absorption.
The genoprotective effects, evidenced by reduced micronucleus frequency, underscore the antioxidant capabilities of rooibos, contributing to cellular health and genetic stability. While the study highlights promising applications for rooibos extracts in aquafeed, particularly given their local sourcing potential, the observed hepatic alterations necessitate further investigation into optimal inclusion levels and long-term physiological impacts. Future research should focus on elucidating specific molecular mechanisms, conducting comprehensive gut health and immune response analyses, and evaluating effects on product quality. This will ensure the sustainable and effective integration of A. linearis extracts into aquaculture practices, contributing to both fish health and food security.
By Grace Okuthe, Bongile Bhomela, Noluyolo Vundisa
Read more:
- Optimizing tilapia biofloc technology systems, part 1
- Optimizing tilapia biofloc technology systems, part 2
- Optimizing tilapia biofloc technology systems, part 3