Introduction
Rainbow trout (Oncorhynchus mykiss) is one of the most important cultured species because of its high adaptability to various environmental conditions, rapid growth in intensive farming systems, and habituation to commercial aquafeeds (Parchami et al., 2022). Fish oil had been a main raw ingredient in aquafeeds for a long time; however, its availability has become increasingly limited (Gasco et al., 2018). In Iran, fat powder, a by-product of oil extraction, has potential for mass production. Nevertheless, the incorporation of fat powder into fish diets may adversely affect fish health due to the presence of saturated fatty acids, low digestibility, and associated side effects (Keramat Amirkolaie et al., 2014; Adhami and Keramat Amirkolaie, 2016). The complete or partial replacement of fish oil with these saturated oils can negatively impact lipid lipolysis and immune function (Abedian Kenari et al., 2010; Adhami and Keramat Amirkolaie, 2016). Furthermore, diets high in unsaturated fatty acids are associated with increased lipid peroxidation in mitochondria, leading to liver cell damage (Abedian Kenari et al., 2011). Taurine plays a crucial role in fish physiology, including bile acid conjugation, immune regulation, osmoregulation, antioxidant effects, and the development and regeneration of the nervous system (Salze and Davis, 2015; Xu et al., 2020). It also regulates liver oxidative status (Martins et al., 2019), aids in detoxification, and stimulates immune responses in bony fish (Cheng et al., 2018; Dehghani et al., 2020). Recognized as a potent antioxidant, taurine mitigates oxidative stress by acting as a non-specific scavenger of harmful reactive oxygen species, thus protecting cells from oxidative damage and enhancing the synthesis of antioxidant enzymes (Hosseini et al., 2017). This research aims to evaluate the potential of taurine as a valuable additive in aquaculture feed. The current study is designed to assess the effects of taurine on blood indices, immune system function, antioxidant enzyme activity, and liver function in rainbow trout (O. mykiss) fed with a diet containing fat powder.
Methodology
A total of 225 rainbow trout with an average initial weight of 12.00±0.03 g was randomly assigned to 15 tanks, each containing 150 liters of water for 58 days. Five experimental diets were formulated: a fish oil and canola oil-based diet (positive control), fat powder-based diet (approximately 70% of the fat source was derived from fat powder) supplemented with 0% (negative control), 0.5%, 1%, and 2% taurine. The fish were fed three times daily at 8:00, 12:00, and 18:00 until apparent satiation. At the end of the trial, blood samples were taken from the caudal fin of each fish. The samples were allowed to clot and were subsequently centrifuged at 4600 rpm for 10 minutes to obtain serum. Hematological parameters, including red blood cell (RBC) and white blood cell (WBC) counts, hemoglobin concentration, and hematocrit levels, were measured. Additionally, serum biochemical parameters including cholesterol, triglycerides, total protein, glucose, albumin were determined. Hepatic enzyme (aspartate aminotransferase (AST) and alanine aminotransferase (ALT)), antioxidant indices (superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT) and malondialdehyde (MDA)), and immune indices (complement, lysozyme and immunoglobulin) were also assessed to evaluate the health status of fish fed diet containing taurine and fat powder.
Results
According to the results, the fish fed with 2% taurine indicated the highest levels of WBC and RBC (p<0.05). The highest values of hematocrit and MCV were obtained in fish fed 1% taurine (P<0.05). MCH and MCHC amounts elevated in the negative control group and at the 0.5% taurine dietary level, respectively (p<0.05). In contrast, the concentrations of hemoglobin were not significantly affected by the experimental diets (p>0.05). The serum biochemical parameters showed that the inclusion of 2% taurine led to the highest levels of cholesterol, triglycerides, total protein, and glucose; However, the albumin content remained unaffected (p>0.05). The concentration of triglyceride reduced with the addition of 0.5% taurine to the diet, while the lowest levels of cholesterol and glucose were found in the negative control group (p<0.05). The highest concentration of total protein was observed with the supplementation of 2% taurine (p<0.05). Furthermore, the activities of AST and ALT in negative control were significantly higher than those of fish fed 2% taurine (p<0.05). Antioxidant indices demonstrated significant differences in SOD, GPx, CAT, and MDA among the experimental groups (P<0.05). The highest levels of SOD and GPx were obtained by inclusion of 1.5% and 2% taurine, while the lowest amounts were observed in the negative control (p<0.05). Immunological parameter indicated that the addition of 2% taurine to diet containing fat powder resulted in an improvement of immunoglobulin, ACH50 and Lysozyme compared to the negative control (p<0.05).
Discussion and conclusion
The results of the current study suggest that reduced damage to the blood cells of fish fed a diet supplemented with 2% taurine may lead to an increase in hematocrit, RBC and WBC counts. Previous research indicates that taurine deficiency can contribute to blood cell destruction (Takagi et al., 2006, 2011). Furthermore, taurine plays a crucial role in synthesizing bile acids from cholesterol, which may reduce the need for bile acids and promote greater cholesterol reabsorption. This implies a decrease in hepatic absorption for bile acid synthesis in fish fed with 2% taurine, consistent with findings in rosy barb fish (Pethia conchonius), which taurine significantly increased triglyceride and cholesterol levels (Nejatizadegan et al., 2020). Taurine also interacts synergistically with insulin and insulin-like substances to enhance glucose and amino acid uptake in fish cells (Michelato et al., 2018). Additionally, taurine actively participates in glucose metabolism (Bañuelos-Vargas et al., 2014). The reduction in ALT and AST levels indicates that there are no adverse effects when taurine added to diet containing fat powder, aligning with similar findings reported in common carp (Cyprinus carpio L.) (Liu et al., 2024). At the same time, the improvement in antioxidant capacity and reduction in fat oxidation demonstrate the protective effects of taurine. In Asian swamp eel (Monopterus albus), the addition of 0.2% taurine to oxidized fish oil was shown to increase antioxidant enzyme activities (SOD, GPx, CAT) while reducing MDA levels (Zhang et al., 2022). The enhancement of immune indices, including ACH50, Lysozyme, and immunoglobulin levels, underscores the importance of taurine in maintaining immune function. Conversely, taurine deficiency has been shown to disrupt immune function (Schuller-Levis et al., 1990). In conclusion, the addition of 2% taurine positively affects bile salt production, fat digestion, and overall fat metabolism. It mitigates the negative effects of fat powder on liver function while improving blood parameters, immune function, and antioxidant activity.
Conflict of interest
There is no conflict of interest between authors.
Acknowledgment
This research was carried out with the assistance of the members of the Fisheries Department at Sari Agricultural Sciences and Natural Resources University, who provided laboratory and workshop facilities for this experiment. We would also like to thank the esteemed management of the Livestock and Aquatic Feed Company for their sincere cooperation in providing the raw materials for the fish diet. Special thanks to the esteemed manager of Kimia Growth Industry for supplying the fat powder.
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