اثر افزودن تورین و پودر چربی به جیره غذایی بر شاخص‌های رشد، ترکیب بیوشیمیایی بدن و هضم‌پذیری چربی قزل‌آلای رنگین کمان انگشت قد (Oncorhynchus mykiss)

Introduction: Rainbow trout, Oncorhynchus mykiss belongs to the Salmonid family, one of the important domestic species in freshwater. High-energy diets with approximate levels of 45-50% protein and 18-24% fat are considered to meet their demands. However, due to the limited availability of fish oil, it cannot be considered a sustainable fat source in aquaculture. Meanwhile, fat powder can be used as an available and sustainable alternative in the diet, but due to the presence of saturated fatty acids, it has low digestibility, which may adversely affect fish health and growth. Thus, suitable additives might increase the possibility of replacing fat powder in the diet. Additionally, the amino acid taurine plays a crucial role in the formation of bile salts, which are essential for the digestion and absorption of intestinal fats. This study aims to investigate the effect of supplementary taurine on the growth, body composition, and fat digestibility of rainbow trout fed fat powder, with a focus on optimizing aquaculture productivity and sustainability.
Materials and Methods: Experimental diets included a positive control containing fish oil, canola oil and other diets containing fat powder (about 70% of the fat source was provided) supplemented with taurine at doses of 0 (negative control), 5, 10, and 20 g/kg (T₀, T₅, T₁₀ and T₂₀, respectively). A total of 225 rainbow trout with an average initial weight of 12 ± 0.03 g were distributed in 15 rearing tanks based on a completely randomized design for 58 days. Fish were fed three times a day at 8:00, 12:00 and 18:00 until apparent satiation. After 24 hours of stop feeding, the approximate amount of body composition (sampling of three fish from each tank at the end of the experiment) and items including protein, moisture, total fat and ash were measured followed by the procedure of AOAC (2005). At the end of the experiment, in order to evaluate the apparent digestibility of nutrients, fish were fed with diets containing chromic oxide. Then, feed and feces were collected and analyzed according to the method described by Austreng (1978). SPSS software (version 19.0) was used for statistical analysis. Data were presented as the mean of 3 replicates ± standard deviation using the Duncan post-hoc test to compare means of each treatment.
Results: The results of growth parameters showed that T₂₀ exhibited in a significant increase in final weight, percentage of weight gain and specific growth rate compared to the negative control (T₀; P<0.05) Feed conversion ratio, feed intake, protein efficiency, condition factor and survival rate were not affected by the experimental diets (P<0.05). According to the result of body composition, fat, protein and ash contents were affected by the different levels of taurine (P<0.05), but moisture did not show a significant difference between the experimental treatments (P<0.05). T₂₀ resulted in an increase in protein and fat compared to the negative control (P<0.05). Body fat content was higher in the positive control than in the other groups (P<0.05). The lowest body fat content was related to T₂₀. (P<0.05). Body protein was improved in T₂₀ similar to the positive control (P<0.05), and the lowest amount of protein was found in the negative control (P<0.05). The ash content in the positive control was significantly higher than in the negative control (P<0.05). Furthermore, T₂₀ led to an increase in protein digestion, while the lowest value was observed in the negative control (P<0.05). The fat digestibility was negatively affected by fat powder inclusion. However, it could be improved to some extent when taurine was supplemented. The highest ash content was found in T₂₀ whereas the lowest in the negative control (P<0.05).
Discussion: The results of the present study indicate that the addition of 20 g/kg taurine to a diet containing fat powder (T₂₀) improves the growth performance of fish. This improvement is likely due to the enhanced fat digestion and better fat utilization as an energy source. Furthermore, T₂₀ resulted in elevated body protein and fat content. Although the body fat in fish treated with the positive control was higher than in the other treatments, T₂₀ seems to fully compensate the body protein levels. The increased body fat suggests improved digestibility of fat powder under the influence of taurine. Taurine has a significant role in enhancing fat digestion by binding to bile acids, leading to the formation of bile salts. These bile salts are stored in the gallbladder and released into the intestine as required, facilitating the digestion and absorption of fats. Taurine increases protein synthesis by accelerating glycolysis and fulfills the growth requirements of the fish body through intense oxidative glucose breakdown or its conversion into amino acids. The current findings showed that protein digestibility improved by diets containing 10 and 20 g/kg taurine (T₁₀ and T₂₀). On the other hand, the inclusion of fish oil in the diet, due to its polyunsaturated fatty acids with multiple double bonds, demonstrates greater digestibility compared to fat powders. The observed reduction in digestibility in the negative control group can be attributed to the insufficiency of bile salts, but this could be compensated by the addition of taurine.
Conclusion: Given the results, it can be concluded that the addition of taurine successfully improved the digestibility of fat powder in the diet of rainbow trout. As a result, taurine enhanced fat absorption, and at the level of 20 g/kg taurine, it also improved protein digestibility. These improvements led to enhanced growth performance and increased body protein in rainbow trout.